table of contents
GCC(1) | GNU | GCC(1) |
NAME¶
gcc - GNU project C and C++ compiler
SYNOPSIS¶
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...
Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc.
DESCRIPTION¶
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" allow you to stop this process at an intermediate stage. For example, the -c option says not to run the linker. Then the output consists of object files output by the assembler.
Other options are passed on to one or more stages of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.
The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or machine-gcc-version to run a specific version of GCC. When you compile C++ programs, you should invoke GCC as g++ instead.
The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dv is very different from -d -v.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant.
Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This manual documents only one of these two forms, whichever one is not the default.
OPTIONS¶
Option Summary¶
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
- Overall Options
- -c -S -E -o file -x language -v -### --help[=class[,...]] --target-help --version -pass-exit-codes -pipe -specs=file -wrapper @file -ffile-prefix-map=old=new -fplugin=file -fplugin-arg-name=arg -fdump-ada-spec[-slim] -fada-spec-parent=unit -fdump-go-spec=file
- C Language Options
- -ansi -std=standard -fgnu89-inline -fpermitted-flt-eval-methods=standard -aux-info filename -fallow-parameterless-variadic-functions -fno-asm -fno-builtin -fno-builtin-function -fgimple -fhosted -ffreestanding -fopenacc -fopenmp -fopenmp-simd -fms-extensions -fplan9-extensions -fsso-struct=endianness -fallow-single-precision -fcond-mismatch -flax-vector-conversions -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char
- C++ Language Options
- -fabi-version=n -fno-access-control -faligned-new=n -fargs-in-order=n -fcheck-new -fconstexpr-depth=n -fconstexpr-loop-limit=n -ffriend-injection -fno-elide-constructors -fno-enforce-eh-specs -ffor-scope -fno-for-scope -fno-gnu-keywords -fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines -fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching -fno-nonansi-builtins -fnothrow-opt -fno-operator-names -fno-optional-diags -fpermissive -fno-pretty-templates -frepo -fno-rtti -fsized-deallocation -ftemplate-backtrace-limit=n -ftemplate-depth=n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fvisibility-inlines-hidden -fvisibility-ms-compat -fext-numeric-literals -Wabi=n -Wabi-tag -Wconversion-null -Wctor-dtor-privacy -Wdelete-non-virtual-dtor -Wliteral-suffix -Wmultiple-inheritance -Wnamespaces -Wnarrowing -Wnoexcept -Wnoexcept-type -Wclass-memaccess -Wnon-virtual-dtor -Wreorder -Wregister -Weffc++ -Wstrict-null-sentinel -Wtemplates -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo -Wvirtual-inheritance
- Objective-C and Objective-C++ Language Options
- -fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers -fobjc-abi-version=n -fobjc-call-cxx-cdtors -fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck -fobjc-std=objc1 -fno-local-ivars -fivar-visibility=[public|protected|private|package] -freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept -Wno-protocol -Wselector -Wstrict-selector-match -Wundeclared-selector
- Diagnostic Message Formatting Options
- -fmessage-length=n -fdiagnostics-show-location=[once|every-line] -fdiagnostics-color=[auto|never|always] -fno-diagnostics-show-option -fno-diagnostics-show-caret -fdiagnostics-parseable-fixits -fdiagnostics-generate-patch -fdiagnostics-show-template-tree -fno-elide-type -fno-show-column
- Warning Options
- -fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w -Wextra -Wall -Waddress -Waggregate-return -Waligned-new -Walloc-zero -Walloc-size-larger-than=n -Walloca -Walloca-larger-than=n -Wno-aggressive-loop-optimizations -Warray-bounds -Warray-bounds=n -Wno-attributes -Wbidi-chars=[none|unpaired|any] -Wbool-compare -Wbool-operation -Wno-builtin-declaration-mismatch -Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat -Wc++-compat -Wc++11-compat -Wc++14-compat -Wcast-align -Wcast-align=strict -Wcast-function-type -Wcast-qual -Wchar-subscripts -Wchkp -Wcatch-value -Wcatch-value=n -Wclobbered -Wcomment -Wconditionally-supported -Wconversion -Wcoverage-mismatch -Wno-cpp -Wdangling-else -Wdate-time -Wdelete-incomplete -Wno-deprecated -Wno-deprecated-declarations -Wno-designated-init -Wdisabled-optimization -Wno-discarded-qualifiers -Wno-discarded-array-qualifiers -Wno-div-by-zero -Wdouble-promotion -Wduplicated-branches -Wduplicated-cond -Wempty-body -Wenum-compare -Wno-endif-labels -Wexpansion-to-defined -Werror -Werror=* -Wextra-semi -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral -Wformat-overflow=n -Wformat-security -Wformat-signedness -Wformat-truncation=n -Wformat-y2k -Wframe-address -Wframe-larger-than=len -Wno-free-nonheap-object -Wjump-misses-init -Wif-not-aligned -Wignored-qualifiers -Wignored-attributes -Wincompatible-pointer-types -Wimplicit -Wimplicit-fallthrough -Wimplicit-fallthrough=n -Wimplicit-function-declaration -Wimplicit-int -Winit-self -Winline -Wno-int-conversion -Wint-in-bool-context -Wno-int-to-pointer-cast -Winvalid-memory-model -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than=len -Wlogical-op -Wlogical-not-parentheses -Wlong-long -Wmain -Wmaybe-uninitialized -Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation -Wmissing-attributes -Wmissing-braces -Wmissing-field-initializers -Wmissing-include-dirs -Wno-multichar -Wmultistatement-macros -Wnonnull -Wnonnull-compare -Wnormalized=[none|id|nfc|nfkc] -Wnull-dereference -Wodr -Wno-overflow -Wopenmp-simd -Woverride-init-side-effects -Woverlength-strings -Wpacked -Wpacked-bitfield-compat -Wpacked-not-aligned -Wpadded -Wparentheses -Wno-pedantic-ms-format -Wplacement-new -Wplacement-new=n -Wpointer-arith -Wpointer-compare -Wno-pointer-to-int-cast -Wno-pragmas -Wredundant-decls -Wrestrict -Wno-return-local-addr -Wreturn-type -Wsequence-point -Wshadow -Wno-shadow-ivar -Wshadow=global, -Wshadow=local, -Wshadow=compatible-local -Wshift-overflow -Wshift-overflow=n -Wshift-count-negative -Wshift-count-overflow -Wshift-negative-value -Wsign-compare -Wsign-conversion -Wfloat-conversion -Wno-scalar-storage-order -Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wsizeof-array-argument -Wstack-protector -Wstack-usage=len -Wstrict-aliasing -Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=n -Wstringop-overflow=n -Wstringop-truncation -Wsuggest-attribute=[pure|const|noreturn|format|malloc] -Wsuggest-final-types -Wsuggest-final-methods -Wsuggest-override -Wmissing-format-attribute -Wsubobject-linkage -Wswitch -Wswitch-bool -Wswitch-default -Wswitch-enum -Wswitch-unreachable -Wsync-nand -Wsystem-headers -Wtautological-compare -Wtrampolines -Wtrigraphs -Wtype-limits -Wundef -Wuninitialized -Wunknown-pragmas -Wunsuffixed-float-constants -Wunused -Wunused-function -Wunused-label -Wunused-local-typedefs -Wunused-macros -Wunused-parameter -Wno-unused-result -Wunused-value -Wunused-variable -Wunused-const-variable -Wunused-const-variable=n -Wunused-but-set-parameter -Wunused-but-set-variable -Wuseless-cast -Wvariadic-macros -Wvector-operation-performance -Wvla -Wvla-larger-than=n -Wvolatile-register-var -Wwrite-strings -Wzero-as-null-pointer-constant -Whsa
- C and Objective-C-only Warning Options
- -Wbad-function-cast -Wmissing-declarations -Wmissing-parameter-type -Wmissing-prototypes -Wnested-externs -Wold-style-declaration -Wold-style-definition -Wstrict-prototypes -Wtraditional -Wtraditional-conversion -Wdeclaration-after-statement -Wpointer-sign
- Debugging Options
- -g -glevel -gdwarf -gdwarf-version -ggdb -grecord-gcc-switches -gno-record-gcc-switches -gstabs -gstabs+ -gstrict-dwarf -gno-strict-dwarf -gas-loc-support -gno-as-loc-support -gas-locview-support -gno-as-locview-support -gcolumn-info -gno-column-info -gstatement-frontiers -gno-statement-frontiers -gvariable-location-views -gno-variable-location-views -ginternal-reset-location-views -gno-internal-reset-location-views -ginline-points -gno-inline-points -gvms -gxcoff -gxcoff+ -gz[=type] -fdebug-prefix-map=old=new -fdebug-types-section -fno-eliminate-unused-debug-types -femit-struct-debug-baseonly -femit-struct-debug-reduced -femit-struct-debug-detailed[=spec-list] -feliminate-unused-debug-symbols -femit-class-debug-always -fno-merge-debug-strings -fno-dwarf2-cfi-asm -fvar-tracking -fvar-tracking-assignments
- Optimization Options
- -faggressive-loop-optimizations -falign-functions[=n] -falign-jumps[=n] -falign-labels[=n] -falign-loops[=n] -fassociative-math -fauto-profile -fauto-profile[=path] -fauto-inc-dec -fbranch-probabilities -fbranch-target-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves -fcombine-stack-adjustments -fconserve-stack -fcompare-elim -fcprop-registers -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range -fdata-sections -fdce -fdelayed-branch -fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively -fdevirtualize-at-ltrans -fdse -fearly-inlining -fipa-sra -fexpensive-optimizations -ffat-lto-objects -ffast-math -ffinite-math-only -ffloat-store -fexcess-precision=style -fforward-propagate -ffp-contract=style -ffunction-sections -fgcse -fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity -fgcse-sm -fhoist-adjacent-loads -fif-conversion -fif-conversion2 -findirect-inlining -finline-functions -finline-functions-called-once -finline-limit=n -finline-small-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp -fipa-vrp -fipa-pta -fipa-profile -fipa-pure-const -fipa-reference -fipa-icf -fira-algorithm=algorithm -flive-patching=level -fira-region=region -fira-hoist-pressure -fira-loop-pressure -fno-ira-share-save-slots -fno-ira-share-spill-slots -fisolate-erroneous-paths-dereference -fisolate-erroneous-paths-attribute -fivopts -fkeep-inline-functions -fkeep-static-functions -fkeep-static-consts -flimit-function-alignment -flive-range-shrinkage -floop-block -floop-interchange -floop-strip-mine -floop-unroll-and-jam -floop-nest-optimize -floop-parallelize-all -flra-remat -flto -flto-compression-level -flto-partition=alg -fmerge-all-constants -fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves -fmove-loop-invariants -fno-branch-count-reg -fno-defer-pop -fno-fp-int-builtin-inexact -fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 -fno-printf-return-value -fno-sched-interblock -fno-sched-spec -fno-signed-zeros -fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-sibling-calls -fpartial-inlining -fpeel-loops -fpredictive-commoning -fprefetch-loop-arrays -fprofile-correction -fprofile-use -fprofile-use=path -fprofile-values -fprofile-reorder-functions -freciprocal-math -free -frename-registers -freorder-blocks -freorder-blocks-algorithm=algorithm -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -freschedule-modulo-scheduled-loops -frounding-math -fsched2-use-superblocks -fsched-pressure -fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n] -fsched-group-heuristic -fsched-critical-path-heuristic -fsched-spec-insn-heuristic -fsched-rank-heuristic -fsched-last-insn-heuristic -fsched-dep-count-heuristic -fschedule-fusion -fschedule-insns -fschedule-insns2 -fsection-anchors -fselective-scheduling -fselective-scheduling2 -fsel-sched-pipelining -fsel-sched-pipelining-outer-loops -fsemantic-interposition -fshrink-wrap -fshrink-wrap-separate -fsignaling-nans -fsingle-precision-constant -fsplit-ivs-in-unroller -fsplit-loops -fsplit-paths -fsplit-wide-types -fssa-backprop -fssa-phiopt -fstdarg-opt -fstore-merging -fstrict-aliasing -fthread-jumps -ftracer -ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -fcode-hoisting -ftree-loop-if-convert -ftree-loop-im -ftree-phiprop -ftree-loop-distribution -ftree-loop-distribute-patterns -ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize -ftree-loop-vectorize -ftree-parallelize-loops=n -ftree-pre -ftree-partial-pre -ftree-pta -ftree-reassoc -ftree-sink -ftree-slsr -ftree-sra -ftree-switch-conversion -ftree-tail-merge -ftree-ter -ftree-vectorize -ftree-vrp -funconstrained-commons -funit-at-a-time -funroll-all-loops -funroll-loops -funsafe-math-optimizations -funswitch-loops -fipa-ra -fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb -fwhole-program -fwpa -fuse-linker-plugin --param name=value -O -O0 -O1 -O2 -O3 -Os -Ofast -Og
- Program Instrumentation Options
- -p -pg -fprofile-arcs --coverage -ftest-coverage -fprofile-abs-path -fprofile-dir=path -fprofile-generate -fprofile-generate=path -fsanitize=style -fsanitize-recover -fsanitize-recover=style -fasan-shadow-offset=number -fsanitize-sections=s1,s2,... -fsanitize-undefined-trap-on-error -fbounds-check -fcheck-pointer-bounds -fchkp-check-incomplete-type -fchkp-first-field-has-own-bounds -fchkp-narrow-bounds -fchkp-narrow-to-innermost-array -fchkp-optimize -fchkp-use-fast-string-functions -fchkp-use-nochk-string-functions -fchkp-use-static-bounds -fchkp-use-static-const-bounds -fchkp-treat-zero-dynamic-size-as-infinite -fchkp-check-read -fchkp-check-read -fchkp-check-write -fchkp-store-bounds -fchkp-instrument-calls -fchkp-instrument-marked-only -fchkp-use-wrappers -fchkp-flexible-struct-trailing-arrays -fcf-protection=[full|branch|return|none] -fstack-protector -fstack-protector-all -fstack-protector-strong -fstack-protector-explicit -fstack-check -fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit -fsplit-stack -fvtable-verify=[std|preinit|none] -fvtv-counts -fvtv-debug -finstrument-functions -finstrument-functions-exclude-function-list=sym,sym,... -finstrument-functions-exclude-file-list=file,file,...
- Preprocessor Options
- -Aquestion=answer -A-question[=answer] -C -CC -Dmacro[=defn] -dD -dI -dM -dN -dU -fdebug-cpp -fdirectives-only -fdollars-in-identifiers -fexec-charset=charset -fextended-identifiers -finput-charset=charset -fmacro-prefix-map=old=new -fno-canonical-system-headers -fpch-deps -fpch-preprocess -fpreprocessed -ftabstop=width -ftrack-macro-expansion -fwide-exec-charset=charset -fworking-directory -H -imacros file -include file -M -MD -MF -MG -MM -MMD -MP -MQ -MT -no-integrated-cpp -P -pthread -remap -traditional -traditional-cpp -trigraphs -Umacro -undef -Wp,option -Xpreprocessor option
- Assembler Options
- -Wa,option -Xassembler option
- Linker Options
- object-file-name -fuse-ld=linker -llibrary -nostartfiles -nodefaultlibs -nostdlib -pie -pthread -rdynamic -s -static -static-pie -static-libgcc -static-libstdc++ -static-libasan -static-libtsan -static-liblsan -static-libubsan -static-libmpx -static-libmpxwrappers -shared -shared-libgcc -symbolic -T script -Wl,option -Xlinker option -u symbol -z keyword
- Directory Options
- -Bprefix -Idir -I- -idirafter dir -imacros file -imultilib dir -iplugindir=dir -iprefix file -iquote dir -isysroot dir -isystem dir -iwithprefix dir -iwithprefixbefore dir -Ldir -no-canonical-prefixes --no-sysroot-suffix -nostdinc -nostdinc++ --sysroot=dir
- Code Generation Options
- -fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions -fdelete-dead-exceptions -funwind-tables -fasynchronous-unwind-tables -fno-gnu-unique -finhibit-size-directive -fno-common -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-plt -fno-jump-tables -frecord-gcc-switches -freg-struct-return -fshort-enums -fshort-wchar -fverbose-asm -fpack-struct[=n] -fleading-underscore -ftls-model=model -fstack-reuse=reuse_level -ftrampolines -ftrapv -fwrapv -fvisibility=[default|internal|hidden|protected] -fstrict-volatile-bitfields -fsync-libcalls
- Developer Options
- -dletters -dumpspecs -dumpmachine -dumpversion -dumpfullversion -fchecking -fchecking=n -fdbg-cnt-list -fdbg-cnt=counter-value-list -fdisable-ipa-pass_name -fdisable-rtl-pass_name -fdisable-rtl-pass-name=range-list -fdisable-tree-pass_name -fdisable-tree-pass-name=range-list -fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links -fdump-final-insns[=file] -fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline -fdump-lang-all -fdump-lang-switch -fdump-lang-switch-options -fdump-lang-switch-options=filename -fdump-passes -fdump-rtl-pass -fdump-rtl-pass=filename -fdump-statistics -fdump-tree-all -fdump-tree-switch -fdump-tree-switch-options -fdump-tree-switch-options=filename -fcompare-debug[=opts] -fcompare-debug-second -fenable-kind-pass -fenable-kind-pass=range-list -fira-verbose=n -flto-report -flto-report-wpa -fmem-report-wpa -fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report -fopt-info -fopt-info-options[=file] -fprofile-report -frandom-seed=string -fsched-verbose=n -fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose -fstats -fstack-usage -ftime-report -ftime-report-details -fvar-tracking-assignments-toggle -gtoggle -print-file-name=library -print-libgcc-file-name -print-multi-directory -print-multi-lib -print-multi-os-directory -print-prog-name=program -print-search-dirs -Q -print-sysroot -print-sysroot-headers-suffix -save-temps -save-temps=cwd -save-temps=obj -time[=file]
- Machine-Dependent Options
- AArch64 Options -mabi=name -mbig-endian
-mlittle-endian -mgeneral-regs-only -mcmodel=tiny
-mcmodel=small -mcmodel=large -mstrict-align
-momit-leaf-frame-pointer -mtls-dialect=desc
-mtls-dialect=traditional -mtls-size=size
-mfix-cortex-a53-835769 -mfix-cortex-a53-843419
-mlow-precision-recip-sqrt -mlow-precision-sqrt -mlow-precision-div
-mpc-relative-literal-loads
-msign-return-address=scope -march=name
-mcpu=name -mtune=name
-moverride=string -mverbose-cost-dump
-moutline-atomics
Adapteva Epiphany Options -mhalf-reg-file -mprefer-short-insn-regs -mbranch-cost=num -mcmove -mnops=num -msoft-cmpsf -msplit-lohi -mpost-inc -mpost-modify -mstack-offset=num -mround-nearest -mlong-calls -mshort-calls -msmall16 -mfp-mode=mode -mvect-double -max-vect-align=num -msplit-vecmove-early -m1reg-reg
ARC Options -mbarrel-shifter -mjli-always -mcpu=cpu -mA6 -mARC600 -mA7 -mARC700 -mdpfp -mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr -mea -mno-mpy -mmul32x16 -mmul64 -matomic -mnorm -mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap -mcrc -mdsp-packa -mdvbf -mlock -mmac-d16 -mmac-24 -mrtsc -mswape -mtelephony -mxy -misize -mannotate-align -marclinux -marclinux_prof -mlong-calls -mmedium-calls -msdata -mirq-ctrl-saved -mrgf-banked-regs -mlpc-width=width -G num -mvolatile-cache -mtp-regno=regno -malign-call -mauto-modify-reg -mbbit-peephole -mno-brcc -mcase-vector-pcrel -mcompact-casesi -mno-cond-exec -mearly-cbranchsi -mexpand-adddi -mindexed-loads -mlra -mlra-priority-none -mlra-priority-compact mlra-priority-noncompact -mno-millicode -mmixed-code -mq-class -mRcq -mRcw -msize-level=level -mtune=cpu -mmultcost=num -munalign-prob-threshold=probability -mmpy-option=multo -mdiv-rem -mcode-density -mll64 -mfpu=fpu -mrf16
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check -mno-apcs-stack-check -mapcs-reentrant -mno-apcs-reentrant -msched-prolog -mno-sched-prolog -mlittle-endian -mbig-endian -mbe8 -mbe32 -mfloat-abi=name -mfp16-format=name -mthumb-interwork -mno-thumb-interwork -mcpu=name -march=name -mfpu=name -mtune=name -mprint-tune-info -mstructure-size-boundary=n -mabort-on-noreturn -mlong-calls -mno-long-calls -msingle-pic-base -mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport -mpoke-function-name -mthumb -marm -mflip-thumb -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking -mtp=name -mtls-dialect=dialect -mword-relocations -mfix-cortex-m3-ldrd -munaligned-access -mneon-for-64bits -mslow-flash-data -masm-syntax-unified -mrestrict-it -mverbose-cost-dump -mpure-code -mcmse
AVR Options -mmcu=mcu -mabsdata -maccumulate-args -mbranch-cost=cost -mcall-prologues -mgas-isr-prologues -mint8 -mn_flash=size -mno-interrupts -mmain-is-OS_task -mrelax -mrmw -mstrict-X -mtiny-stack -mfract-convert-truncate -mshort-calls -nodevicelib -nodevicespecs -Waddr-space-convert -Wmisspelled-isr
Blackfin Options -mcpu=cpu[-sirevision] -msim -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer -mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly -mlow-64k -mno-low64k -mstack-check-l1 -mid-shared-library -mno-id-shared-library -mshared-library-id=n -mleaf-id-shared-library -mno-leaf-id-shared-library -msep-data -mno-sep-data -mlong-calls -mno-long-calls -mfast-fp -minline-plt -mmulticore -mcorea -mcoreb -msdram -micplb
C6X Options -mbig-endian -mlittle-endian -march=cpu -msim -msdata=sdata-type
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n -melinux-stacksize=n -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt -melf -maout -melinux -mlinux -sim -sim2 -mmul-bug-workaround -mno-mul-bug-workaround
CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32 -mbit-ops -mdata-model=model
Darwin Options -all_load -allowable_client -arch -arch_errors_fatal -arch_only -bind_at_load -bundle -bundle_loader -client_name -compatibility_version -current_version -dead_strip -dependency-file -dylib_file -dylinker_install_name -dynamic -dynamiclib -exported_symbols_list -filelist -flat_namespace -force_cpusubtype_ALL -force_flat_namespace -headerpad_max_install_names -iframework -image_base -init -install_name -keep_private_externs -multi_module -multiply_defined -multiply_defined_unused -noall_load -no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs -noprebind -noseglinkedit -pagezero_size -prebind -prebind_all_twolevel_modules -private_bundle -read_only_relocs -sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder -segaddr -segs_read_only_addr -segs_read_write_addr -seg_addr_table -seg_addr_table_filename -seglinkedit -segprot -segs_read_only_addr -segs_read_write_addr -single_module -static -sub_library -sub_umbrella -twolevel_namespace -umbrella -undefined -unexported_symbols_list -weak_reference_mismatches -whatsloaded -F -gused -gfull -mmacosx-version-min=version -mkernel -mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -mieee -mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode -mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants -mcpu=cpu-type -mtune=cpu-type -mbwx -mmax -mfix -mcix -mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data -mlarge-data -msmall-text -mlarge-text -mmemory-latency=time
FR30 Options -msmall-model -mno-lsim
FT32 Options -msim -mlra -mnodiv -mft32b -mcompress -mnopm
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float -malloc-cc -mfixed-cc -mdword -mno-dword -mdouble -mno-double -mmedia -mno-media -mmuladd -mno-muladd -mfdpic -minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp -mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move -moptimize-membar -mno-optimize-membar -mscc -mno-scc -mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec -mno-nested-cond-exec -mtomcat-stats -mTLS -mtls -mcpu=cpu
GNU/Linux Options -mglibc -muclibc -mmusl -mbionic -mandroid -tno-android-cc -tno-android-ld
H8/300 Options -mrelax -mh -ms -mn -mexr -mno-exr -mint32 -malign-300
HPPA Options -march=architecture-type -mcaller-copies -mdisable-fpregs -mdisable-indexing -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld -mfixed-range=register-range -mjump-in-delay -mlinker-opt -mlong-calls -mlong-load-store -mno-disable-fpregs -mno-disable-indexing -mno-fast-indirect-calls -mno-gas -mno-jump-in-delay -mno-long-load-store -mno-portable-runtime -mno-soft-float -mno-space-regs -msoft-float -mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime -mschedule=cpu-type -mspace-regs -msio -mwsio -munix=unix-std -nolibdld -static -threads
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop -mregister-names -msdata -mno-sdata -mconstant-gp -mauto-pic -mfused-madd -minline-float-divide-min-latency -minline-float-divide-max-throughput -mno-inline-float-divide -minline-int-divide-min-latency -minline-int-divide-max-throughput -mno-inline-int-divide -minline-sqrt-min-latency -minline-sqrt-max-throughput -mno-inline-sqrt -mdwarf2-asm -mearly-stop-bits -mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-type -milp32 -mlp64 -msched-br-data-spec -msched-ar-data-spec -msched-control-spec -msched-br-in-data-spec -msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc -msched-spec-control-ldc -msched-prefer-non-data-spec-insns -msched-prefer-non-control-spec-insns -msched-stop-bits-after-every-cycle -msched-count-spec-in-critical-path -msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost -msched-max-memory-insns-hard-limit -msched-max-memory-insns=max-insns
LM32 Options -mbarrel-shift-enabled -mdivide-enabled -mmultiply-enabled -msign-extend-enabled -muser-enabled
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops -missue-rate=number -mbranch-cost=number -mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func -mflush-func=name -mno-flush-trap -mflush-trap=number -G num
M32C Options -mcpu=cpu -msim -memregs=number
M680x0 Options -march=arch -mcpu=cpu -mtune=tune -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200 -m5206e -m528x -m5307 -m5407 -mcfv4e -mbitfield -mno-bitfield -mc68000 -mc68020 -mnobitfield -mrtd -mno-rtd -mdiv -mno-div -mshort -mno-short -mhard-float -m68881 -msoft-float -mpcrel -malign-int -mstrict-align -msep-data -mno-sep-data -mshared-library-id=n -mid-shared-library -mno-id-shared-library -mxgot -mno-xgot -mlong-jump-table-offsets
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates -mno-relax-immediates -mwide-bitfields -mno-wide-bitfields -m4byte-functions -mno-4byte-functions -mcallgraph-data -mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment
MeP Options -mabsdiff -mall-opts -maverage -mbased=n -mbitops -mc=n -mclip -mconfig=name -mcop -mcop32 -mcop64 -mivc2 -mdc -mdiv -meb -mel -mio-volatile -ml -mleadz -mm -mminmax -mmult -mno-opts -mrepeat -ms -msatur -msdram -msim -msimnovec -mtf -mtiny=n
MicroBlaze Options -msoft-float -mhard-float -msmall-divides -mcpu=cpu -mmemcpy -mxl-soft-mul -mxl-soft-div -mxl-barrel-shift -mxl-pattern-compare -mxl-stack-check -mxl-gp-opt -mno-clearbss -mxl-multiply-high -mxl-float-convert -mxl-float-sqrt -mbig-endian -mlittle-endian -mxl-reorder -mxl-mode-app-model
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips32r3 -mips32r5 -mips32r6 -mips64 -mips64r2 -mips64r3 -mips64r5 -mips64r6 -mips16 -mno-mips16 -mflip-mips16 -minterlink-compressed -mno-interlink-compressed -minterlink-mips16 -mno-interlink-mips16 -mabi=abi -mabicalls -mno-abicalls -mshared -mno-shared -mplt -mno-plt -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfpxx -mfp64 -mhard-float -msoft-float -mno-float -msingle-float -mdouble-float -modd-spreg -mno-odd-spreg -mabs=mode -mnan=encoding -mdsp -mno-dsp -mdspr2 -mno-dspr2 -mmcu -mmno-mcu -meva -mno-eva -mvirt -mno-virt -mxpa -mno-xpa -mmicromips -mno-micromips -mmsa -mno-msa -mfpu=fpu-type -msmartmips -mno-smartmips -mpaired-single -mno-paired-single -mdmx -mno-mdmx -mips3d -mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64 -mlong32 -msym32 -mno-sym32 -Gnum -mlocal-sdata -mno-local-sdata -mextern-sdata -mno-extern-sdata -mgpopt -mno-gopt -membedded-data -mno-embedded-data -muninit-const-in-rodata -mno-uninit-const-in-rodata -mcode-readable=setting -msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs -mcheck-zero-division -mno-check-zero-division -mdivide-traps -mdivide-breaks -mload-store-pairs -mno-load-store-pairs -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls -mmad -mno-mad -mimadd -mno-imadd -mfused-madd -mno-fused-madd -nocpp -mfix-24k -mno-fix-24k -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 -mfix-r10000 -mno-fix-r10000 -mfix-rm7000 -mno-fix-rm7000 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mno-fix-vr4130 -mfix-sb1 -mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-cost=num -mbranch-likely -mno-branch-likely -mcompact-branches=policy -mfp-exceptions -mno-fp-exceptions -mvr4130-align -mno-vr4130-align -msynci -mno-synci -mlxc1-sxc1 -mno-lxc1-sxc1 -mmadd4 -mno-madd4 -mrelax-pic-calls -mno-relax-pic-calls -mmcount-ra-address -mframe-header-opt -mno-frame-header-opt
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict -mbase-addresses -mno-base-addresses -msingle-exit -mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mno-am33 -mam33 -mam33-2 -mam34 -mtune=cpu-type -mreturn-pointer-on-d0 -mno-crt0 -mrelax -mliw -msetlb
Moxie Options -meb -mel -mmul.x -mno-crt0
MSP430 Options -msim -masm-hex -mmcu= -mcpu= -mlarge -msmall -mrelax -mwarn-mcu -mcode-region= -mdata-region= -msilicon-errata= -msilicon-errata-warn= -mhwmult= -minrt
NDS32 Options -mbig-endian -mlittle-endian -mreduced-regs -mfull-regs -mcmov -mno-cmov -mext-perf -mno-ext-perf -mext-perf2 -mno-ext-perf2 -mext-string -mno-ext-string -mv3push -mno-v3push -m16bit -mno-16bit -misr-vector-size=num -mcache-block-size=num -march=arch -mcmodel=code-model -mctor-dtor -mrelax
Nios II Options -G num -mgpopt=option -mgpopt -mno-gpopt -mgprel-sec=regexp -mr0rel-sec=regexp -mel -meb -mno-bypass-cache -mbypass-cache -mno-cache-volatile -mcache-volatile -mno-fast-sw-div -mfast-sw-div -mhw-mul -mno-hw-mul -mhw-mulx -mno-hw-mulx -mno-hw-div -mhw-div -mcustom-insn=N -mno-custom-insn -mcustom-fpu-cfg=name -mhal -msmallc -msys-crt0=name -msys-lib=name -march=arch -mbmx -mno-bmx -mcdx -mno-cdx
Nvidia PTX Options -m32 -m64 -mmainkernel -moptimize
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mbcopy -mbcopy-builtin -mint32 -mno-int16 -mint16 -mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32 -mabshi -mno-abshi -mbranch-expensive -mbranch-cheap -munix-asm -mdec-asm
picoChip Options -mae=ae_type -mvliw-lookahead=N -msymbol-as-address -mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
PowerPC SPE Options -mcpu=cpu-type -mtune=cpu-type -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -m32 -mxl-compat -mno-xl-compat -malign-power -malign-natural -msoft-float -mhard-float -mmultiple -mno-multiple -msingle-float -mdouble-float -mupdate -mno-update -mavoid-indexed-addresses -mno-avoid-indexed-addresses -mstrict-align -mno-strict-align -mrelocatable -mno-relocatable -mrelocatable-lib -mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian -msingle-pic-base -mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type -minsert-sched-nops=scheme -mcall-sysv -mcall-netbsd -maix-struct-return -msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt -mblock-move-inline-limit=num -misel -mno-isel -misel=yes -misel=no -mspe -mno-spe -mspe=yes -mspe=no -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double -mprototype -mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -G num -mrecip -mrecip=opt -mno-recip -mrecip-precision -mno-recip-precision -mpointers-to-nested-functions -mno-pointers-to-nested-functions -msave-toc-indirect -mno-save-toc-indirect -mcompat-align-parm -mno-compat-align-parm -mfloat128 -mno-float128 -mgnu-attribute -mno-gnu-attribute -mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset
RISC-V Options -mbranch-cost=N-instruction -mplt -mno-plt -mabi=ABI-string -mfdiv -mno-fdiv -mdiv -mno-div -march=ISA-string -mtune=processor-string -mpreferred-stack-boundary=num -msmall-data-limit=N-bytes -msave-restore -mno-save-restore -mstrict-align -mno-strict-align -mcmodel=medlow -mcmodel=medany -mexplicit-relocs -mno-explicit-relocs -mrelax -mno-relax
RL78 Options -msim -mmul=none -mmul=g13 -mmul=g14 -mallregs -mcpu=g10 -mcpu=g13 -mcpu=g14 -mg10 -mg13 -mg14 -m64bit-doubles -m32bit-doubles -msave-mduc-in-interrupts
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mpowerpc64 -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mpopcntd -mno-popcntd -mfprnd -mno-fprnd -mcmpb -mno-cmpb -mmfpgpr -mno-mfpgpr -mhard-dfp -mno-hard-dfp -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat -mpe -malign-power -malign-natural -msoft-float -mhard-float -mmultiple -mno-multiple -msingle-float -mdouble-float -msimple-fpu -mupdate -mno-update -mavoid-indexed-addresses -mno-avoid-indexed-addresses -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align -mstrict-align -mno-strict-align -mrelocatable -mno-relocatable -mrelocatable-lib -mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian -mdynamic-no-pic -maltivec -mswdiv -msingle-pic-base -mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type -minsert-sched-nops=scheme -mcall-aixdesc -mcall-eabi -mcall-freebsd -mcall-linux -mcall-netbsd -mcall-openbsd -mcall-sysv -mcall-sysv-eabi -mcall-sysv-noeabi -mtraceback=traceback_type -maix-struct-return -msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt -mblock-move-inline-limit=num -mblock-compare-inline-limit=num -mblock-compare-inline-loop-limit=num -mstring-compare-inline-limit=num -misel -mno-isel -misel=yes -misel=no -mpaired -mvrsave -mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mprototype -mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mreadonly-in-sdata -mvxworks -G num -mrecip -mrecip=opt -mno-recip -mrecip-precision -mno-recip-precision -mveclibabi=type -mfriz -mno-friz -mpointers-to-nested-functions -mno-pointers-to-nested-functions -msave-toc-indirect -mno-save-toc-indirect -mpower8-fusion -mno-mpower8-fusion -mpower8-vector -mno-power8-vector -mcrypto -mno-crypto -mhtm -mno-htm -mquad-memory -mno-quad-memory -mquad-memory-atomic -mno-quad-memory-atomic -mcompat-align-parm -mno-compat-align-parm -mfloat128 -mno-float128 -mfloat128-hardware -mno-float128-hardware -mgnu-attribute -mno-gnu-attribute -mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset
RX Options -m64bit-doubles -m32bit-doubles -fpu -nofpu -mcpu= -mbig-endian-data -mlittle-endian-data -msmall-data -msim -mno-sim -mas100-syntax -mno-as100-syntax -mrelax -mmax-constant-size= -mint-register= -mpid -mallow-string-insns -mno-allow-string-insns -mjsr -mno-warn-multiple-fast-interrupts -msave-acc-in-interrupts
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float -mhard-dfp -mno-hard-dfp -mlong-double-64 -mlong-double-128 -mbackchain -mno-backchain -mpacked-stack -mno-packed-stack -msmall-exec -mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch -mhtm -mvx -mzvector -mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard -mhotpatch=halfwords,halfwords
Score Options -meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u -mscore7 -mscore7d
SH Options -m1 -m2 -m2e -m2a-nofpu -m2a-single-only -m2a-single -m2a -m3 -m3e -m4-nofpu -m4-single-only -m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mrenesas -mno-renesas -mnomacsave -mieee -mno-ieee -mbitops -misize -minline-ic_invalidate -mpadstruct -mprefergot -musermode -multcost=number -mdiv=strategy -mdivsi3_libfunc=name -mfixed-range=register-range -maccumulate-outgoing-args -matomic-model=atomic-model -mbranch-cost=num -mzdcbranch -mno-zdcbranch -mcbranch-force-delay-slot -mfused-madd -mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra -mpretend-cmove -mtas
Solaris 2 Options -mclear-hwcap -mno-clear-hwcap -mimpure-text -mno-impure-text -pthreads
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mmemory-model=mem-model -m32 -m64 -mapp-regs -mno-app-regs -mfaster-structs -mno-faster-structs -mflat -mno-flat -mfpu -mno-fpu -mhard-float -msoft-float -mhard-quad-float -msoft-quad-float -mstack-bias -mno-stack-bias -mstd-struct-return -mno-std-struct-return -munaligned-doubles -mno-unaligned-doubles -muser-mode -mno-user-mode -mv8plus -mno-v8plus -mvis -mno-vis -mvis2 -mno-vis2 -mvis3 -mno-vis3 -mvis4 -mno-vis4 -mvis4b -mno-vis4b -mcbcond -mno-cbcond -mfmaf -mno-fmaf -mfsmuld -mno-fsmuld -mpopc -mno-popc -msubxc -mno-subxc -mfix-at697f -mfix-ut699 -mfix-ut700 -mfix-gr712rc -mlra -mno-lra
SPU Options -mwarn-reloc -merror-reloc -msafe-dma -munsafe-dma -mbranch-hints -msmall-mem -mlarge-mem -mstdmain -mfixed-range=register-range -mea32 -mea64 -maddress-space-conversion -mno-address-space-conversion -mcache-size=cache-size -matomic-updates -mno-atomic-updates
System V Options -Qy -Qn -YP,paths -Ym,dir
TILE-Gx Options -mcpu=CPU -m32 -m64 -mbig-endian -mlittle-endian -mcmodel=code-model
TILEPro Options -mcpu=cpu -m32
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function -mno-prolog-function -mspace -mtda=n -msda=n -mzda=n -mapp-regs -mno-app-regs -mdisable-callt -mno-disable-callt -mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e -mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps -msoft-float -mhard-float -mgcc-abi -mrh850-abi -mbig-switch
VAX Options -mg -mgnu -munix
Visium Options -mdebug -msim -mfpu -mno-fpu -mhard-float -msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode -muser-mode
VMS Options -mvms-return-codes -mdebug-main=prefix -mmalloc64 -mpointer-size=size
VxWorks Options -mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy -Xbind-now
x86 Options -mtune=cpu-type -march=cpu-type -mtune-ctrl=feature-list -mdump-tune-features -mno-default -mfpmath=unit -masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387 -m80387 -mhard-float -msoft-float -mno-wide-multiply -mrtd -malign-double -mpreferred-stack-boundary=num -mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe -mcrc32 -mrecip -mrecip=opt -mvzeroupper -mprefer-avx128 -mprefer-vector-width=opt -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f -mavx512pf -mavx512er -mavx512cd -mavx512vl -mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi -msha -maes -mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mprefetchwt1 -mclflushopt -mclwb -mxsavec -mxsaves -msse4a -m3dnow -m3dnowa -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -madx -mlzcnt -mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm -mlwp -mmpx -mmwaitx -mclzero -mpku -mthreads -mgfni -mvaes -mshstk -mforce-indirect-call -mavx512vbmi2 -mvpclmulqdq -mavx512bitalg -mmovdiri -mmovdir64b -mavx512vpopcntdq -mavx5124fmaps -mavx512vnni -mavx5124vnniw -mprfchw -mrdpid -mrdseed -msgx -mms-bitfields -mno-align-stringops -minline-all-stringops -minline-stringops-dynamically -mstringop-strategy=alg -mmemcpy-strategy=strategy -mmemset-strategy=strategy -mpush-args -maccumulate-outgoing-args -m128bit-long-double -m96bit-long-double -mlong-double-64 -mlong-double-80 -mlong-double-128 -mregparm=num -msseregparm -mveclibabi=type -mvect8-ret-in-mem -mpc32 -mpc64 -mpc80 -mstackrealign -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model -mabi=name -maddress-mode=mode -m32 -m64 -mx32 -m16 -miamcu -mlarge-data-threshold=num -msse2avx -mfentry -mrecord-mcount -mnop-mcount -m8bit-idiv -mavx256-split-unaligned-load -mavx256-split-unaligned-store -malign-data=type -mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset -mstack-protector-guard-symbol=symbol -mmitigate-rop -mgeneral-regs-only -mcall-ms2sysv-xlogues -mindirect-branch=choice -mfunction-return=choice -mindirect-branch-register -mharden-sls=choice -mindirect-branch-cs-prefix
x86 Windows Options -mconsole -mcygwin -mno-cygwin -mdll -mnop-fun-dllimport -mthread -municode -mwin32 -mwindows -fno-set-stack-executable
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mforce-no-pic -mserialize-volatile -mno-serialize-volatile -mtext-section-literals -mno-text-section-literals -mauto-litpools -mno-auto-litpools -mtarget-align -mno-target-align -mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Options Controlling the Kind of Output¶
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done:
- file.c
- C source code that must be preprocessed.
- file.i
- C source code that should not be preprocessed.
- file.ii
- C++ source code that should not be preprocessed.
- file.m
- Objective-C source code. Note that you must link with the libobjc library to make an Objective-C program work.
- file.mi
- Objective-C source code that should not be preprocessed.
- file.mm
- file.M
- Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ program work. Note that .M refers to a literal capital M.
- file.mii
- Objective-C++ source code that should not be preprocessed.
- file.h
- C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).
- file.cc
- file.cp
- file.cxx
- file.cpp
- file.CPP
- file.c++
- file.C
- C++ source code that must be preprocessed. Note that in .cxx, the last two letters must both be literally x. Likewise, .C refers to a literal capital C.
- file.mm
- file.M
- Objective-C++ source code that must be preprocessed.
- file.mii
- Objective-C++ source code that should not be preprocessed.
- file.hh
- file.H
- file.hp
- file.hxx
- file.hpp
- file.HPP
- file.h++
- file.tcc
- C++ header file to be turned into a precompiled header or Ada spec.
- file.f
- file.for
- file.ftn
- Fixed form Fortran source code that should not be preprocessed.
- file.F
- file.FOR
- file.fpp
- file.FPP
- file.FTN
- Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
- file.f90
- file.f95
- file.f03
- file.f08
- Free form Fortran source code that should not be preprocessed.
- file.F90
- file.F95
- file.F03
- file.F08
- Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
- file.go
- Go source code.
- file.brig
- BRIG files (binary representation of HSAIL).
- file.ads
- Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.
- file.adb
- Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.
- file.s
- Assembler code.
- file.S
- file.sx
- Assembler code that must be preprocessed.
- other
- An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
- -x language
- Specify explicitly the language for the following input files
(rather than letting the compiler choose a default based on the file name
suffix). This option applies to all following input files until the next
-x option. Possible values for language are:
c c-header cpp-output c++ c++-header c++-cpp-output objective-c objective-c-header objective-c-cpp-output objective-c++ objective-c++-header objective-c++-cpp-output assembler assembler-with-cpp ada f77 f77-cpp-input f95 f95-cpp-input go brig
- -x none
- Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if -x has not been used at all).
If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all.
- -c
- Compile or assemble the source files, but do not link. The linking stage
simply is not done. The ultimate output is in the form of an object file
for each source file.
By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
- -S
- Stop after the stage of compilation proper; do not assemble. The output is
in the form of an assembler code file for each non-assembler input file
specified.
By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
- -E
- Stop after the preprocessing stage; do not run the compiler proper. The
output is in the form of preprocessed source code, which is sent to the
standard output.
Input files that don't require preprocessing are ignored.
- -o file
- Place output in file file. This applies to whatever sort of output
is being produced, whether it be an executable file, an object file, an
assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output.
- -v
- Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.
- -###
- Like -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or "./-_". This is useful for shell scripts to capture the driver-generated command lines.
- --help
- Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. If the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed.
- --target-help
- Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed.
- --help={class|[^]qualifier}[,...]
- Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. These are the supported classes:
- optimizers
- Display all of the optimization options supported by the compiler.
- warnings
- Display all of the options controlling warning messages produced by the compiler.
- target
- Display target-specific options. Unlike the --target-help option however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended --help= syntax.
- params
- Display the values recognized by the --param option.
- language
- Display the options supported for language, where language is the name of one of the languages supported in this version of GCC.
- common
- Display the options that are common to all languages.
These are the supported qualifiers:
- undocumented
- Display only those options that are undocumented.
- joined
- Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as: --help=target.
- separate
- Display options taking an argument that appears as a separate word following the original option, such as: -o output-file.
Thus for example to display all the undocumented target-specific switches supported by the compiler, use:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of inverted qualifiers.
Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:
--help=target,optimizers
The --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed.
If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the --help= option is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c The following options are target specific: -mabi= 2 -mabort-on-noreturn [disabled] -mapcs [disabled]
The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts diff /tmp/O2-opts /tmp/O3-opts | grep enabled
- --version
- Display the version number and copyrights of the invoked GCC.
- -pass-exit-codes
- Normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code. If you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return 4 if an internal compiler error is encountered.
- -pipe
- Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble.
- -specs=file
- Process file after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command line, and they are processed in order, from left to right.
- -wrapper
- Invoke all subcommands under a wrapper program. The name of the wrapper
program and its parameters are passed as a comma separated list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 ....
- -ffile-prefix-map=old=new
- When compiling files residing in directory old, record any references to them in the result of the compilation as if the files resided in directory new instead. Specifying this option is equivalent to specifying all the individual -f*-prefix-map options. This can be used to make reproducible builds that are location independent. See also -fmacro-prefix-map and -fdebug-prefix-map.
- -fplugin=name.so
- Load the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the compiler. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (See -fplugin-arg-name-key=value below). Each plugin should define the callback functions specified in the Plugins API.
- -fplugin-arg-name-key=value
- Define an argument called key with a value of value for the plugin called name.
- -fdump-ada-spec[-slim]
- For C and C++ source and include files, generate corresponding Ada specs.
- -fada-spec-parent=unit
- In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of parent unit.
- -fdump-go-spec=file
- For input files in any language, generate corresponding Go declarations in file. This generates Go "const", "type", "var", and "func" declarations which may be a useful way to start writing a Go interface to code written in some other language.
- @file
- Read command-line options from file. The options read are inserted
in place of the original @file option. If file does not
exist, or cannot be read, then the option will be treated literally, and
not removed.
Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively.
Compiling C++ Programs¶
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use the suffix .ii. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).
However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats .c, .h and .i files as C++ source files instead of C source files unless -x is used. This program is also useful when precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs.
Options Controlling C Dialect¶
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:
- -ansi
- In C mode, this is equivalent to -std=c90. In C++ mode, it is
equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the "asm" and "typeof" keywords, and predefined macros such as "unix" and "vax" that identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++ style // comments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and "__typeof__" continue to work despite -ansi. You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with -ansi. Alternate predefined macros such as "__unix__" and "__vax__" are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -Wpedantic is required in addition to -ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things.
Functions that are normally built in but do not have semantics defined by ISO C (such as "alloca" and "ffs") are not built-in functions when -ansi is used.
- -std=
- Determine the language standard. This option is currently only supported
when compiling C or C++.
The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the "asm" and "typeof" keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a "?:" expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are
- c90
- c89
- iso9899:1990
- Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.
- iso9899:199409
- ISO C90 as modified in amendment 1.
- c99
- c9x
- iso9899:1999
- iso9899:199x
- ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <http://gcc.gnu.org/c99status.html> for more information. The names c9x and iso9899:199x are deprecated.
- c11
- c1x
- iso9899:2011
- ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated.
- c17
- c18
- iso9899:2017
- iso9899:2018
- ISO C17, the 2017 revision of the ISO C standard (expected to be published in 2018). This standard is same as C11 except for corrections of defects (all of which are also applied with -std=c11) and a new value of "__STDC_VERSION__", and so is supported to the same extent as C11.
- gnu90
- gnu89
- GNU dialect of ISO C90 (including some C99 features).
- gnu99
- gnu9x
- GNU dialect of ISO C99. The name gnu9x is deprecated.
- gnu11
- gnu1x
- GNU dialect of ISO C11. The name gnu1x is deprecated.
- gnu17
- gnu18
- GNU dialect of ISO C17. This is the default for C code.
- c++98
- c++03
- The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code.
- gnu++98
- gnu++03
- GNU dialect of -std=c++98.
- c++11
- c++0x
- The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.
- gnu++11
- gnu++0x
- GNU dialect of -std=c++11. The name gnu++0x is deprecated.
- c++14
- c++1y
- The 2014 ISO C++ standard plus amendments. The name c++1y is deprecated.
- gnu++14
- gnu++1y
- GNU dialect of -std=c++14. This is the default for C++ code. The name gnu++1y is deprecated.
- c++17
- c++1z
- The 2017 ISO C++ standard plus amendments. The name c++1z is deprecated.
- gnu++17
- gnu++1z
- GNU dialect of -std=c++17. The name gnu++1z is deprecated.
- c++2a
- The next revision of the ISO C++ standard, tentatively planned for 2020. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
- gnu++2a
- GNU dialect of -std=c++2a. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
- -fgnu89-inline
- The option -fgnu89-inline tells GCC to use the traditional GNU
semantics for "inline" functions when in
C99 mode.
Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which semantics are in effect for "inline" functions.
- -fpermitted-flt-eval-methods=style
- ISO/IEC TS 18661-3 defines new permissible values for
"FLT_EVAL_METHOD" that indicate that
operations and constants with a semantic type that is an interchange or
extended format should be evaluated to the precision and range of that
type. These new values are a superset of those permitted under C99/C11,
which does not specify the meaning of other positive values of
"FLT_EVAL_METHOD". As such, code
conforming to C11 may not have been written expecting the possibility of
the new values.
-fpermitted-flt-eval-methods specifies whether the compiler should allow only the values of "FLT_EVAL_METHOD" specified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3.
style is either "c11" or "ts-18661-3" as appropriate.
The default when in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. The default when in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3.
- -aux-info filename
- Output to the given filename prototyped declarations for all functions
declared and/or defined in a translation unit, including those in header
files. This option is silently ignored in any language other than C.
Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.
- -fallow-parameterless-variadic-functions
- Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very useful as it is not possible to read the arguments. This is only supported for C as this construct is allowed by C++.
- -fno-asm
- Do not recognize "asm",
"inline" or
"typeof" as a keyword, so that code can
use these words as identifiers. You can use the keywords
"__asm__",
"__inline__" and
"__typeof__" instead. -ansi
implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm" and "inline" are standard keywords. You may want to use the -fno-gnu-keywords flag instead, which has the same effect. In C99 mode (-std=c99 or -std=gnu99), this switch only affects the "asm" and "typeof" keywords, since "inline" is a standard keyword in ISO C99.
- -fno-builtin
- -fno-builtin-function
- Don't recognize built-in functions that do not begin with
__builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to "alloca" may become single instructions which adjust the stack directly, and calls to "memcpy" may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to "printf" when "printf" is built in and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with __builtin_. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n)) #define strcpy(d, s) __builtin_strcpy ((d), (s))
- -fgimple
- Enable parsing of function definitions marked with "__GIMPLE". This is an experimental feature that allows unit testing of GIMPLE passes.
- -fhosted
- Assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is one in which the entire standard library is available, and in which "main" has a return type of "int". Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.
- -ffreestanding
- Assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at "main". The most obvious example is an OS kernel. This is equivalent to -fno-hosted.
- -fopenacc
- Enable handling of OpenACC directives "#pragma acc" in C/C++ and "!$acc" in Fortran. When -fopenacc is specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.0 <https://www.openacc.org>. This option implies -pthread, and thus is only supported on targets that have support for -pthread.
- -fopenacc-dim=geom
- Specify default compute dimensions for parallel offload regions that do not explicitly specify. The geom value is a triple of ':'-separated sizes, in order 'gang', 'worker' and, 'vector'. A size can be omitted, to use a target-specific default value.
- -fopenmp
- Enable handling of OpenMP directives "#pragma omp" in C/C++ and "!$omp" in Fortran. When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5 <http://www.openmp.org/>. This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd.
- -fopenmp-simd
- Enable handling of OpenMP's SIMD directives with "#pragma omp" in C/C++ and "!$omp" in Fortran. Other OpenMP directives are ignored.
- -fgnu-tm
- When the option -fgnu-tm is specified, the compiler generates code
for the Linux variant of Intel's current Transactional Memory ABI
specification document (Revision 1.1, May 6 2009). This is an experimental
feature whose interface may change in future versions of GCC, as the
official specification changes. Please note that not all architectures are
supported for this feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).
- -fms-extensions
- Accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar to previous types declarations.
typedef int UOW; struct ABC { UOW UOW; };
Some cases of unnamed fields in structures and unions are only accepted with this option.
Note that this option is off for all targets but x86 targets using ms-abi.
- -fplan9-extensions
- Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. This is only supported for C, not C++.
- -fcond-mismatch
- Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C++.
- -flax-vector-conversions
- Allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code.
- -funsigned-char
- Let the type "char" be unsigned, like
"unsigned char".
Each kind of machine has a default for what "char" should be. It is either like "unsigned char" by default or like "signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char" when it depends on the signedness of an object. But many programs have been written to use plain "char" and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed char" or "unsigned char", even though its behavior is always just like one of those two.
- -fsigned-char
- Let the type "char" be signed, like
"signed char".
Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char.
- -fsigned-bitfields
- -funsigned-bitfields
- -fno-signed-bitfields
- -fno-unsigned-bitfields
- These options control whether a bit-field is signed or unsigned, when the declaration does not use either "signed" or "unsigned". By default, such a bit-field is signed, because this is consistent: the basic integer types such as "int" are signed types.
- -fsso-struct=endianness
- Set the default scalar storage order of structures and unions to the
specified endianness. The accepted values are big-endian,
little-endian and native for the native endianness of the
target (the default). This option is not supported for C++.
Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target.
Options Controlling C++ Dialect¶
This section describes the command-line options that are only meaningful for C++ programs. You can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.C
In this example, only -fstrict-enums is an option meant only for C++ programs; you can use the other options with any language supported by GCC.
Some options for compiling C programs, such as -std, are also relevant for C++ programs.
Here is a list of options that are only for compiling C++ programs:
- -fabi-version=n
- Use version n of the C++ ABI. The default is version 0.
Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed.
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as a template argument.
Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.
Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment of "nullptr_t".
Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors.
Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor.
Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12.
See also -Wabi.
- -fabi-compat-version=n
- On targets that support strong aliases, G++ works around mangling changes
by creating an alias with the correct mangled name when defining a symbol
with an incorrect mangled name. This switch specifies which ABI version to
use for the alias.
With -fabi-version=0 (the default), this defaults to 11 (GCC 7 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning.
- -fno-access-control
- Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.
- -faligned-new
- Enable support for C++17 "new" of types
that require more alignment than "void* ::operator
new(std::size_t)" provides. A numeric argument such as
"-faligned-new=32" can be used to
specify how much alignment (in bytes) is provided by that function, but
few users will need to override the default of
"alignof(std::max_align_t)".
This flag is enabled by default for -std=c++17.
- -fcheck-new
- Check that the pointer returned by "operator new" is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that "operator new" only returns 0 if it is declared "throw()", in which case the compiler always checks the return value even without this option. In all other cases, when "operator new" has a non-empty exception specification, memory exhaustion is signalled by throwing "std::bad_alloc". See also new (nothrow).
- -fconcepts
- Enable support for the C++ Extensions for Concepts Technical
Specification, ISO 19217 (2015), which allows code like
template <class T> concept bool Addable = requires (T t) { t + t; }; template <Addable T> T add (T a, T b) { return a + b; }
- -fconstexpr-depth=n
- Set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512.
- -fconstexpr-loop-limit=n
- Set the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18).
- -fdeduce-init-list
- Enable deduction of a template type parameter as
"std::initializer_list" from a
brace-enclosed initializer list, i.e.
template <class T> auto forward(T t) -> decltype (realfn (t)) { return realfn (t); } void f() { forward({1,2}); // call forward<std::initializer_list<int>> }
This deduction was implemented as a possible extension to the originally proposed semantics for the C++11 standard, but was not part of the final standard, so it is disabled by default. This option is deprecated, and may be removed in a future version of G++.
- -ffriend-injection
- Inject friend functions into the enclosing namespace, so that they are
visible outside the scope of the class in which they are declared. Friend
functions were documented to work this way in the old Annotated C++
Reference Manual. However, in ISO C++ a friend function that is not
declared in an enclosing scope can only be found using argument dependent
lookup. GCC defaults to the standard behavior.
This option is deprecated and will be removed.
- -fno-elide-constructors
- The C++ standard allows an implementation to omit creating a temporary
that is only used to initialize another object of the same type.
Specifying this option disables that optimization, and forces G++ to call
the copy constructor in all cases. This option also causes G++ to call
trivial member functions which otherwise would be expanded inline.
In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions.
- -fno-enforce-eh-specs
- Don't generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining "NDEBUG". This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time.
- -fextern-tls-init
- -fno-extern-tls-init
- The C++11 and OpenMP standards allow
"thread_local" and
"threadprivate" variables to have
dynamic (runtime) initialization. To support this, any use of such a
variable goes through a wrapper function that performs any necessary
initialization. When the use and definition of the variable are in the
same translation unit, this overhead can be optimized away, but when the
use is in a different translation unit there is significant overhead even
if the variable doesn't actually need dynamic initialization. If the
programmer can be sure that no use of the variable in a non-defining TU
needs to trigger dynamic initialization (either because the variable is
statically initialized, or a use of the variable in the defining TU will
be executed before any uses in another TU), they can avoid this overhead
with the -fno-extern-tls-init option.
On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol aliases, the default is -fno-extern-tls-init.
- -ffor-scope
- -fno-for-scope
- If -ffor-scope is specified, the scope of variables declared in a
for-init-statement is limited to the
"for" loop itself, as specified by the
C++ standard. If -fno-for-scope is specified, the scope of
variables declared in a for-init-statement extends to the end of
the enclosing scope, as was the case in old versions of G++, and other
(traditional) implementations of C++.
This option is deprecated and the associated non-standard functionality will be removed.
- -fno-gnu-keywords
- Do not recognize "typeof" as a keyword, so that code can use this word as an identifier. You can use the keyword "__typeof__" instead. This option is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc.
- -fno-implicit-templates
- Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations.
- -fno-implicit-inline-templates
- Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations.
- -fno-implement-inlines
- To save space, do not emit out-of-line copies of inline functions controlled by "#pragma implementation". This causes linker errors if these functions are not inlined everywhere they are called.
- -fms-extensions
- Disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.
- -fnew-inheriting-ctors
- Enable the P0136 adjustment to the semantics of C++11 constructor inheritance. This is part of C++17 but also considered to be a Defect Report against C++11 and C++14. This flag is enabled by default unless -fabi-version=10 or lower is specified.
- -fnew-ttp-matching
- Enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters. This flag is enabled by default for -std=c++17.
- -fno-nonansi-builtins
- Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include "ffs", "alloca", "_exit", "index", "bzero", "conjf", and other related functions.
- -fnothrow-opt
- Treat a "throw()" exception specification as if it were a "noexcept" specification to reduce or eliminate the text size overhead relative to a function with no exception specification. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to "terminate" rather than "unexpected".
- -fno-operator-names
- Do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor" as synonyms as keywords.
- -fno-optional-diags
- Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.
- -fpermissive
- Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive allows some nonconforming code to compile.
- -fno-pretty-templates
- When an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. "void f(T) [with T = int]" rather than "void f(int)") so that it's clear which template is involved. When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them.
- -frepo
- Enable automatic template instantiation at link time. This option also implies -fno-implicit-templates.
- -fno-rtti
- Disable generation of information about every class with virtual functions for use by the C++ run-time type identification features ("dynamic_cast" and "typeid"). If you don't use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. The "dynamic_cast" operator can still be used for casts that do not require run-time type information, i.e. casts to "void *" or to unambiguous base classes.
- -fsized-deallocation
- Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept; void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under -std=c++14 and above. The flag -Wsized-deallocation warns about places that might want to add a definition.
- -fstrict-enums
- Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type.
- -fstrong-eval-order
- Evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17. Enabled by default with -std=c++17. -fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default without -std=c++17.
- -ftemplate-backtrace-limit=n
- Set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10.
- -ftemplate-depth=n
- Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations.
- -fno-threadsafe-statics
- Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.
- -fuse-cxa-atexit
- Register destructors for objects with static storage duration with the "__cxa_atexit" function rather than the "atexit" function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports "__cxa_atexit".
- -fno-use-cxa-get-exception-ptr
- Don't use the "__cxa_get_exception_ptr" runtime routine. This causes "std::uncaught_exception" to be incorrect, but is necessary if the runtime routine is not available.
- -fvisibility-inlines-hidden
- This switch declares that the user does not attempt to compare pointers to
inline functions or methods where the addresses of the two functions are
taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with "__attribute__ ((visibility ("hidden")))" so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates.
The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary.
- -fvisibility-ms-compat
- This flag attempts to use visibility settings to make GCC's C++ linkage
model compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
- 1.
- It sets the default visibility to "hidden", like -fvisibility=hidden.
- 2.
- Types, but not their members, are not hidden by default.
- 3.
- The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.
In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently.
- -fno-weak
- Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++.
- -nostdinc++
- Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
- -Wabi (C, Objective-C, C++ and Objective-C++ only)
- Warn when G++ it generates code that is probably not compatible with the
vendor-neutral C++ ABI. Since G++ now defaults to updating the ABI with
each major release, normally -Wabi will warn only if there is a
check added later in a release series for an ABI issue discovered since
the initial release. -Wabi will warn about more things if an older
ABI version is selected (with -fabi-version=n).
-Wabi can also be used with an explicit version number to warn about compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.
If an explicit version number is provided and -fabi-compat-version is not specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but -fabi-compat-version is specified, that version number is used for ABI warnings.
Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated is compatible.
You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.
Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:
- A template with a non-type template parameter of reference type was
mangled incorrectly:
extern int N; template <int &> struct S {}; void n (S<N>) {2}
This was fixed in -fabi-version=3.
- SIMD vector types declared using "__attribute
((vector_size))" were mangled in a non-standard way that does
not allow for overloading of functions taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
- "__attribute ((const))" and
"noreturn" were mangled as type
qualifiers, and "decltype" of a plain
declaration was folded away.
These mangling issues were fixed in -fabi-version=5.
- Scoped enumerators passed as arguments to a variadic function are promoted
like unscoped enumerators, causing
"va_arg" to complain. On most targets
this does not actually affect the parameter passing ABI, as there is no
way to pass an argument smaller than
"int".
Also, the ABI changed the mangling of template argument packs, "const_cast", "static_cast", prefix increment/decrement, and a class scope function used as a template argument.
These issues were corrected in -fabi-version=6.
- Lambdas in default argument scope were mangled incorrectly, and the ABI
changed the mangling of "nullptr_t".
These issues were corrected in -fabi-version=7.
- When mangling a function type with function-cv-qualifiers, the
un-qualified function type was incorrectly treated as a substitution
candidate.
This was fixed in -fabi-version=8, the default for GCC 5.1.
- "decltype(nullptr)" incorrectly had an
alignment of 1, leading to unaligned accesses. Note that this did not
affect the ABI of a function with a
"nullptr_t" parameter, as parameters
have a minimum alignment.
This was fixed in -fabi-version=9, the default for GCC 5.2.
- Target-specific attributes that affect the identity of a type, such as
ia32 calling conventions on a function type (stdcall, regparm, etc.), did
not affect the mangled name, leading to name collisions when function
pointers were used as template arguments.
This was fixed in -fabi-version=10, the default for GCC 6.1.
It also warns about psABI-related changes. The known psABI changes at this point include:
- *
- For SysV/x86-64, unions with "long
double" members are passed in memory as specified in psABI.
For example:
union U { long double ld; int i; };
"union U" is always passed in memory.
- -Wabi-tag (C++ and Objective-C++ only)
- Warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more information about ABI tags.
- -Wctor-dtor-privacy (C++ and Objective-C++ only)
- Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor.
- -Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
- Warn when "delete" is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by -Wall.
- -Wliteral-suffix (C++ and Objective-C++ only)
- Warn when a string or character literal is followed by a ud-suffix which
does not begin with an underscore. As a conforming extension, GCC treats
such suffixes as separate preprocessing tokens in order to maintain
backwards compatibility with code that uses formatting macros from
"<inttypes.h>". For example:
#define __STDC_FORMAT_MACROS #include <inttypes.h> #include <stdio.h> int main() { int64_t i64 = 123; printf("My int64: %" PRId64"\n", i64); }
In this case, "PRId64" is treated as a separate preprocessing token.
Additionally, warn when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization.
This warning is enabled by default.
- -Wlto-type-mismatch
- During the link-time optimization warn about type mismatches in global declarations from different compilation units. Requires -flto to be enabled. Enabled by default.
- -Wno-narrowing (C++ and Objective-C++ only)
- For C++11 and later standards, narrowing conversions are diagnosed by
default, as required by the standard. A narrowing conversion from a
constant produces an error, and a narrowing conversion from a non-constant
produces a warning, but -Wno-narrowing suppresses the diagnostic.
Note that this does not affect the meaning of well-formed code; narrowing
conversions are still considered ill-formed in SFINAE contexts.
With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
- -Wnoexcept (C++ and Objective-C++ only)
- Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. "throw()" or "noexcept") but is known by the compiler to never throw an exception.
- -Wnoexcept-type (C++ and Objective-C++ only)
- Warn if the C++17 feature making
"noexcept" part of a function type
changes the mangled name of a symbol relative to C++14. Enabled by
-Wabi and -Wc++17-compat.
As an example:
template <class T> void f(T t) { t(); }; void g() noexcept; void h() { f(g); }
In C++14, "f" calls "f<void(*)()>", but in C++17 it calls "f<void(*)()noexcept>".
- -Wclass-memaccess (C++ and Objective-C++ only)
- Warn when the destination of a call to a raw memory function such as
"memset" or
"memcpy" is an object of class type, and
when writing into such an object might bypass the class non-trivial or
deleted constructor or copy assignment, violate const-correctness or
encapsulation, or corrupt virtual table pointers. Modifying the
representation of such objects may violate invariants maintained by member
functions of the class. For example, the call to
"memset" below is undefined because it
modifies a non-trivial class object and is, therefore, diagnosed. The safe
way to either initialize or clear the storage of objects of such types is
by using the appropriate constructor or assignment operator, if one is
available.
std::string str = "abc"; memset (&str, 0, sizeof str);
The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the pointer to the class object to "void *" or to a type that can be safely accessed by the raw memory function suppresses the warning.
- -Wnon-virtual-dtor (C++ and Objective-C++ only)
- Warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if -Weffc++ is specified.
- -Wregister (C++ and Objective-C++ only)
- Warn on uses of the "register" storage class specifier, except when it is part of the GNU Explicit Register Variables extension. The use of the "register" keyword as storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with -std=c++17.
- -Wreorder (C++ and Objective-C++ only)
- Warn when the order of member initializers given in the code does not
match the order in which they must be executed. For instance:
struct A { int i; int j; A(): j (0), i (1) { } };
The compiler rearranges the member initializers for "i" and "j" to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by -Wall.
- -fext-numeric-literals (C++ and Objective-C++ only)
- Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...).
The following -W... options are not affected by -Wall.
- -Weffc++ (C++ and Objective-C++ only)
- Warn about violations of the following style guidelines from Scott Meyers' Effective C++ series of books:
- Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
- Prefer initialization to assignment in constructors.
- Have "operator=" return a reference to *this.
- Don't try to return a reference when you must return an object.
- Distinguish between prefix and postfix forms of increment and decrement operators.
- Never overload "&&", "||", or ",".
This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too.
When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings.
- -Wstrict-null-sentinel (C++ and Objective-C++ only)
- Warn about the use of an uncasted "NULL" as sentinel. When compiling only with GCC this is a valid sentinel, as "NULL" is defined to "__null". Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers.
- -Wno-non-template-friend (C++ and Objective-C++ only)
- Disable warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such as friend int foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the warning exists to diagnose compatibility problems, and is enabled by default.
- -Wold-style-cast (C++ and Objective-C++ only)
- Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts ("dynamic_cast", "static_cast", "reinterpret_cast", and "const_cast") are less vulnerable to unintended effects and much easier to search for.
- -Woverloaded-virtual (C++ and Objective-C++ only)
- Warn when a function declaration hides virtual functions from a base
class. For example, in:
struct A { virtual void f(); }; struct B: public A { void f(int); };
the "A" class version of "f" is hidden in "B", and code like:
B* b; b->f();
fails to compile.
- -Wno-pmf-conversions (C++ and Objective-C++ only)
- Disable the diagnostic for converting a bound pointer to member function to a plain pointer.
- -Wsign-promo (C++ and Objective-C++ only)
- Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior.
- -Wtemplates (C++ and Objective-C++ only)
- Warn when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also instantiate or specialize templates.
- -Wmultiple-inheritance (C++ and Objective-C++ only)
- Warn when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use multiple inheritance.
- -Wvirtual-inheritance
- Warn when a class is defined with a virtual direct base class. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use virtual inheritance.
- -Wnamespaces
- Warn when a namespace definition is opened. Some coding rules disallow namespaces, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also use using directives and qualified names.
- -Wno-terminate (C++ and Objective-C++ only)
- Disable the warning about a throw-expression that will immediately result in a call to "terminate".
Options Controlling Objective-C and Objective-C++ Dialects¶
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.
This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs. You can also use most of the language-independent GNU compiler options. For example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use the other options with any language supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:
- -fconstant-string-class=class-name
- Use class-name as the name of the class to instantiate for each literal string specified with the syntax "@"..."". The default class name is "NXConstantString" if the GNU runtime is being used, and "NSConstantString" if the NeXT runtime is being used (see below). The -fconstant-cfstrings option, if also present, overrides the -fconstant-string-class setting and cause "@"..."" literals to be laid out as constant CoreFoundation strings.
- -fgnu-runtime
- Generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems.
- -fnext-runtime
- Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin and Mac OS X. The macro "__NEXT_RUNTIME__" is predefined if (and only if) this option is used.
- -fno-nil-receivers
- Assume that all Objective-C message dispatches ("[receiver message:arg]") in this translation unit ensure that the receiver is not "nil". This allows for more efficient entry points in the runtime to be used. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.
- -fobjc-abi-version=n
- Use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines.
- -fobjc-call-cxx-cdtors
- For each Objective-C class, check if any of its instance variables is a
C++ object with a non-trivial default constructor. If so, synthesize a
special "- (id) .cxx_construct" instance
method which runs non-trivial default constructors on any such instance
variables, in order, and then return
"self". Similarly, check if any instance
variable is a C++ object with a non-trivial destructor, and if so,
synthesize a special "- (void)
.cxx_destruct" method which runs all such default destructors,
in reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct" methods thusly generated only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy. The "- (id) .cxx_construct" methods are invoked by the runtime immediately after a new object instance is allocated; the "- (void) .cxx_destruct" methods are invoked immediately before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the "- (id) .cxx_construct" and "- (void) .cxx_destruct" methods.
- -fobjc-direct-dispatch
- Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.
- -fobjc-exceptions
- Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).
- -fobjc-gc
- Enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags.
- -fobjc-nilcheck
- For the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using -fno-objc-nilcheck. Class methods and super calls are never checked for nil in this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used.
- -fobjc-std=objc1
- Conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC.
- -freplace-objc-classes
- Emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
- -fzero-link
- When compiling for the NeXT runtime, the compiler ordinarily replaces calls to "objc_getClass("...")" (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance. Specifying the -fzero-link flag suppresses this behavior and causes calls to "objc_getClass("...")" to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. The GNU runtime currently always retains calls to "objc_get_class("...")" regardless of command-line options.
- -fno-local-ivars
- By default instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. Specifying the -fno-local-ivars flag disables this behavior thus avoiding variable shadowing issues.
- -fivar-visibility=[public|protected|private|package]
- Set the default instance variable visibility to the specified option so that instance variables declared outside the scope of any access modifier directives default to the specified visibility.
- -gen-decls
- Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.
- -Wassign-intercept (Objective-C and Objective-C++ only)
- Warn whenever an Objective-C assignment is being intercepted by the garbage collector.
- -Wno-protocol (Objective-C and Objective-C++ only)
- If a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them.
- -Wselector (Objective-C and Objective-C++ only)
- Warn if multiple methods of different types for the same selector are found during compilation. The check is performed on the list of methods in the final stage of compilation. Additionally, a check is performed for each selector appearing in a "@selector(...)" expression, and a corresponding method for that selector has been found during compilation. Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation is not reached, for example because an error is found during compilation, or because the -fsyntax-only option is being used.
- -Wstrict-selector-match (Objective-C and Objective-C++ only)
- Warn if multiple methods with differing argument and/or return types are found for a given selector when attempting to send a message using this selector to a receiver of type "id" or "Class". When this flag is off (which is the default behavior), the compiler omits such warnings if any differences found are confined to types that share the same size and alignment.
- -Wundeclared-selector (Objective-C and Objective-C++ only)
- Warn if a "@selector(...)" expression referring to an undeclared selector is found. A selector is considered undeclared if no method with that name has been declared before the "@selector(...)" expression, either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. This option always performs its checks as soon as a "@selector(...)" expression is found, while -Wselector only performs its checks in the final stage of compilation. This also enforces the coding style convention that methods and selectors must be declared before being used.
- -print-objc-runtime-info
- Generate C header describing the largest structure that is passed by value, if any.
Options to Control Diagnostic Messages Formatting¶
Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). You can use the options described below to control the formatting algorithm for diagnostic messages, e.g. how many characters per line, how often source location information should be reported. Note that some language front ends may not honor these options.
- -fmessage-length=n
- Try to format error messages so that they fit on lines of about n characters. If n is zero, then no line-wrapping is done; each error message appears on a single line. This is the default for all front ends.
- -fdiagnostics-show-location=once
- Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior.
- -fdiagnostics-show-location=every-line
- Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line.
- -fdiagnostics-color[=WHEN]
- -fno-diagnostics-color
- Use color in diagnostics. WHEN is never, always, or
auto. The default depends on how the compiler has been configured,
it can be any of the above WHEN options or also never if
GCC_COLORS environment variable isn't present in the environment,
and auto otherwise. auto means to use color only when the
standard error is a terminal. The forms -fdiagnostics-color and
-fno-diagnostics-color are aliases for
-fdiagnostics-color=always and -fdiagnostics-color=never,
respectively.
The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\ quote=01:fixit-insert=32:fixit-delete=31:\ diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\ type-diff=01;32
where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red. Setting GCC_COLORS to the empty string disables colors. Supported capabilities are as follows.
- "error="
- SGR substring for error: markers.
- "warning="
- SGR substring for warning: markers.
- "note="
- SGR substring for note: markers.
- "range1="
- SGR substring for first additional range.
- "range2="
- SGR substring for second additional range.
- "locus="
- SGR substring for location information, file:line or file:line:column etc.
- "quote="
- SGR substring for information printed within quotes.
- "fixit-insert="
- SGR substring for fix-it hints suggesting text to be inserted or replaced.
- "fixit-delete="
- SGR substring for fix-it hints suggesting text to be deleted.
- "diff-filename="
- SGR substring for filename headers within generated patches.
- "diff-hunk="
- SGR substring for the starts of hunks within generated patches.
- "diff-delete="
- SGR substring for deleted lines within generated patches.
- "diff-insert="
- SGR substring for inserted lines within generated patches.
- "type-diff="
- SGR substring for highlighting mismatching types within template arguments in the C++ frontend.
- -fno-diagnostics-show-option
- By default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying the -fno-diagnostics-show-option flag suppresses that behavior.
- -fno-diagnostics-show-caret
- By default, each diagnostic emitted includes the original source line and a caret ^ indicating the column. This option suppresses this information. The source line is truncated to n characters, if the -fmessage-length=n option is given. When the output is done to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if not set, to the terminal width.
- -fdiagnostics-parseable-fixits
- Emit fix-it hints in a machine-parseable format, suitable for consumption
by IDEs. For each fix-it, a line will be printed after the relevant
diagnostic, starting with the string "fix-it:". For example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"
The location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for the initial column. In the above example, bytes 3 through 20 of line 45 of "test.c" are to be replaced with the given string:
00000000011111111112222222222 12345678901234567890123456789 gtk_widget_showall (dlg); ^^^^^^^^^^^^^^^^^^ gtk_widget_show_all
The filename and replacement string escape backslash as "\\", tab as "\t", newline as "\n", double quotes as "\"", non-printable characters as octal (e.g. vertical tab as "\013").
An empty replacement string indicates that the given range is to be removed. An empty range (e.g. "45:3-45:3") indicates that the string is to be inserted at the given position.
- -fdiagnostics-generate-patch
- Print fix-it hints to stderr in unified diff format, after any diagnostics
are printed. For example:
--- test.c +++ test.c @ -42,5 +42,5 @ void show_cb(GtkDialog *dlg) { - gtk_widget_showall(dlg); + gtk_widget_show_all(dlg); }
The diff may or may not be colorized, following the same rules as for diagnostics (see -fdiagnostics-color).
- -fdiagnostics-show-template-tree
- In the C++ frontend, when printing diagnostics showing mismatching
template types, such as:
could not convert 'std::map<int, std::vector<double> >()' from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and differing parts of the types, such as:
map< [...], vector< [double != float]>>
The parts that differ are highlighted with color ("double" and "float" in this case).
- -fno-elide-type
- By default when the C++ frontend prints diagnostics showing mismatching
template types, common parts of the types are printed as "[...]"
to simplify the error message. For example:
could not convert 'std::map<int, std::vector<double> >()' from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the -fdiagnostics-show-template-tree flag.
- -fno-show-column
- Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu.
Options to Request or Suppress Warnings¶
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error.
The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.
- -fsyntax-only
- Check the code for syntax errors, but don't do anything beyond that.
- -fmax-errors=n
- Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option.
- -w
- Inhibit all warning messages.
- -Werror
- Make all warnings into errors.
- -Werror=
- Make the specified warning into an error. The specifier for a warning is
appended; for example -Werror=switch turns the warnings controlled
by -Wswitch into errors. This switch takes a negative form, to be
used to negate -Werror for specific warnings; for example
-Wno-error=switch makes -Wswitch warnings not be errors,
even when -Werror is in effect.
The warning message for each controllable warning includes the option that controls the warning. That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything.
- -Wfatal-errors
- This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages.
You can request many specific warnings with options beginning with -W, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further options, such as -Wunused-value. The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of their position in the command-line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas take effect as if they appeared at the end of the command-line.
When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC emits a diagnostic stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present.
The effectiveness of some warnings depends on optimizations also being enabled. For example -Wsuggest-final-types is more effective with link-time optimization and -Wmaybe-uninitialized will not warn at all unless optimization is enabled.
- -Wpedantic
- -pedantic
- Issue all the warnings demanded by strict ISO C and ISO C++; reject all
programs that use forbidden extensions, and some other programs that do
not follow ISO C and ISO C++. For ISO C, follows the version of the ISO C
standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected.
-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. Pedantic warnings are also disabled in the expression that follows "__extension__". However, only system header files should use these escape routes; application programs should avoid them.
Some users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all---only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -Wpedantic. We don't have plans to support such a feature in the near future.
Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -Wpedantic are given where they are required by the base standard. (It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)
- -pedantic-errors
- Give an error whenever the base standard (see -Wpedantic) requires a diagnostic, in some cases where there is undefined behavior at compile-time and in some other cases that do not prevent compilation of programs that are valid according to the standard. This is not equivalent to -Werror=pedantic, since there are errors enabled by this option and not enabled by the latter and vice versa.
- -Wall
- This enables all the warnings about constructions that some users consider
questionable, and that are easy to avoid (or modify to prevent the
warning), even in conjunction with macros. This also enables some
language-specific warnings described in C++ Dialect Options
and Objective-C and Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress -Warray-bounds=1 (only with -O2) -Wbool-compare -Wbool-operation -Wc++11-compat -Wc++14-compat -Wcatch-value (C++ and Objective-C++ only) -Wchar-subscripts -Wcomment -Wduplicate-decl-specifier (C and Objective-C only) -Wenum-compare (in C/ObjC; this is on by default in C++) -Wformat -Wint-in-bool-context -Wimplicit (C and Objective-C only) -Wimplicit-int (C and Objective-C only) -Wimplicit-function-declaration (C and Objective-C only) -Winit-self (only for C++) -Wlogical-not-parentheses -Wmain (only for C/ObjC and unless -ffreestanding) -Wmaybe-uninitialized -Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation (only for C/C++) -Wmissing-attributes -Wmissing-braces (only for C/ObjC) -Wmultistatement-macros -Wnarrowing (only for C++) -Wnonnull -Wnonnull-compare -Wopenmp-simd -Wparentheses -Wpointer-sign -Wreorder -Wrestrict -Wreturn-type -Wsequence-point -Wsign-compare (only in C++) -Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1 -Wstringop-truncation -Wswitch -Wtautological-compare -Wtrigraphs -Wuninitialized -Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value -Wunused-variable -Wvolatile-register-var
Note that some warning flags are not implied by -Wall. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra but many of them must be enabled individually.
- -Wextra
- This enables some extra warning flags that are not enabled by
-Wall. (This option used to be called -W. The older name is
still supported, but the newer name is more descriptive.)
-Wclobbered -Wcast-function-type -Wempty-body -Wignored-qualifiers -Wimplicit-fallthrough=3 -Wmissing-field-initializers -Wmissing-parameter-type (C only) -Wold-style-declaration (C only) -Woverride-init -Wsign-compare (C only) -Wtype-limits -Wuninitialized -Wshift-negative-value (in C++03 and in C99 and newer) -Wunused-parameter (only with -Wunused or -Wall) -Wunused-but-set-parameter (only with -Wunused or -Wall)
The option -Wextra also prints warning messages for the following cases:
- A pointer is compared against integer zero with "<", "<=", ">", or ">=".
- (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
- (C++ only) Ambiguous virtual bases.
- (C++ only) Subscripting an array that has been declared "register".
- (C++ only) Taking the address of a variable that has been declared "register".
- (C++ only) A base class is not initialized in the copy constructor of a derived class.
- -Wchar-subscripts
- Warn if an array subscript has type "char". This is a common cause of error, as programmers often forget that this type is signed on some machines. This warning is enabled by -Wall.
- -Wchkp
- Warn about an invalid memory access that is found by Pointer Bounds Checker (-fcheck-pointer-bounds).
- -Wno-coverage-mismatch
- Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is changed between compiling with -fprofile-gen and with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error. -Wno-coverage-mismatch can be used to disable the warning or -Wno-error=coverage-mismatch can be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended.
- -Wno-cpp
- (C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
- -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
- Give a warning when a value of type
"float" is implicitly promoted to
"double". CPUs with a 32-bit
"single-precision" floating-point unit implement
"float" in hardware, but emulate
"double" in software. On such a machine,
doing computations using "double" values
is much more expensive because of the overhead required for software
emulation.
It is easy to accidentally do computations with "double" because floating-point literals are implicitly of type "double". For example, in:
float area(float radius) { return 3.14159 * radius * radius; }
the compiler performs the entire computation with "double" because the floating-point literal is a "double".
- -Wduplicate-decl-specifier (C and Objective-C only)
- Warn if a declaration has duplicate "const", "volatile", "restrict" or "_Atomic" specifier. This warning is enabled by -Wall.
- -Wformat
- -Wformat=n
- Check calls to "printf" and
"scanf", etc., to make sure that the
arguments supplied have types appropriate to the format string specified,
and that the conversions specified in the format string make sense. This
includes standard functions, and others specified by format attributes, in
the "printf",
"scanf",
"strftime" and
"strfmon" (an X/Open extension, not in
the C standard) families (or other target-specific families). Which
functions are checked without format attributes having been specified
depends on the standard version selected, and such checks of functions
without the attribute specified are disabled by -ffreestanding or
-fno-builtin.
The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations. However, if -Wpedantic is used with -Wformat, warnings are given about format features not in the selected standard version (but not for "strfmon" formats, since those are not in any version of the C standard).
- -Wformat=1
- -Wformat
- Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall.
- -Wno-format-contains-nul
- If -Wformat is specified, do not warn about format strings that contain NUL bytes.
- -Wno-format-extra-args
- If -Wformat is specified, do not warn about excess arguments to a
"printf" or
"scanf" format function. The C standard
specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that are specified with $ operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to "va_arg" to skip the unused arguments. However, in the case of "scanf" formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed.
- -Wformat-overflow
- -Wformat-overflow=level
- Warn about calls to formatted input/output functions such as "sprintf" and "vsprintf" that might overflow the destination buffer. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives.
- -Wformat-overflow
- -Wformat-overflow=1
- Level 1 of -Wformat-overflow enabled by -Wformat
employs a conservative approach that warns only about calls that most
likely overflow the buffer. At this level, numeric arguments to format
directives with unknown values are assumed to have the value of one, and
strings of unknown length to be empty. Numeric arguments that are known to
be bounded to a subrange of their type, or string arguments whose output
is bounded either by their directive's precision or by a finite set of
string literals, are assumed to take on the value within the range that
results in the most bytes on output. For example, the call to
"sprintf" below is diagnosed because
even with both a and b equal to zero, the terminating NUL
character ('\0') appended by the function to the
destination buffer will be written past its end. Increasing the size of
the buffer by a single byte is sufficient to avoid the warning, though it
may not be sufficient to avoid the overflow.
void f (int a, int b) { char buf [13]; sprintf (buf, "a = %i, b = %i\n", a, b); }
- -Wformat-overflow=2
- Level 2 warns also about calls that might overflow the destination
buffer given an argument of sufficient length or magnitude. At level
2, unknown numeric arguments are assumed to have the minimum
representable value for signed types with a precision greater than 1, and
the maximum representable value otherwise. Unknown string arguments whose
length cannot be assumed to be bounded either by the directive's
precision, or by a finite set of string literals they may evaluate to, or
the character array they may point to, are assumed to be 1 character long.
At level 2, the call in the example above is again diagnosed, but this time because with a equal to a 32-bit "INT_MIN" the first %i directive will write some of its digits beyond the end of the destination buffer. To make the call safe regardless of the values of the two variables, the size of the destination buffer must be increased to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note following the warning.
An alternative to increasing the size of the destination buffer is to constrain the range of formatted values. The maximum length of string arguments can be bounded by specifying the precision in the format directive. When numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, choosing an appropriate length modifier to the format specifier will reduce the required buffer size. For example, if a and b in the example above can be assumed to be within the precision of the "short int" type then using either the %hi format directive or casting the argument to "short" reduces the maximum required size of the buffer to 24 bytes.
void f (int a, int b) { char buf [23]; sprintf (buf, "a = %hi, b = %i\n", a, (short)b); }
- -Wno-format-zero-length
- If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed.
- -Wformat=2
- Enable -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.
- -Wformat-nonliteral
- If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a "va_list".
- -Wformat-security
- If -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to "printf" and "scanf" functions where the format string is not a string literal and there are no format arguments, as in "printf (foo);". This may be a security hole if the format string came from untrusted input and contains %n. (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.)
- -Wformat-signedness
- If -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa.
- -Wformat-truncation
- -Wformat-truncation=level
- Warn about calls to formatted input/output functions such as "snprintf" and "vsnprintf" that might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic -Wformat-overflow.
- -Wformat-truncation
- -Wformat-truncation=1
- Level 1 of -Wformat-truncation enabled by -Wformat employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation.
- -Wformat-truncation=2
- Level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude.
- -Wformat-y2k
- If -Wformat is specified, also warn about "strftime" formats that may yield only a two-digit year.
- -Wnonnull
- Warn about passing a null pointer for arguments marked as requiring a
non-null value by the "nonnull" function
attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.
- -Wnonnull-compare
- Warn when comparing an argument marked with the
"nonnull" function attribute against
null inside the function.
-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.
- -Wnull-dereference
- Warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when -fdelete-null-pointer-checks is active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used.
- -Winit-self (C, C++, Objective-C and Objective-C++ only)
- Warn about uninitialized variables that are initialized with themselves.
Note this option can only be used with the -Wuninitialized option.
For example, GCC warns about "i" being uninitialized in the following snippet only when -Winit-self has been specified:
int f() { int i = i; return i; }
This warning is enabled by -Wall in C++.
- -Wimplicit-int (C and Objective-C only)
- Warn when a declaration does not specify a type. This warning is enabled by -Wall.
- -Wimplicit-function-declaration (C and Objective-C only)
- Give a warning whenever a function is used before being declared. In C99 mode (-std=c99 or -std=gnu99), this warning is enabled by default and it is made into an error by -pedantic-errors. This warning is also enabled by -Wall.
- -Wimplicit (C and Objective-C only)
- Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
- -Wimplicit-fallthrough
- -Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and -Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0.
- -Wimplicit-fallthrough=n
- Warn when a switch case falls through. For example:
switch (cond) { case 1: a = 1; break; case 2: a = 2; case 3: a = 3; break; }
This warning does not warn when the last statement of a case cannot fall through, e.g. when there is a return statement or a call to function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes into account control flow statements, such as ifs, and only warns when appropriate. E.g.
switch (cond) { case 1: if (i > 3) { bar (5); break; } else if (i < 1) { bar (0); } else return; default: ... }
Since there are occasions where a switch case fall through is desirable, GCC provides an attribute, "__attribute__ ((fallthrough))", that is to be used along with a null statement to suppress this warning that would normally occur:
switch (cond) { case 1: bar (0); __attribute__ ((fallthrough)); default: ... }
C++17 provides a standard way to suppress the -Wimplicit-fallthrough warning using "[[fallthrough]];" instead of the GNU attribute. In C++11 or C++14 users can use "[[gnu::fallthrough]];", which is a GNU extension. Instead of these attributes, it is also possible to add a fallthrough comment to silence the warning. The whole body of the C or C++ style comment should match the given regular expressions listed below. The option argument n specifies what kind of comments are accepted:
- *<-Wimplicit-fallthrough=0 disables the warning altogether.>
- *<-Wimplicit-fallthrough=1 matches ".*" regular>
- expression, any comment is used as fallthrough comment.
- *<-Wimplicit-fallthrough=2 case insensitively matches>
- ".*falls?[ \t-]*thr(ough|u).*" regular expression.
- *<-Wimplicit-fallthrough=3 case sensitively matches one of the>
- following regular expressions:
- *<"-fallthrough">
- *<"@fallthrough@">
- *<"lint -fallthrough[ \t]*">
- *<"[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?">
- *<"[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?">
- *<"[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?">
- *<-Wimplicit-fallthrough=4 case sensitively matches one of the>
- following regular expressions:
- *<"-fallthrough">
- *<"@fallthrough@">
- *<"lint -fallthrough[ \t]*">
- *<"[ \t]*FALLTHR(OUGH|U)[ \t]*">
- *<-Wimplicit-fallthrough=5 doesn't recognize any comments as>
- fallthrough comments, only attributes disable the warning.
The comment needs to be followed after optional whitespace and other comments by "case" or "default" keywords or by a user label that precedes some "case" or "default" label.
switch (cond) { case 1: bar (0); /* FALLTHRU */ default: ... }
The -Wimplicit-fallthrough=3 warning is enabled by -Wextra.
- -Wif-not-aligned (C, C++, Objective-C and Objective-C++ only)
- Control if warning triggered by the "warn_if_not_aligned" attribute should be issued. This is enabled by default. Use -Wno-if-not-aligned to disable it.
- -Wignored-qualifiers (C and C++ only)
- Warn if the return type of a function has a type qualifier such as
"const". For ISO C such a type qualifier
has no effect, since the value returned by a function is not an lvalue.
For C++, the warning is only emitted for scalar types or
"void". ISO C prohibits qualified
"void" return types on function
definitions, so such return types always receive a warning even without
this option.
This warning is also enabled by -Wextra.
- -Wignored-attributes (C and C++ only)
- Warn when an attribute is ignored. This is different from the -Wattributes option in that it warns whenever the compiler decides to drop an attribute, not that the attribute is either unknown, used in a wrong place, etc. This warning is enabled by default.
- -Wmain
- Warn if the type of "main" is suspicious. "main" should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic.
- -Wmisleading-indentation (C and C++ only)
- Warn when the indentation of the code does not reflect the block
structure. Specifically, a warning is issued for
"if",
"else",
"while", and
"for" clauses with a guarded statement
that does not use braces, followed by an unguarded statement with the same
indentation.
In the following example, the call to "bar" is misleadingly indented as if it were guarded by the "if" conditional.
if (some_condition ()) foo (); bar (); /* Gotcha: this is not guarded by the "if". */
In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements line up (defaulting to 8).
The warning is not issued for code involving multiline preprocessor logic such as the following example.
if (flagA) foo (0); #if SOME_CONDITION_THAT_DOES_NOT_HOLD if (flagB) #endif foo (1);
The warning is not issued after a "#line" directive, since this typically indicates autogenerated code, and no assumptions can be made about the layout of the file that the directive references.
This warning is enabled by -Wall in C and C++.
- -Wmissing-attributes
- Warn when a declaration of a function is missing one or more attributes
that a related function is declared with and whose absence may adversely
affect the correctness or efficiency of generated code. For example, in
C++, the warning is issued when an explicit specialization of a primary
template declared with attribute
"alloc_align",
"alloc_size",
"assume_aligned",
"format",
"format_arg",
"malloc", or
"nonnull" is declared without it.
Attributes "deprecated",
"error", and
"warning" suppress the warning..
-Wmissing-attributes is enabled by -Wall.
For example, since the declaration of the primary function template below makes use of both attribute "malloc" and "alloc_size" the declaration of the explicit specialization of the template is diagnosed because it is missing one of the attributes.
template <class T> T* __attribute__ ((malloc, alloc_size (1))) allocate (size_t); template <> void* __attribute__ ((malloc)) // missing alloc_size allocate<void> (size_t);
- -Wmissing-braces
- Warn if an aggregate or union initializer is not fully bracketed. In the
following example, the initializer for
"a" is not fully bracketed, but that for
"b" is fully bracketed. This warning is
enabled by -Wall in C.
int a[2][2] = { 0, 1, 2, 3 }; int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
- -Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
- Warn if a user-supplied include directory does not exist.
- -Wmultistatement-macros
- Warn about unsafe multiple statement macros that appear to be guarded by a
clause such as "if",
"else",
"for",
"switch", or
"while", in which only the first
statement is actually guarded after the macro is expanded.
For example:
#define DOIT x++; y++ if (c) DOIT;
will increment "y" unconditionally, not just when "c" holds. The can usually be fixed by wrapping the macro in a do-while loop:
#define DOIT do { x++; y++; } while (0) if (c) DOIT;
This warning is enabled by -Wall in C and C++.
- -Wparentheses
- Warn if parentheses are omitted in certain contexts, such as when there is
an assignment in a context where a truth value is expected, or when
operators are nested whose precedence people often get confused about.
Also warn if a comparison like "x<=y<=z" appears; this is equivalent to "(x<=y ? 1 : 0) <= z", which is a different interpretation from that of ordinary mathematical notation.
Also warn for dangerous uses of the GNU extension to "?:" with omitted middle operand. When the condition in the "?": operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead.
For C++ this also warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt at a function call instead of a declaration:
{ // Declares a local variable called mymutex. std::unique_lock<std::mutex> (mymutex); // User meant std::unique_lock<std::mutex> lock (mymutex); }
This warning is enabled by -Wall.
- -Wsequence-point
- Warn about code that may have undefined semantics because of violations of
sequence point rules in the C and C++ standards.
The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a "&&", "||", "? :" or "," (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.". If a program breaks these rules, the results on any particular implementation are entirely unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and "a[i++] = i;". Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.
The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this warning will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++.
The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at <http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++.
- -Wno-return-local-addr
- Do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns.
- -Wreturn-type
- Warn whenever a function is defined with a return type that defaults to
"int". Also warn about any
"return" statement with no return value
in a function whose return type is not
"void" (falling off the end of the
function body is considered returning without a value).
For C only, warn about a "return" statement with an expression in a function whose return type is "void", unless the expression type is also "void". As a GNU extension, the latter case is accepted without a warning unless -Wpedantic is used.
For C++, a function without return type always produces a diagnostic message, even when -Wno-return-type is specified. The only exceptions are "main" and functions defined in system headers.
This warning is enabled by default for C++ and is enabled by -Wall.
- -Wshift-count-negative
- Warn if shift count is negative. This warning is enabled by default.
- -Wshift-count-overflow
- Warn if shift count >= width of type. This warning is enabled by default.
- -Wshift-negative-value
- Warn if left shifting a negative value. This warning is enabled by -Wextra in C99 and C++11 modes (and newer).
- -Wshift-overflow
- -Wshift-overflow=n
- Warn about left shift overflows. This warning is enabled by default in C99 and C++11 modes (and newer).
- -Wshift-overflow=1
- This is the warning level of -Wshift-overflow and is enabled by default in C99 and C++11 modes (and newer). This warning level does not warn about left-shifting 1 into the sign bit. (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.)
- -Wshift-overflow=2
- This warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode is active.
- -Wswitch
- Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the named codes of that enumeration. (The presence of a "default" label prevents this warning.) "case" labels outside the enumeration range also provoke warnings when this option is used (even if there is a "default" label). This warning is enabled by -Wall.
- -Wswitch-default
- Warn whenever a "switch" statement does not have a "default" case.
- -Wswitch-enum
- Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the named codes of that enumeration. "case" labels outside the enumeration range also provoke warnings when this option is used. The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there is a "default" label.
- -Wswitch-bool
- Warn whenever a "switch" statement has
an index of boolean type and the case values are outside the range of a
boolean type. It is possible to suppress this warning by casting the
controlling expression to a type other than
"bool". For example:
switch ((int) (a == 4)) { ... }
This warning is enabled by default for C and C++ programs.
- -Wswitch-unreachable
- Warn whenever a "switch" statement
contains statements between the controlling expression and the first case
label, which will never be executed. For example:
switch (cond) { i = 15; ... case 5: ... }
-Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label is just a declaration:
switch (cond) { int i; ... case 5: i = 5; ... }
This warning is enabled by default for C and C++ programs.
- -Wsync-nand (C and C++ only)
- Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch" built-in functions are used. These functions changed semantics in GCC 4.4.
- -Wunused-but-set-parameter
- Warn whenever a function parameter is assigned to, but otherwise unused
(aside from its declaration).
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused together with -Wextra.
- -Wunused-but-set-variable
- Warn whenever a local variable is assigned to, but otherwise unused (aside
from its declaration). This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused, which is enabled by -Wall.
- -Wunused-function
- Warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by -Wall.
- -Wunused-label
- Warn whenever a label is declared but not used. This warning is enabled by
-Wall.
To suppress this warning use the "unused" attribute.
- -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
- Warn when a typedef locally defined in a function is not used. This warning is enabled by -Wall.
- -Wunused-parameter
- Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the "unused" attribute.
- -Wno-unused-result
- Do not warn if a caller of a function marked with attribute "warn_unused_result" does not use its return value. The default is -Wunused-result.
- -Wunused-variable
- Warn whenever a local or static variable is unused aside from its
declaration. This option implies -Wunused-const-variable=1 for C,
but not for C++. This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
- -Wunused-const-variable
- -Wunused-const-variable=n
- Warn whenever a constant static variable is unused aside from its
declaration. -Wunused-const-variable=1 is enabled by
-Wunused-variable for C, but not for C++. In C this declares
variable storage, but in C++ this is not an error since const variables
take the place of "#define"s.
To suppress this warning use the "unused" attribute.
- -Wunused-const-variable=1
- This is the warning level that is enabled by -Wunused-variable for C. It warns only about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included.
- -Wunused-const-variable=2
- This warning level also warns for unused constant static variables in headers (excluding system headers). This is the warning level of -Wunused-const-variable and must be explicitly requested since in C++ this isn't an error and in C it might be harder to clean up all headers included.
- -Wunused-value
- Warn whenever a statement computes a result that is explicitly not used.
To suppress this warning cast the unused expression to
"void". This includes an
expression-statement or the left-hand side of a comma expression that
contains no side effects. For example, an expression such as
"x[i,j]" causes a warning, while
"x[(void)i,j]" does not.
This warning is enabled by -Wall.
- -Wunused
- All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter.
- -Wuninitialized
- Warn if an automatic variable is used without first being initialized or
if a variable may be clobbered by a
"setjmp" call. In C++, warn if a
non-static reference or non-static
"const" member appears in a class
without constructors.
If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the -Winit-self option.
These warnings occur for individual uninitialized or clobbered elements of structure, union or array variables as well as for variables that are uninitialized or clobbered as a whole. They do not occur for variables or elements declared "volatile". Because these warnings depend on optimization, the exact variables or elements for which there are warnings depends on the precise optimization options and version of GCC used.
Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.
- -Winvalid-memory-model
- Warn for invocations of __atomic Builtins, __sync Builtins,
and the C11 atomic generic functions with a memory consistency argument
that is either invalid for the operation or outside the range of values of
the "memory_order" enumeration. For
example, since the "__atomic_store" and
"__atomic_store_n" built-ins are only
defined for the relaxed, release, and sequentially consistent memory
orders the following code is diagnosed:
void store (int *i) { __atomic_store_n (i, 0, memory_order_consume); }
-Winvalid-memory-model is enabled by default.
- -Wmaybe-uninitialized
- For an automatic (i.e. local) variable, if there exists a path from the
function entry to a use of the variable that is initialized, but there
exist some other paths for which the variable is not initialized, the
compiler emits a warning if it cannot prove the uninitialized paths are
not executed at run time.
These warnings are only possible in optimizing compilation, because otherwise GCC does not keep track of the state of variables.
These warnings are made optional because GCC may not be able to determine when the code is correct in spite of appearing to have an error. Here is one example of how this can happen:
{ int x; switch (y) { case 1: x = 1; break; case 2: x = 4; break; case 3: x = 5; } foo (x); }
If the value of "y" is always 1, 2 or 3, then "x" is always initialized, but GCC doesn't know this. To suppress the warning, you need to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might be changed by a call to "longjmp". The compiler sees only the calls to "setjmp". It cannot know where "longjmp" will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because "longjmp" cannot in fact be called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return as "noreturn".
This warning is enabled by -Wall or -Wextra.
- -Wunknown-pragmas
- Warn when a "#pragma" directive is encountered that is not understood by GCC. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the -Wall command-line option.
- -Wno-pragmas
- Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas.
- -Wstrict-aliasing
- This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in -Wall. It is equivalent to -Wstrict-aliasing=3
- -Wstrict-aliasing=n
- This option is only active when -fstrict-aliasing is active. It
warns about code that might break the strict aliasing rules that the
compiler is using for optimization. Higher levels correspond to higher
accuracy (fewer false positives). Higher levels also correspond to more
effort, similar to the way -O works. -Wstrict-aliasing is
equivalent to -Wstrict-aliasing=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but -fstrict-aliasing still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only.
Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: "*(int*)&some_float". If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.
- -Wstrict-overflow
- -Wstrict-overflow=n
- This option is only active when signed overflow is undefined. It warns
about cases where the compiler optimizes based on the assumption that
signed overflow does not occur. Note that it does not warn about all cases
where the code might overflow: it only warns about cases where the
compiler implements some optimization. Thus this warning depends on the
optimization level.
An optimization that assumes that signed overflow does not occur is perfectly safe if the values of the variables involved are such that overflow never does, in fact, occur. Therefore this warning can easily give a false positive: a warning about code that is not actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all.
- -Wstrict-overflow=1
- Warn about cases that are both questionable and easy to avoid. For example the compiler simplifies "x + 1 > x" to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and must be explicitly requested.
- -Wstrict-overflow=2
- Also warn about other cases where a comparison is simplified to a constant. For example: "abs (x) >= 0". This can only be simplified when signed integer overflow is undefined, because "abs (INT_MIN)" overflows to "INT_MIN", which is less than zero. -Wstrict-overflow (with no level) is the same as -Wstrict-overflow=2.
- -Wstrict-overflow=3
- Also warn about other cases where a comparison is simplified. For example: "x + 1 > 1" is simplified to "x > 0".
- -Wstrict-overflow=4
- Also warn about other simplifications not covered by the above cases. For example: "(x * 10) / 5" is simplified to "x * 2".
- -Wstrict-overflow=5
- Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: "x + 2 > y" is simplified to "x + 1 >= y". This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives.
- -Wstringop-overflow
- -Wstringop-overflow=type
- Warn for calls to string manipulation functions such as
"memcpy" and
"strcpy" that are determined to overflow
the destination buffer. The optional argument is one greater than the type
of Object Size Checking to perform to determine the size of the
destination. The argument is meaningful only for functions that operate on
character arrays but not for raw memory functions like
"memcpy" which always make use of Object
Size type-0. The option also warns for calls that specify a size in excess
of the largest possible object or at most "SIZE_MAX
/ 2" bytes. The option produces the best results with
optimization enabled but can detect a small subset of simple buffer
overflows even without optimization in calls to the GCC built-in functions
like "__builtin_memcpy" that correspond
to the standard functions. In any case, the option warns about just a
subset of buffer overflows detected by the corresponding overflow checking
built-ins. For example, the option will issue a warning for the
"strcpy" call below because it copies at
least 5 characters (the string "blue"
including the terminating NUL) into the buffer of size 4.
enum Color { blue, purple, yellow }; const char* f (enum Color clr) { static char buf [4]; const char *str; switch (clr) { case blue: str = "blue"; break; case purple: str = "purple"; break; case yellow: str = "yellow"; break; } return strcpy (buf, str); // warning here }
Option -Wstringop-overflow=2 is enabled by default.
- -Wstringop-overflow
- -Wstringop-overflow=1
- The -Wstringop-overflow=1 option uses type-zero Object Size Checking to determine the sizes of destination objects. This is the default setting of the option. At this setting the option will not warn for writes past the end of subobjects of larger objects accessed by pointers unless the size of the largest surrounding object is known. When the destination may be one of several objects it is assumed to be the largest one of them. On Linux systems, when optimization is enabled at this setting the option warns for the same code as when the "_FORTIFY_SOURCE" macro is defined to a non-zero value.
- -Wstringop-overflow=2
- The -Wstringop-overflow=2 option uses type-one Object Size Checking to determine the sizes of destination objects. At this setting the option will warn about overflows when writing to members of the largest complete objects whose exact size is known. It will, however, not warn for excessive writes to the same members of unknown objects referenced by pointers since they may point to arrays containing unknown numbers of elements.
- -Wstringop-overflow=3
- The -Wstringop-overflow=3 option uses type-two Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing the smallest object or data member. This is the most restrictive setting of the option that may result in warnings for safe code.
- -Wstringop-overflow=4
- The -Wstringop-overflow=4 option uses type-three Object Size Checking to determine the sizes of destination objects. At this setting the option will warn about overflowing any data members, and when the destination is one of several objects it uses the size of the largest of them to decide whether to issue a warning. Similarly to -Wstringop-overflow=3 this setting of the option may result in warnings for benign code.
- -Wstringop-truncation
- Warn for calls to bounded string manipulation functions such as
"strncat",
"strncpy", and
"stpncpy" that may either truncate the
copied string or leave the destination unchanged.
In the following example, the call to "strncat" specifies a bound that is less than the length of the source string. As a result, the copy of the source will be truncated and so the call is diagnosed. To avoid the warning use "bufsize - strlen (buf) - 1)" as the bound.
void append (char *buf, size_t bufsize) { strncat (buf, ".txt", 3); }
As another example, the following call to "strncpy" results in copying to "d" just the characters preceding the terminating NUL, without appending the NUL to the end. Assuming the result of "strncpy" is necessarily a NUL-terminated string is a common mistake, and so the call is diagnosed. To avoid the warning when the result is not expected to be NUL-terminated, call "memcpy" instead.
void copy (char *d, const char *s) { strncpy (d, s, strlen (s)); }
In the following example, the call to "strncpy" specifies the size of the destination buffer as the bound. If the length of the source string is equal to or greater than this size the result of the copy will not be NUL-terminated. Therefore, the call is also diagnosed. To avoid the warning, specify "sizeof buf - 1" as the bound and set the last element of the buffer to "NUL".
void copy (const char *s) { char buf[80]; strncpy (buf, s, sizeof buf); ... }
In situations where a character array is intended to store a sequence of bytes with no terminating "NUL" such an array may be annotated with attribute "nonstring" to avoid this warning. Such arrays, however, are not suitable arguments to functions that expect "NUL"-terminated strings. To help detect accidental misuses of such arrays GCC issues warnings unless it can prove that the use is safe.
Option -Wstringop-truncation is enabled by -Wall.
- -Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]
- Warn for cases where adding an attribute may be beneficial. The attributes currently supported are listed below.
- -Wsuggest-attribute=pure
- -Wsuggest-attribute=const
- -Wsuggest-attribute=noreturn
- -Wsuggest-attribute=malloc
- Warn about functions that might be candidates for attributes "pure", "const" or "noreturn" or "malloc". The compiler only warns for functions visible in other compilation units or (in the case of "pure" and "const") if it cannot prove that the function returns normally. A function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, calling "abort" or trapping. This analysis requires option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels improve the accuracy of the analysis.
- -Wsuggest-attribute=format
- -Wmissing-format-attribute
- Warn about function pointers that might be candidates for
"format" attributes. Note these are only
possible candidates, not absolute ones. GCC guesses that function pointers
with "format" attributes that are used
in assignment, initialization, parameter passing or return statements
should have a corresponding "format"
attribute in the resulting type. I.e. the left-hand side of the assignment
or initialization, the type of the parameter variable, or the return type
of the containing function respectively should also have a
"format" attribute to avoid the warning.
GCC also warns about function definitions that might be candidates for "format" attributes. Again, these are only possible candidates. GCC guesses that "format" attributes might be appropriate for any function that calls a function like "vprintf" or "vscanf", but this might not always be the case, and some functions for which "format" attributes are appropriate may not be detected.
- -Wsuggest-attribute=cold
- Warn about functions that might be candidates for "cold" attribute. This is based on static detection and generally will only warn about functions which always leads to a call to another "cold" function such as wrappers of C++ "throw" or fatal error reporting functions leading to "abort".
- -Wsuggest-final-types
- Warn about types with virtual methods where code quality would be improved if the type were declared with the C++11 "final" specifier, or, if possible, declared in an anonymous namespace. This allows GCC to more aggressively devirtualize the polymorphic calls. This warning is more effective with link time optimization, where the information about the class hierarchy graph is more complete.
- -Wsuggest-final-methods
- Warn about virtual methods where code quality would be improved if the method were declared with the C++11 "final" specifier, or, if possible, its type were declared in an anonymous namespace or with the "final" specifier. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. It is recommended to first consider suggestions of -Wsuggest-final-types and then rebuild with new annotations.
- -Wsuggest-override
- Warn about overriding virtual functions that are not marked with the override keyword.
- -Walloc-zero
- Warn about calls to allocation functions decorated with attribute "alloc_size" that specify zero bytes, including those to the built-in forms of the functions "aligned_alloc", "alloca", "calloc", "malloc", and "realloc". Because the behavior of these functions when called with a zero size differs among implementations (and in the case of "realloc" has been deprecated) relying on it may result in subtle portability bugs and should be avoided.
- -Walloc-size-larger-than=n
- Warn about calls to functions decorated with attribute "alloc_size" that attempt to allocate objects larger than the specified number of bytes, or where the result of the size computation in an integer type with infinite precision would exceed "SIZE_MAX / 2". The option argument n may end in one of the standard suffixes designating a multiple of bytes such as "kB" and "KiB" for kilobyte and kibibyte, respectively, "MB" and "MiB" for megabyte and mebibyte, and so on. -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled by specifying n of SIZE_MAX or more.
- -Walloca
- This option warns on all uses of "alloca" in the source.
- -Walloca-larger-than=n
- This option warns on calls to "alloca"
that are not bounded by a controlling predicate limiting its argument of
integer type to at most n bytes, or calls to
"alloca" where the bound is unknown.
Arguments of non-integer types are considered unbounded even if they
appear to be constrained to the expected range.
For example, a bounded case of "alloca" could be:
void func (size_t n) { void *p; if (n <= 1000) p = alloca (n); else p = malloc (n); f (p); }
In the above example, passing "-Walloca-larger-than=1000" would not issue a warning because the call to "alloca" is known to be at most 1000 bytes. However, if "-Walloca-larger-than=500" were passed, the compiler would emit a warning.
Unbounded uses, on the other hand, are uses of "alloca" with no controlling predicate constraining its integer argument. For example:
void func () { void *p = alloca (n); f (p); }
If "-Walloca-larger-than=500" were passed, the above would trigger a warning, but this time because of the lack of bounds checking.
Note, that even seemingly correct code involving signed integers could cause a warning:
void func (signed int n) { if (n < 500) { p = alloca (n); f (p); } }
In the above example, n could be negative, causing a larger than expected argument to be implicitly cast into the "alloca" call.
This option also warns when "alloca" is used in a loop.
This warning is not enabled by -Wall, and is only active when -ftree-vrp is active (default for -O2 and above).
See also -Wvla-larger-than=n.
- -Warray-bounds
- -Warray-bounds=n
- This option is only active when -ftree-vrp is active (default for -O2 and above). It warns about subscripts to arrays that are always out of bounds. This warning is enabled by -Wall.
- -Warray-bounds=1
- This is the warning level of -Warray-bounds and is enabled by -Wall; higher levels are not, and must be explicitly requested.
- -Warray-bounds=2
- This warning level also warns about out of bounds access for arrays at the end of a struct and for arrays accessed through pointers. This warning level may give a larger number of false positives and is deactivated by default.
- -Wattribute-alias
- Warn about declarations using the "alias" and similar attributes whose target is incompatible with the type of the alias.
- -Wbidi-chars=[none|unpaired|any]
- Warn about possibly misleading UTF-8 bidirectional control characters in
comments, string literals, character constants, and identifiers. Such
characters can change left-to-right writing direction into right-to-left
(and vice versa), which can cause confusion between the logical order and
visual order. This may be dangerous; for instance, it may seem that a
piece of code is not commented out, whereas it in fact is.
There are three levels of warning supported by GCC. The default is -Wbidi-chars=unpaired, which warns about improperly terminated bidi contexts. -Wbidi-chars=none turns the warning off. -Wbidi-chars=any warns about any use of bidirectional control characters.
- -Wbool-compare
- Warn about boolean expression compared with an integer value different
from
"true"/"false".
For instance, the following comparison is always false:
int n = 5; ... if ((n > 1) == 2) { ... }
This warning is enabled by -Wall.
- -Wbool-operation
- Warn about suspicious operations on expressions of a boolean type. For
instance, bitwise negation of a boolean is very likely a bug in the
program. For C, this warning also warns about incrementing or decrementing
a boolean, which rarely makes sense. (In C++, decrementing a boolean is
always invalid. Incrementing a boolean is invalid in C++17, and deprecated
otherwise.)
This warning is enabled by -Wall.
- -Wduplicated-branches
- Warn when an if-else has identical branches. This warning detects cases
like
if (p != NULL) return 0; else return 0;
It doesn't warn when both branches contain just a null statement. This warning also warn for conditional operators:
int i = x ? *p : *p;
- -Wduplicated-cond
- Warn about duplicated conditions in an if-else-if chain. For instance,
warn for the following code:
if (p->q != NULL) { ... } else if (p->q != NULL) { ... }
- -Wframe-address
- Warn when the __builtin_frame_address or __builtin_return_address is called with an argument greater than 0. Such calls may return indeterminate values or crash the program. The warning is included in -Wall.
- -Wno-discarded-qualifiers (C and Objective-C only)
- Do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if a "const char *" variable is passed to a function that takes a "char *" parameter. This option can be used to suppress such a warning.
- -Wno-discarded-array-qualifiers (C and Objective-C only)
- Do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler warns if a "const int (*)[]" variable is passed to a function that takes a "int (*)[]" parameter. This option can be used to suppress such a warning.
- -Wno-incompatible-pointer-types (C and Objective-C only)
- Do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases not covered by -Wno-pointer-sign, which warns for pointer argument passing or assignment with different signedness.
- -Wno-int-conversion (C and Objective-C only)
- Do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about implicit conversions; for explicit conversions the warnings -Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast may be used.
- -Wno-div-by-zero
- Do not warn about compile-time integer division by zero. Floating-point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs.
- -Wsystem-headers
- Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. However, note that using -Wall in conjunction with this option does not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.
- -Wtautological-compare
- Warn if a self-comparison always evaluates to true or false. This warning
detects various mistakes such as:
int i = 1; ... if (i > i) { ... }
This warning also warns about bitwise comparisons that always evaluate to true or false, for instance:
if ((a & 16) == 10) { ... }
will always be false.
This warning is enabled by -Wall.
- -Wtrampolines
- Warn about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. For some targets, it is made up of data only and thus requires no special treatment. But, for most targets, it is made up of code and thus requires the stack to be made executable in order for the program to work properly.
- -Wfloat-equal
- Warn if floating-point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken.
- -Wtraditional (C and Objective-C only)
- Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided.
- Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not.
- In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but ignores because the # does not appear as the first character on the line. It also suggests you hide directives like "#pragma" not understood by traditional C by indenting them. Some traditional implementations do not recognize "#elif", so this option suggests avoiding it altogether.
- A function-like macro that appears without arguments.
- The unary plus operator.
- The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in "<limits.h>". Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
- A function declared external in one block and then used after the end of the block.
- A "switch" statement has an operand of type "long".
- A non-"static" function declaration follows a "static" one. This construct is not accepted by some traditional C compilers.
- The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
- Usage of ISO string concatenation is detected.
- Initialization of automatic aggregates.
- Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
- Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. "__STDC__" to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.
- Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.
- Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros, "PARAMS" and "VPARAMS". This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
- -Wtraditional-conversion (C and Objective-C only)
- Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion.
- -Wdeclaration-after-statement (C and Objective-C only)
- Warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90.
- -Wshadow
- Warn whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C) or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. Same as -Wshadow=global.
- -Wno-shadow-ivar (Objective-C only)
- Do not warn whenever a local variable shadows an instance variable in an Objective-C method.
- -Wshadow=global
- The default for -Wshadow. Warns for any (global) shadowing.
- -Wshadow=local
- Warn when a local variable shadows another local variable or parameter. This warning is enabled by -Wshadow=global.
- -Wshadow=compatible-local
- Warn when a local variable shadows another local variable or parameter
whose type is compatible with that of the shadowing variable. In C++, type
compatibility here means the type of the shadowing variable can be
converted to that of the shadowed variable. The creation of this flag (in
addition to -Wshadow=local) is based on the idea that when a local
variable shadows another one of incompatible type, it is most likely
intentional, not a bug or typo, as shown in the following example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i) { for (int i = 0; i < N; ++i) { ... } ... }
Since the two variable "i" in the example above have incompatible types, enabling only -Wshadow=compatible-local will not emit a warning. Because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking will catch that and emit an error or warning. So not warning (about shadowing) in this case will not lead to undetected bugs. Use of this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing.
This warning is enabled by -Wshadow=local.
- -Wlarger-than=len
- Warn whenever an object of larger than len bytes is defined.
- -Wframe-larger-than=len
- Warn if the size of a function frame is larger than len bytes. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than len even if you do not get a warning. In addition, any space allocated via "alloca", variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning.
- -Wno-free-nonheap-object
- Do not warn when attempting to free an object that was not allocated on the heap.
- -Wstack-usage=len
- Warn if the stack usage of a function might be larger than len
bytes. The computation done to determine the stack usage is conservative.
Any space allocated via "alloca",
variable-length arrays, or related constructs is included by the compiler
when determining whether or not to issue a warning.
The message is in keeping with the output of -fstack-usage.
- If the stack usage is fully static but exceeds the specified amount, it's:
warning: stack usage is 1120 bytes
- If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
- If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded
- -Wno-pedantic-ms-format (MinGW targets only)
- When used in combination with -Wformat and -pedantic without GNU extensions, this option disables the warnings about non-ISO "printf" / "scanf" format width specifiers "I32", "I64", and "I" used on Windows targets, which depend on the MS runtime.
- -Waligned-new
- Warn about a new-expression of a type that requires greater alignment than
the "alignof(std::max_align_t)" but uses
an allocation function without an explicit alignment parameter. This
option is enabled by -Wall.
Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions.
- -Wplacement-new
- -Wplacement-new=n
- Warn about placement new expressions with undefined behavior, such as
constructing an object in a buffer that is smaller than the type of the
object. For example, the placement new expression below is diagnosed
because it attempts to construct an array of 64 integers in a buffer only
64 bytes large.
char buf [64]; new (buf) int[64];
This warning is enabled by default.
- -Wplacement-new=1
- This is the default warning level of -Wplacement-new. At this level
the warning is not issued for some strictly undefined constructs that GCC
allows as extensions for compatibility with legacy code. For example, the
following "new" expression is not
diagnosed at this level even though it has undefined behavior according to
the C++ standard because it writes past the end of the one-element array.
struct S { int n, a[1]; }; S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]); new (s->a)int [32]();
- -Wplacement-new=2
- At this level, in addition to diagnosing all the same constructs as at
level 1, a diagnostic is also issued for placement new expressions that
construct an object in the last member of structure whose type is an array
of a single element and whose size is less than the size of the object
being constructed. While the previous example would be diagnosed, the
following construct makes use of the flexible member array extension to
avoid the warning at level 2.
struct S { int n, a[]; }; S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]); new (s->a)int [32]();
- -Wpointer-arith
- Warn about anything that depends on the "size of" a function type or of "void". GNU C assigns these types a size of 1, for convenience in calculations with "void *" pointers and pointers to functions. In C++, warn also when an arithmetic operation involves "NULL". This warning is also enabled by -Wpedantic.
- -Wpointer-compare
- Warn if a pointer is compared with a zero character constant. This usually
means that the pointer was meant to be dereferenced. For example:
const char *p = foo (); if (p == '\0') return 42;
Note that the code above is invalid in C++11.
This warning is enabled by default.
- -Wtype-limits
- Warn if a comparison is always true or always false due to the limited range of the data type, but do not warn for constant expressions. For example, warn if an unsigned variable is compared against zero with "<" or ">=". This warning is also enabled by -Wextra.
- -Wcomment
- -Wcomments
- Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a // comment. This warning is enabled by -Wall.
- -Wtrigraphs
- Warn if any trigraphs are encountered that might change the meaning of the
program. Trigraphs within comments are not warned about, except those that
would form escaped newlines.
This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs.
- -Wundef
- Warn if an undefined identifier is evaluated in an "#if" directive. Such identifiers are replaced with zero.
- -Wexpansion-to-defined
- Warn whenever defined is encountered in the expansion of a macro (including the case where the macro is expanded by an #if directive). Such usage is not portable. This warning is also enabled by -Wpedantic and -Wextra.
- -Wunused-macros
- Warn about macros defined in the main file that are unused. A macro is
used if it is expanded or tested for existence at least once. The
preprocessor also warns if the macro has not been used at the time it is
redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then the preprocessor reports it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning #endif
- -Wno-endif-labels
- Do not warn whenever an "#else" or an
"#endif" are followed by text. This
sometimes happens in older programs with code of the form
#if FOO ... #else FOO ... #endif FOO
The second and third "FOO" should be in comments. This warning is on by default.
- -Wbad-function-cast (C and Objective-C only)
- Warn when a function call is cast to a non-matching type. For example, warn if a call to a function returning an integer type is cast to a pointer type.
- -Wc90-c99-compat (C and Objective-C only)
- Warn about features not present in ISO C90, but present in ISO C99. For instance, warn about use of variable length arrays, "long long" type, "bool" type, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows "__extension__".
- -Wc99-c11-compat (C and Objective-C only)
- Warn about features not present in ISO C99, but present in ISO C11. For instance, warn about use of anonymous structures and unions, "_Atomic" type qualifier, "_Thread_local" storage-class specifier, "_Alignas" specifier, "Alignof" operator, "_Generic" keyword, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows "__extension__".
- -Wc++-compat (C and Objective-C only)
- Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request for implicit conversion from "void *" to a pointer to non-"void" type.
- -Wc++11-compat (C++ and Objective-C++ only)
- Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall.
- -Wc++14-compat (C++ and Objective-C++ only)
- Warn about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. This warning is enabled by -Wall.
- -Wc++17-compat (C++ and Objective-C++ only)
- Warn about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. This warning is enabled by -Wall.
- -Wcast-qual
- Warn whenever a pointer is cast so as to remove a type qualifier from the
target type. For example, warn if a "const char
*" is cast to an ordinary "char
*".
Also warn when making a cast that introduces a type qualifier in an unsafe way. For example, casting "char **" to "const char **" is unsafe, as in this example:
/* p is char ** value. */ const char **q = (const char **) p; /* Assignment of readonly string to const char * is OK. */ *q = "string"; /* Now char** pointer points to read-only memory. */ **p = 'b';
- -Wcast-align
- Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a "char *" is cast to an "int *" on machines where integers can only be accessed at two- or four-byte boundaries.
- -Wcast-align=strict
- Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a "char *" is cast to an "int *" regardless of the target machine.
- -Wcast-function-type
- Warn when a function pointer is cast to an incompatible function pointer. In a cast involving function types with a variable argument list only the types of initial arguments that are provided are considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types are ignored, like "int" vs. "long" on ILP32 targets. Likewise type qualifiers are ignored. The function type "void (*) (void)" is special and matches everything, which can be used to suppress this warning. In a cast involving pointer to member types this warning warns whenever the type cast is changing the pointer to member type. This warning is enabled by -Wextra.
- -Wwrite-strings
- When compiling C, give string constants the type
"const
char[length]"
so that copying the address of one into a
non-"const" "char
*" pointer produces a warning. These warnings help you find at
compile time code that can try to write into a string constant, but only
if you have been very careful about using
"const" in declarations and prototypes.
Otherwise, it is just a nuisance. This is why we did not make -Wall
request these warnings.
When compiling C++, warn about the deprecated conversion from string literals to "char *". This warning is enabled by default for C++ programs.
- -Wcatch-value
- -Wcatch-value=n (C++ and Objective-C++ only)
- Warn about catch handlers that do not catch via reference. With -Wcatch-value=1 (or -Wcatch-value for short) warn about polymorphic class types that are caught by value. With -Wcatch-value=2 warn about all class types that are caught by value. With -Wcatch-value=3 warn about all types that are not caught by reference. -Wcatch-value is enabled by -Wall.
- -Wclobbered
- Warn for variables that might be changed by "longjmp" or "vfork". This warning is also enabled by -Wextra.
- -Wconditionally-supported (C++ and Objective-C++ only)
- Warn for conditionally-supported (C++11 [intro.defs]) constructs.
- -Wconversion
- Warn for implicit conversions that may alter a value. This includes
conversions between real and integer, like "abs
(x)" when "x" is
"double"; conversions between signed and
unsigned, like "unsigned ui = -1"; and
conversions to smaller types, like "sqrtf
(M_PI)". Do not warn for explicit casts like
"abs ((int) x)"
and "ui = (unsigned) -1", or if the
value is not changed by the conversion like in "abs
(2.0)". Warnings about conversions between signed and unsigned
integers can be disabled by using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-defined conversions; and conversions that never use a type conversion operator: conversions to "void", the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly enabled.
- -Wno-conversion-null (C++ and Objective-C++ only)
- Do not warn for conversions between "NULL" and non-pointer types. -Wconversion-null is enabled by default.
- -Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
- Warn when a literal 0 is used as null pointer constant. This can be useful to facilitate the conversion to "nullptr" in C++11.
- -Wsubobject-linkage (C++ and Objective-C++ only)
- Warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. If a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation because the meaning of B is different in each translation unit. If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the main .C file, as those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default.
- -Wdangling-else
- Warn about constructions where there may be confusion to which
"if" statement an
"else" branch belongs. Here is an
example of such a case:
{ if (a) if (b) foo (); else bar (); }
In C/C++, every "else" branch belongs to the innermost possible "if" statement, which in this example is "if (b)". This is often not what the programmer expected, as illustrated in the above example by indentation the programmer chose. When there is the potential for this confusion, GCC issues a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost "if" statement so there is no way the "else" can belong to the enclosing "if". The resulting code looks like this:
{ if (a) { if (b) foo (); else bar (); } }
This warning is enabled by -Wparentheses.
- -Wdate-time
- Warn when macros "__TIME__", "__DATE__" or "__TIMESTAMP__" are encountered as they might prevent bit-wise-identical reproducible compilations.
- -Wdelete-incomplete (C++ and Objective-C++ only)
- Warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default.
- -Wuseless-cast (C++ and Objective-C++ only)
- Warn when an expression is casted to its own type.
- -Wempty-body
- Warn if an empty body occurs in an "if", "else" or "do while" statement. This warning is also enabled by -Wextra.
- -Wenum-compare
- Warn about a comparison between values of different enumerated types. In C++ enumerated type mismatches in conditional expressions are also diagnosed and the warning is enabled by default. In C this warning is enabled by -Wall.
- -Wextra-semi (C++, Objective-C++ only)
- Warn about redundant semicolon after in-class function definition.
- -Wjump-misses-init (C, Objective-C only)
- Warn if a "goto" statement or a
"switch" statement jumps forward across
the initialization of a variable, or jumps backward to a label after the
variable has been initialized. This only warns about variables that are
initialized when they are declared. This warning is only supported for C
and Objective-C; in C++ this sort of branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option.
- -Wsign-compare
- Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. In C++, this warning is also enabled by -Wall. In C, it is also enabled by -Wextra.
- -Wsign-conversion
- Warn for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this option is enabled also by -Wconversion.
- -Wfloat-conversion
- Warn for implicit conversions that reduce the precision of a real value. This includes conversions from real to integer, and from higher precision real to lower precision real values. This option is also enabled by -Wconversion.
- -Wno-scalar-storage-order
- Do not warn on suspicious constructs involving reverse scalar storage order.
- -Wsized-deallocation (C++ and Objective-C++ only)
- Warn about a definition of an unsized deallocation function
void operator delete (void *) noexcept; void operator delete[] (void *) noexcept;
without a definition of the corresponding sized deallocation function
void operator delete (void *, std::size_t) noexcept; void operator delete[] (void *, std::size_t) noexcept;
or vice versa. Enabled by -Wextra along with -fsized-deallocation.
- -Wsizeof-pointer-div
- Warn for suspicious divisions of two sizeof expressions that divide the pointer size by the element size, which is the usual way to compute the array size but won't work out correctly with pointers. This warning warns e.g. about "sizeof (ptr) / sizeof (ptr[0])" if "ptr" is not an array, but a pointer. This warning is enabled by -Wall.
- -Wsizeof-pointer-memaccess
- Warn for suspicious length parameters to certain string and memory
built-in functions if the argument uses
"sizeof". This warning triggers for
example for "memset (ptr, 0, sizeof
(ptr));" if "ptr" is not an
array, but a pointer, and suggests a possible fix, or about
"memcpy (&foo, ptr, sizeof
(&foo));". -Wsizeof-pointer-memaccess also warns
about calls to bounded string copy functions like
"strncat" or
"strncpy" that specify as the bound a
"sizeof" expression of the source array.
For example, in the following function the call to
"strncat" specifies the size of the
source string as the bound. That is almost certainly a mistake and so the
call is diagnosed.
void make_file (const char *name) { char path[PATH_MAX]; strncpy (path, name, sizeof path - 1); strncat (path, ".text", sizeof ".text"); ... }
The -Wsizeof-pointer-memaccess option is enabled by -Wall.
- -Wsizeof-array-argument
- Warn when the "sizeof" operator is applied to a parameter that is declared as an array in a function definition. This warning is enabled by default for C and C++ programs.
- -Wmemset-elt-size
- Warn for suspicious calls to the "memset" built-in function, if the first argument references an array, and the third argument is a number equal to the number of elements, but not equal to the size of the array in memory. This indicates that the user has omitted a multiplication by the element size. This warning is enabled by -Wall.
- -Wmemset-transposed-args
- Warn for suspicious calls to the "memset" built-in function, if the second argument is not zero and the third argument is zero. This warns e.g.@ about "memset (buf, sizeof buf, 0)" where most probably "memset (buf, 0, sizeof buf)" was meant instead. The diagnostics is only emitted if the third argument is literal zero. If it is some expression that is folded to zero, a cast of zero to some type, etc., it is far less likely that the user has mistakenly exchanged the arguments and no warning is emitted. This warning is enabled by -Wall.
- -Waddress
- Warn about suspicious uses of memory addresses. These include using the address of a function in a conditional expression, such as "void func(void); if (func)", and comparisons against the memory address of a string literal, such as "if (x == "abc")". Such uses typically indicate a programmer error: the address of a function always evaluates to true, so their use in a conditional usually indicate that the programmer forgot the parentheses in a function call; and comparisons against string literals result in unspecified behavior and are not portable in C, so they usually indicate that the programmer intended to use "strcmp". This warning is enabled by -Wall.
- -Wlogical-op
- Warn about suspicious uses of logical operators in expressions. This
includes using logical operators in contexts where a bit-wise operator is
likely to be expected. Also warns when the operands of a logical operator
are the same:
extern int a; if (a < 0 && a < 0) { ... }
- -Wlogical-not-parentheses
- Warn about logical not used on the left hand side operand of a comparison.
This option does not warn if the right operand is considered to be a
boolean expression. Its purpose is to detect suspicious code like the
following:
int a; ... if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the LHS into parentheses:
if ((!a) > 1) { ... }
This warning is enabled by -Wall.
- -Waggregate-return
- Warn if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.)
- -Wno-aggressive-loop-optimizations
- Warn if in a loop with constant number of iterations the compiler detects undefined behavior in some statement during one or more of the iterations.
- -Wno-attributes
- Do not warn if an unexpected "__attribute__" is used, such as unrecognized attributes, function attributes applied to variables, etc. This does not stop errors for incorrect use of supported attributes.
- -Wno-builtin-declaration-mismatch
- Warn if a built-in function is declared with the wrong signature or as non-function. This warning is enabled by default.
- -Wno-builtin-macro-redefined
- Do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of "__TIMESTAMP__", "__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".
- -Wstrict-prototypes (C and Objective-C only)
- Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration that specifies the argument types.)
- -Wold-style-declaration (C and Objective-C only)
- Warn for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like "static" are not the first things in a declaration. This warning is also enabled by -Wextra.
- -Wold-style-definition (C and Objective-C only)
- Warn if an old-style function definition is used. A warning is given even if there is a previous prototype.
- -Wmissing-parameter-type (C and Objective-C only)
- A function parameter is declared without a type specifier in K&R-style
functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
- -Wmissing-prototypes (C and Objective-C only)
- Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. Use this option to detect global functions that do not have a matching prototype declaration in a header file. This option is not valid for C++ because all function declarations provide prototypes and a non-matching declaration declares an overload rather than conflict with an earlier declaration. Use -Wmissing-declarations to detect missing declarations in C++.
- -Wmissing-declarations
- Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations; use -Wmissing-prototypes to detect missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces.
- -Wmissing-field-initializers
- Warn if a structure's initializer has some fields missing. For example,
the following code causes such a warning, because
"x.h" is implicitly zero:
struct s { int f, g, h; }; struct s x = { 3, 4 };
This option does not warn about designated initializers, so the following modification does not trigger a warning:
struct s { int f, g, h; }; struct s x = { .f = 3, .g = 4 };
In C this option does not warn about the universal zero initializer { 0 }:
struct s { int f, g, h; }; struct s x = { 0 };
Likewise, in C++ this option does not warn about the empty { } initializer, for example:
struct s { int f, g, h; }; s x = { };
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers.
- -Wno-multichar
- Do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's code, as they have implementation-defined values, and should not be used in portable code.
- -Wnormalized=[none|id|nfc|nfkc]
- In ISO C and ISO C++, two identifiers are different if they are different
sequences of characters. However, sometimes when characters outside the
basic ASCII character set are used, you can have two different character
sequences that look the same. To avoid confusion, the ISO 10646 standard
sets out some normalization rules which when applied ensure that
two sequences that look the same are turned into the same sequence. GCC
can warn you if you are using identifiers that have not been normalized;
this option controls that warning.
There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns about any identifier that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It is equivalent to -Wnormalized.
Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. That is, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this, which is why this option is not the default.
You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. You should only do this if you are using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that has been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding convention if the programming environment cannot be fixed to display these characters distinctly.
- -Wno-deprecated
- Do not warn about usage of deprecated features.
- -Wno-deprecated-declarations
- Do not warn about uses of functions, variables, and types marked as deprecated by using the "deprecated" attribute.
- -Wno-overflow
- Do not warn about compile-time overflow in constant expressions.
- -Wno-odr
- Warn about One Definition Rule violations during link-time optimization. Requires -flto-odr-type-merging to be enabled. Enabled by default.
- -Wopenmp-simd
- Warn if the vectorizer cost model overrides the OpenMP simd directive set by user. The -fsimd-cost-model=unlimited option can be used to relax the cost model.
- -Woverride-init (C and Objective-C only)
- Warn if an initialized field without side effects is overridden when using
designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-override-init.
- -Woverride-init-side-effects (C and Objective-C only)
- Warn if an initialized field with side effects is overridden when using designated initializers. This warning is enabled by default.
- -Wpacked
- Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure. Such
structures may be mis-aligned for little benefit. For instance, in this
code, the variable "f.x" in
"struct bar" is misaligned even though
"struct bar" does not itself have the
packed attribute:
struct foo { int x; char a, b, c, d; } __attribute__((packed)); struct bar { char z; struct foo f; };
- -Wpacked-bitfield-compat
- The 4.1, 4.2 and 4.3 series of GCC ignore the
"packed" attribute on bit-fields of type
"char". This has been fixed in GCC 4.4
but the change can lead to differences in the structure layout. GCC
informs you when the offset of such a field has changed in GCC 4.4. For
example there is no longer a 4-bit padding between field
"a" and
"b" in this structure:
struct foo { char a:4; char b:8; } __attribute__ ((packed));
This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning.
- -Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only)
- Warn if a structure field with explicitly specified alignment in a packed
struct or union is misaligned. For example, a warning will be issued on
"struct S", like,
"warning: alignment 1 of
'struct S' is less than 8", in this code:
struct __attribute__ ((aligned (8))) S8 { char a[8]; }; struct __attribute__ ((packed)) S { struct S8 s8; };
This warning is enabled by -Wall.
- -Wpadded
- Warn if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens it is possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller.
- -Wredundant-decls
- Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing.
- -Wno-restrict
- Warn when an object referenced by a
"restrict"-qualified parameter (or, in
C++, a "__restrict"-qualified parameter)
is aliased by another argument, or when copies between such objects
overlap. For example, the call to the
"strcpy" function below attempts to
truncate the string by replacing its initial characters with the last
four. However, because the call writes the terminating NUL into
"a[4]", the copies overlap and the call
is diagnosed.
void foo (void) { char a[] = "abcd1234"; strcpy (a, a + 4); ... }
The -Wrestrict option detects some instances of simple overlap even without optimization but works best at -O2 and above. It is included in -Wall.
- -Wnested-externs (C and Objective-C only)
- Warn if an "extern" declaration is encountered within a function.
- -Wno-inherited-variadic-ctor
- Suppress warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited.
- -Winline
- Warn if a function that is declared as inline cannot be inlined. Even with
this option, the compiler does not warn about failures to inline functions
declared in system headers.
The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear.
- -Wno-invalid-offsetof (C++ and Objective-C++ only)
- Suppress warnings from applying the
"offsetof" macro to a non-POD type.
According to the 2014 ISO C++ standard, applying
"offsetof" to a non-standard-layout type
is undefined. In existing C++ implementations, however,
"offsetof" typically gives meaningful
results. This flag is for users who are aware that they are writing
nonportable code and who have deliberately chosen to ignore the warning
about it.
The restrictions on "offsetof" may be relaxed in a future version of the C++ standard.
- -Wint-in-bool-context
- Warn for suspicious use of integer values where boolean values are expected, such as conditional expressions (?:) using non-boolean integer constants in boolean context, like "if (a <= b ? 2 : 3)". Or left shifting of signed integers in boolean context, like "for (a = 0; 1 << a; a++);". Likewise for all kinds of multiplications regardless of the data type. This warning is enabled by -Wall.
- -Wno-int-to-pointer-cast
- Suppress warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by default.
- -Wno-pointer-to-int-cast (C and Objective-C only)
- Suppress warnings from casts from a pointer to an integer type of a different size.
- -Winvalid-pch
- Warn if a precompiled header is found in the search path but cannot be used.
- -Wlong-long
- Warn if "long long" type is used. This is enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit the warning messages, use -Wno-long-long.
- -Wvariadic-macros
- Warn if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. This is enabled by either -Wpedantic or -Wtraditional. To inhibit the warning messages, use -Wno-variadic-macros.
- -Wvarargs
- Warn upon questionable usage of the macros used to handle variable arguments like "va_start". This is default. To inhibit the warning messages, use -Wno-varargs.
- -Wvector-operation-performance
- Warn if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the performance tuning. Vector operation can be implemented "piecewise", which means that the scalar operation is performed on every vector element; "in parallel", which means that the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and "as a single scalar", which means that vector fits into a scalar type.
- -Wno-virtual-move-assign
- Suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. This is dangerous because if the virtual base is reachable along more than one path, it is moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled.
- -Wvla
- Warn if a variable-length array is used in the code. -Wno-vla prevents the -Wpedantic warning of the variable-length array.
- -Wvla-larger-than=n
- If this option is used, the compiler will warn on uses of variable-length
arrays where the size is either unbounded, or bounded by an argument that
can be larger than n bytes. This is similar to how
-Walloca-larger-than=n works, but with variable-length
arrays.
Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for such arrays.
This warning is not enabled by -Wall, and is only active when -ftree-vrp is active (default for -O2 and above).
See also -Walloca-larger-than=n.
- -Wvolatile-register-var
- Warn if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. This warning is enabled by -Wall.
- -Wdisabled-optimization
- Warn if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers are unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time.
- -Wpointer-sign (C and Objective-C only)
- Warn for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign.
- -Wstack-protector
- This option is only active when -fstack-protector is active. It warns about functions that are not protected against stack smashing.
- -Woverlength-strings
- Warn about string constants that are longer than the "minimum
maximum" length specified in the C standard. Modern compilers
generally allow string constants that are much longer than the standard's
minimum limit, but very portable programs should avoid using longer
strings.
The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.
- -Wunsuffixed-float-constants (C and Objective-C only)
- Issue a warning for any floating constant that does not have a suffix. When used together with -Wsystem-headers it warns about such constants in system header files. This can be useful when preparing code to use with the "FLOAT_CONST_DECIMAL64" pragma from the decimal floating-point extension to C99.
- -Wno-designated-init (C and Objective-C only)
- Suppress warnings when a positional initializer is used to initialize a structure that has been marked with the "designated_init" attribute.
- -Whsa
- Issue a warning when HSAIL cannot be emitted for the compiled function or OpenMP construct.
Options for Debugging Your Program¶
To tell GCC to emit extra information for use by a debugger, in almost all cases you need only to add -g to your other options.
GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally be surprising: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values are already at hand; some statements may execute in different places because they have been moved out of loops. Nevertheless it is possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.
If you are not using some other optimization option, consider using -Og with -g. With no -O option at all, some compiler passes that collect information useful for debugging do not run at all, so that -Og may result in a better debugging experience.
- -g
- Produce debugging information in the operating system's native format
(stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging
information.
On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
- -ggdb
- Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.
- -gdwarf
- -gdwarf-version
- Produce debugging information in DWARF format (if that is supported). The
value of version may be either 2, 3, 4 or 5; the default version
for most targets is 4. DWARF Version 5 is only experimental.
Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit.
GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later. For historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm) retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF.
- -gstabs
- Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output that is not understood by DBX. On System V Release 4 systems this option requires the GNU assembler.
- -gstabs+
- Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.
- -gxcoff
- Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.
- -gxcoff+
- Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.
- -gvms
- Produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems.
- -glevel
- -ggdblevel
- -gstabslevel
- -gxcofflevel
- -gvmslevel
- Request debugging information and also use level to specify how
much information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0 negates -g.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no information about local variables.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3.
-gdwarf does not accept a concatenated debug level, to avoid confusion with -gdwarf-level. Instead use an additional -glevel option to change the debug level for DWARF.
- -feliminate-unused-debug-symbols
- Produce debugging information in stabs format (if that is supported), for only symbols that are actually used.
- -femit-class-debug-always
- Instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes because using this option increases the size of debugging information by as much as a factor of two.
- -fno-merge-debug-strings
- Direct the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default.
- -fdebug-prefix-map=old=new
- When compiling files residing in directory old, record debugging information describing them as if the files resided in directory new instead. This can be used to replace a build-time path with an install-time path in the debug info. It can also be used to change an absolute path to a relative path by using . for new. This can give more reproducible builds, which are location independent, but may require an extra command to tell GDB where to find the source files. See also -ffile-prefix-map.
- -fvar-tracking
- Run variable tracking pass. It computes where variables are stored at each
position in code. Better debugging information is then generated (if the
debugging information format supports this information).
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it.
- -fvar-tracking-assignments
- Annotate assignments to user variables early in the compilation and
attempt to carry the annotations over throughout the compilation all the
way to the end, in an attempt to improve debug information while
optimizing. Use of -gdwarf-4 is recommended along with it.
It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end. By default, this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled.
- -gsplit-dwarf
- Separate as much DWARF debugging information as possible into a separate output file with the extension .dwo. This option allows the build system to avoid linking files with debug information. To be useful, this option requires a debugger capable of reading .dwo files.
- -gpubnames
- Generate DWARF ".debug_pubnames" and ".debug_pubtypes" sections.
- -ggnu-pubnames
- Generate ".debug_pubnames" and ".debug_pubtypes" sections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version 7.
- -fdebug-types-section
- When using DWARF Version 4 or higher, type DIEs can be put into their own ".debug_types" section instead of making them part of the ".debug_info" section. It is more efficient to put them in a separate comdat sections since the linker can then remove duplicates. But not all DWARF consumers support ".debug_types" sections yet and on some objects ".debug_types" produces larger instead of smaller debugging information.
- -grecord-gcc-switches
- -gno-record-gcc-switches
- This switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. It is enabled by default. See also -frecord-gcc-switches for another way of storing compiler options into the object file.
- -gstrict-dwarf
- Disallow using extensions of later DWARF standard version than selected with -gdwarf-version. On most targets using non-conflicting DWARF extensions from later standard versions is allowed.
- -gno-strict-dwarf
- Allow using extensions of later DWARF standard version than selected with -gdwarf-version.
- -gas-loc-support
- Inform the compiler that the assembler supports
".loc" directives. It may then use them
for the assembler to generate DWARF2+ line number tables.
This is generally desirable, because assembler-generated line-number tables are a lot more compact than those the compiler can generate itself.
This option will be enabled by default if, at GCC configure time, the assembler was found to support such directives.
- -gno-as-loc-support
- Force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated.
- gas-locview-support
- Inform the compiler that the assembler supports
"view" assignment and reset assertion
checking in ".loc" directives.
This option will be enabled by default if, at GCC configure time, the assembler was found to support them.
- gno-as-locview-support
- Force GCC to assign view numbers internally, if -gvariable-location-views are explicitly requested.
- -gcolumn-info
- -gno-column-info
- Emit location column information into DWARF debugging information, rather than just file and line. This option is enabled by default.
- -gstatement-frontiers
- -gno-statement-frontiers
- This option causes GCC to create markers in the internal representation at the beginning of statements, and to keep them roughly in place throughout compilation, using them to guide the output of "is_stmt" markers in the line number table. This is enabled by default when compiling with optimization (-Os, -O, -O2, ...), and outputting DWARF 2 debug information at the normal level.
- -gvariable-location-views
- -gvariable-location-views=incompat5
- -gno-variable-location-views
- Augment variable location lists with progressive view numbers implied from
the line number table. This enables debug information consumers to inspect
state at certain points of the program, even if no instructions associated
with the corresponding source locations are present at that point. If the
assembler lacks support for view numbers in line number tables, this will
cause the compiler to emit the line number table, which generally makes
them somewhat less compact. The augmented line number tables and location
lists are fully backward-compatible, so they can be consumed by debug
information consumers that are not aware of these augmentations, but they
won't derive any benefit from them either.
This is enabled by default when outputting DWARF 2 debug information at the normal level, as long as there is assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When assembler support is not available, this may still be enabled, but it will force GCC to output internal line number tables, and if -ginternal-reset-location-views is not enabled, that will most certainly lead to silently mismatching location views.
There is a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, that can be enabled with -gvariable-location-views=incompat5. This option may be removed in the future, is only provided as a reference implementation of the proposed representation. Debug information consumers are not expected to support this extended format, and they would be rendered unable to decode location lists using it.
- -ginternal-reset-location-views
- -gno-internal-reset-location-views
- Attempt to determine location views that can be omitted from location view lists. This requires the compiler to have very accurate insn length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when using an assembler that does not support location view lists. The GNU assembler will flag any such error as a "view number mismatch". This is only enabled on ports that define a reliable estimation function.
- -ginline-points
- -gno-inline-points
- Generate extended debug information for inlined functions. Location view tracking markers are inserted at inlined entry points, so that address and view numbers can be computed and output in debug information. This can be enabled independently of location views, in which case the view numbers won't be output, but it can only be enabled along with statement frontiers, and it is only enabled by default if location views are enabled.
- -gz[=type]
- Produce compressed debug sections in DWARF format, if that is supported. If type is not given, the default type depends on the capabilities of the assembler and linker used. type may be one of none (don't compress debug sections), zlib (use zlib compression in ELF gABI format), or zlib-gnu (use zlib compression in traditional GNU format). If the linker doesn't support writing compressed debug sections, the option is rejected. Otherwise, if the assembler does not support them, -gz is silently ignored when producing object files.
- -femit-struct-debug-baseonly
- Emit debug information for struct-like types only when the base name of
the compilation source file matches the base name of file in which the
struct is defined.
This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger. See -femit-struct-debug-reduced for a less aggressive option. See -femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output.
- -femit-struct-debug-reduced
- Emit debug information for struct-like types only when the base name of
the compilation source file matches the base name of file in which the
type is defined, unless the struct is a template or defined in a system
header.
This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See -femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output.
- -femit-struct-debug-detailed[=spec-list]
- Specify the struct-like types for which the compiler generates debug
information. The intent is to reduce duplicate struct debug information
between different object files within the same program.
This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which serves for most needs.
A specification has the syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to structs that are used directly (dir:) or used indirectly (ind:). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is struct one direct; struct two * indirect;.
The optional second word limits the specification to ordinary structs (ord:) or generic structs (gen:). Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those structs for which the compiler should emit debug information. The values none and any have the normal meaning. The value base means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling foo.c, debug information is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.
You may need to experiment to determine the best settings for your application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF debug output.
- -fno-dwarf2-cfi-asm
- Emit DWARF unwind info as compiler generated ".eh_frame" section instead of using GAS ".cfi_*" directives.
- -fno-eliminate-unused-debug-types
- Normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes it is useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit, for example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space.
Options That Control Optimization¶
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section.
Most optimizations are only enabled if an -O level is set on the command line. Otherwise they are disabled, even if individual optimization flags are specified.
Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level.
- -O
- -O1
- Optimize. Optimizing compilation takes somewhat more time, and a lot more
memory for a large function.
With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments -fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdse -fforward-propagate -fguess-branch-probability -fif-conversion2 -fif-conversion -finline-functions-called-once -fipa-pure-const -fipa-profile -fipa-reference -fmerge-constants -fmove-loop-invariants -fomit-frame-pointer -freorder-blocks -fshrink-wrap -fshrink-wrap-separate -fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-phiprop -ftree-sink -ftree-slsr -ftree-sra -ftree-pta -ftree-ter -funit-at-a-time
- -O2
- Optimize even more. GCC performs nearly all supported optimizations that
do not involve a space-speed tradeoff. As compared to -O, this
option increases both compilation time and the performance of the
generated code.
-O2 turns on all optimization flags specified by -O. It also turns on the following optimization flags: -fthread-jumps -falign-functions -falign-jumps -falign-loops -falign-labels -fcaller-saves -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively -fexpensive-optimizations -fgcse -fgcse-lm -fhoist-adjacent-loads -finline-small-functions -findirect-inlining -fipa-cp -fipa-bit-cp -fipa-vrp -fipa-sra -fipa-icf -fisolate-erroneous-paths-dereference -flra-remat -foptimize-sibling-calls -foptimize-strlen -fpartial-inlining -fpeephole2 -freorder-blocks-algorithm=stc -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -fsched-interblock -fsched-spec -fschedule-insns -fschedule-insns2 -fstore-merging -fstrict-aliasing -ftree-builtin-call-dce -ftree-switch-conversion -ftree-tail-merge -fcode-hoisting -ftree-pre -ftree-vrp -fipa-ra
Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.
- -O3
- Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the following optimization flags: -finline-functions -funswitch-loops -fpredictive-commoning -fgcse-after-reload -ftree-loop-vectorize -ftree-loop-distribution -ftree-loop-distribute-patterns -floop-interchange -floop-unroll-and-jam -fsplit-paths -ftree-slp-vectorize -fvect-cost-model -ftree-partial-pre -fpeel-loops -fipa-cp-clone
- -O0
- Reduce compilation time and make debugging produce the expected results. This is the default.
- -Os
- Optimize for size. -Os enables all -O2 optimizations that do
not typically increase code size.
-Os disables the following optimization flags: -falign-functions -falign-jumps -falign-loops -falign-labels -fprefetch-loop-arrays
It also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size.
- -Ofast
- Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math and the Fortran-specific -fstack-arrays, unless -fmax-stack-var-size is specified, and -fno-protect-parens.
- -Og
- Optimize debugging experience. -Og enables optimizations that do not interfere with debugging. It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience.
If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one you typically use. You can figure out the other form by either removing no- or adding it.
The following options control specific optimizations. They are either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired.
- -fno-defer-pop
- Always pop the arguments to each function call as soon as that function
returns. For machines that must pop arguments after a function call, the
compiler normally lets arguments accumulate on the stack for several
function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
- -fforward-propagate
- Perform a forward propagation pass on RTL. The pass tries to combine two
instructions and checks if the result can be simplified. If loop unrolling
is active, two passes are performed and the second is scheduled after loop
unrolling.
This option is enabled by default at optimization levels -O, -O2, -O3, -Os.
- -ffp-contract=style
- -ffp-contract=off disables floating-point expression contraction.
-ffp-contract=fast enables floating-point expression contraction
such as forming of fused multiply-add operations if the target has native
support for them. -ffp-contract=on enables floating-point
expression contraction if allowed by the language standard. This is
currently not implemented and treated equal to -ffp-contract=off.
The default is -ffp-contract=fast.
- -fomit-frame-pointer
- Omit the frame pointer in functions that don't need one. This avoids the
instructions to save, set up and restore the frame pointer; on many
targets it also makes an extra register available.
On some targets this flag has no effect because the standard calling sequence always uses a frame pointer, so it cannot be omitted.
Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions.
Enabled by default at -O and higher.
- -foptimize-sibling-calls
- Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
- -foptimize-strlen
- Optimize various standard C string functions (e.g.
"strlen",
"strchr" or
"strcpy") and their
"_FORTIFY_SOURCE" counterparts into
faster alternatives.
Enabled at levels -O2, -O3.
- -fno-inline
- Do not expand any functions inline apart from those marked with the
"always_inline" attribute. This is the
default when not optimizing.
Single functions can be exempted from inlining by marking them with the "noinline" attribute.
- -finline-small-functions
- Integrate functions into their callers when their body is smaller than
expected function call code (so overall size of program gets smaller). The
compiler heuristically decides which functions are simple enough to be
worth integrating in this way. This inlining applies to all functions,
even those not declared inline.
Enabled at levels -O2, -O3, -Os.
- -findirect-inlining
- Inline also indirect calls that are discovered to be known at compile time
thanks to previous inlining. This option has any effect only when inlining
itself is turned on by the -finline-functions or
-finline-small-functions options.
Enabled at levels -O3, -Os. Also enabled by -fprofile-use and -fauto-profile.
- -finline-functions
- Consider all functions for inlining, even if they are not declared inline.
The compiler heuristically decides which functions are worth integrating
in this way.
If all calls to a given function are integrated, and the function is declared "static", then the function is normally not output as assembler code in its own right.
Enabled at levels -O2, -O3, -Os.
- -finline-functions-called-once
- Consider all "static" functions called
once for inlining into their caller even if they are not marked
"inline". If a call to a given function
is integrated, then the function is not output as assembler code in its
own right.
Enabled at levels -O1, -O2, -O3 and -Os.
- -fearly-inlining
- Inline functions marked by
"always_inline" and functions whose body
seems smaller than the function call overhead early before doing
-fprofile-generate instrumentation and real inlining pass. Doing so
makes profiling significantly cheaper and usually inlining faster on
programs having large chains of nested wrapper functions.
Enabled by default.
- -fipa-sra
- Perform interprocedural scalar replacement of aggregates, removal of
unused parameters and replacement of parameters passed by reference by
parameters passed by value.
Enabled at levels -O2, -O3 and -Os.
- -finline-limit=n
- By default, GCC limits the size of functions that can be inlined. This
flag allows coarse control of this limit. n is the size of
functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which may be specified individually by using --param name=value. The -finline-limit=n option sets some of these parameters as follows:
- max-inline-insns-single
- is set to n/2.
- max-inline-insns-auto
- is set to n/2.
See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters.
Note: there may be no value to -finline-limit that results in default behavior.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another.
- -fno-keep-inline-dllexport
- This is a more fine-grained version of -fkeep-inline-functions, which applies only to functions that are declared using the "dllexport" attribute or declspec.
- -fkeep-inline-functions
- In C, emit "static" functions that are declared "inline" into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the "extern inline" extension in GNU C90. In C++, emit any and all inline functions into the object file.
- -fkeep-static-functions
- Emit "static" functions into the object file, even if the function is never used.
- -fkeep-static-consts
- Emit variables declared "static const"
when optimization isn't turned on, even if the variables aren't
referenced.
GCC enables this option by default. If you want to force the compiler to check if a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.
- -fmerge-constants
- Attempt to merge identical constants (string constants and floating-point
constants) across compilation units.
This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
- -fmerge-all-constants
- Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior.
- -fmodulo-sched
- Perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations.
- -fmodulo-sched-allow-regmoves
- Perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with -fmodulo-sched enabled.
- -fno-branch-count-reg
- Avoid running a pass scanning for opportunities to use "decrement and
branch" instructions on a count register instead of generating
sequences of instructions that decrement a register, compare it against
zero, and then branch based upon the result. This option is only
meaningful on architectures that support such instructions, which include
x86, PowerPC, IA-64 and S/390. Note that the -fno-branch-count-reg
option doesn't remove the decrement and branch instructions from the
generated instruction stream introduced by other optimization passes.
Enabled by default at -O1 and higher.
The default is -fbranch-count-reg.
- -fno-function-cse
- Do not put function addresses in registers; make each instruction that
calls a constant function contain the function's address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.
The default is -ffunction-cse
- -fno-zero-initialized-in-bss
- If the target supports a BSS section, GCC by default puts variables that
are initialized to zero into BSS. This can save space in the resulting
code.
This option turns off this behavior because some programs explicitly rely on variables going to the data section---e.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.
The default is -fzero-initialized-in-bss.
- -fthread-jumps
- Perform optimizations that check to see if a jump branches to a location
where another comparison subsumed by the first is found. If so, the first
branch is redirected to either the destination of the second branch or a
point immediately following it, depending on whether the condition is
known to be true or false.
Enabled at levels -O2, -O3, -Os.
- -fsplit-wide-types
- When using a type that occupies multiple registers, such as
"long long" on a
32-bit system, split the registers apart and allocate them independently.
This normally generates better code for those types, but may make
debugging more difficult.
Enabled at levels -O, -O2, -O3, -Os.
- -fcse-follow-jumps
- In common subexpression elimination (CSE), scan through jump instructions
when the target of the jump is not reached by any other path. For example,
when CSE encounters an "if" statement
with an "else" clause, CSE follows the
jump when the condition tested is false.
Enabled at levels -O2, -O3, -Os.
- -fcse-skip-blocks
- This is similar to -fcse-follow-jumps, but causes CSE to follow
jumps that conditionally skip over blocks. When CSE encounters a simple
"if" statement with no else clause,
-fcse-skip-blocks causes CSE to follow the jump around the body of
the "if".
Enabled at levels -O2, -O3, -Os.
- -frerun-cse-after-loop
- Re-run common subexpression elimination after loop optimizations are
performed.
Enabled at levels -O2, -O3, -Os.
- -fgcse
- Perform a global common subexpression elimination pass. This pass also
performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
- -fgcse-lm
- When -fgcse-lm is enabled, global common subexpression elimination
attempts to move loads that are only killed by stores into themselves.
This allows a loop containing a load/store sequence to be changed to a
load outside the loop, and a copy/store within the loop.
Enabled by default when -fgcse is enabled.
- -fgcse-sm
- When -fgcse-sm is enabled, a store motion pass is run after global
common subexpression elimination. This pass attempts to move stores out of
loops. When used in conjunction with -fgcse-lm, loops containing a
load/store sequence can be changed to a load before the loop and a store
after the loop.
Not enabled at any optimization level.
- -fgcse-las
- When -fgcse-las is enabled, the global common subexpression
elimination pass eliminates redundant loads that come after stores to the
same memory location (both partial and full redundancies).
Not enabled at any optimization level.
- -fgcse-after-reload
- When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The purpose of this pass is to clean up redundant spilling.
- -faggressive-loop-optimizations
- This option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. This assumes that loop code does not invoke undefined behavior by for example causing signed integer overflows or out-of-bound array accesses. The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations. This option is enabled by default.
- -funconstrained-commons
- This option tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden with longer trailing arrays. This prevents certain optimizations that depend on knowing the array bounds.
- -fcrossjumping
- Perform cross-jumping transformation. This transformation unifies
equivalent code and saves code size. The resulting code may or may not
perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
- -fauto-inc-dec
- Combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that do not have instructions to support this. Enabled by default at -O and higher on architectures that support this.
- -fdce
- Perform dead code elimination (DCE) on RTL. Enabled by default at -O and higher.
- -fdse
- Perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher.
- -fif-conversion
- Attempt to transform conditional jumps into branch-less equivalents. This
includes use of conditional moves, min, max, set flags and abs
instructions, and some tricks doable by standard arithmetics. The use of
conditional execution on chips where it is available is controlled by
-fif-conversion2.
Enabled at levels -O, -O2, -O3, -Os.
- -fif-conversion2
- Use conditional execution (where available) to transform conditional jumps
into branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
- -fdeclone-ctor-dtor
- The C++ ABI requires multiple entry points for constructors and
destructors: one for a base subobject, one for a complete object, and one
for a virtual destructor that calls operator delete afterwards. For a
hierarchy with virtual bases, the base and complete variants are clones,
which means two copies of the function. With this option, the base and
complete variants are changed to be thunks that call a common
implementation.
Enabled by -Os.
- -fdelete-null-pointer-checks
- Assume that programs cannot safely dereference null pointers, and that no
code or data element resides at address zero. This option enables simple
constant folding optimizations at all optimization levels. In addition,
other optimization passes in GCC use this flag to control global dataflow
analyses that eliminate useless checks for null pointers; these assume
that a memory access to address zero always results in a trap, so that if
a pointer is checked after it has already been dereferenced, it cannot be
null.
Note however that in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.
This option is enabled by default on most targets. On Nios II ELF, it defaults to off. On AVR, CR16, and MSP430, this option is completely disabled.
Passes that use the dataflow information are enabled independently at different optimization levels.
- -fdevirtualize
- Attempt to convert calls to virtual functions to direct calls. This is done both within a procedure and interprocedurally as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.
- -fdevirtualize-speculatively
- Attempt to convert calls to virtual functions to speculative direct calls. Based on the analysis of the type inheritance graph, determine for a given call the set of likely targets. If the set is small, preferably of size 1, change the call into a conditional deciding between direct and indirect calls. The speculative calls enable more optimizations, such as inlining. When they seem useless after further optimization, they are converted back into original form.
- -fdevirtualize-at-ltrans
- Stream extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. This option enables more devirtualization but significantly increases the size of streamed data. For this reason it is disabled by default.
- -fexpensive-optimizations
- Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
- -free
- Attempt to remove redundant extension instructions. This is especially
helpful for the x86-64 architecture, which implicitly zero-extends in
64-bit registers after writing to their lower 32-bit half.
Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os.
- -fno-lifetime-dse
- In C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed. Normally dead store elimination will take advantage of this; if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization. To preserve stores before the constructor starts (e.g. because your operator new clears the object storage) but still treat the object as dead after the destructor you, can use -flifetime-dse=1. The default behavior can be explicitly selected with -flifetime-dse=2. -flifetime-dse=0 is equivalent to -fno-lifetime-dse.
- -flive-range-shrinkage
- Attempt to decrease register pressure through register live range shrinkage. This is helpful for fast processors with small or moderate size register sets.
- -fira-algorithm=algorithm
- Use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it is the default because it generates better code.
- -fira-region=region
- Use specified regions for the integrated register allocator. The region argument should be one of the following:
- all
- Use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.
- mixed
- Use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O, -O2, ...).
- one
- Use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0.
- -fira-hoist-pressure
- Use IRA to evaluate register pressure in the code hoisting pass for
decisions to hoist expressions. This option usually results in smaller
code, but it can slow the compiler down.
This option is enabled at level -Os for all targets.
- -fira-loop-pressure
- Use IRA to evaluate register pressure in loops for decisions to move loop
invariants. This option usually results in generation of faster and
smaller code on machines with large register files (>= 32 registers),
but it can slow the compiler down.
This option is enabled at level -O3 for some targets.
- -fno-ira-share-save-slots
- Disable sharing of stack slots used for saving call-used hard registers living through a call. Each hard register gets a separate stack slot, and as a result function stack frames are larger.
- -fno-ira-share-spill-slots
- Disable sharing of stack slots allocated for pseudo-registers. Each pseudo-register that does not get a hard register gets a separate stack slot, and as a result function stack frames are larger.
- -flra-remat
- Enable CFG-sensitive rematerialization in LRA. Instead of loading values
of spilled pseudos, LRA tries to rematerialize (recalculate) values if it
is profitable.
Enabled at levels -O2, -O3, -Os.
- -fdelayed-branch
- If supported for the target machine, attempt to reorder instructions to
exploit instruction slots available after delayed branch instructions.
Enabled at levels -O, -O2, -O3, -Os.
- -fschedule-insns
- If supported for the target machine, attempt to reorder instructions to
eliminate execution stalls due to required data being unavailable. This
helps machines that have slow floating point or memory load instructions
by allowing other instructions to be issued until the result of the load
or floating-point instruction is required.
Enabled at levels -O2, -O3.
- -fschedule-insns2
- Similar to -fschedule-insns, but requests an additional pass of
instruction scheduling after register allocation has been done. This is
especially useful on machines with a relatively small number of registers
and where memory load instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
- -fno-sched-interblock
- Don't schedule instructions across basic blocks. This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
- -fno-sched-spec
- Don't allow speculative motion of non-load instructions. This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
- -fsched-pressure
- Enable register pressure sensitive insn scheduling before register allocation. This only makes sense when scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or higher. Usage of this option can improve the generated code and decrease its size by preventing register pressure increase above the number of available hard registers and subsequent spills in register allocation.
- -fsched-spec-load
- Allow speculative motion of some load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
- -fsched-spec-load-dangerous
- Allow speculative motion of more load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
- -fsched-stalled-insns
- -fsched-stalled-insns=n
- Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass. -fno-sched-stalled-insns means that no insns are moved prematurely, -fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved prematurely. -fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1.
- -fsched-stalled-insns-dep
- -fsched-stalled-insns-dep=n
- Define how many insn groups (cycles) are examined for a dependency on a stalled insn that is a candidate for premature removal from the queue of stalled insns. This has an effect only during the second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1.
- -fsched2-use-superblocks
- When scheduling after register allocation, use superblock scheduling. This
allows motion across basic block boundaries, resulting in faster
schedules. This option is experimental, as not all machine descriptions
used by GCC model the CPU closely enough to avoid unreliable results from
the algorithm.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
- -fsched-group-heuristic
- Enable the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a schedule group. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
- -fsched-critical-path-heuristic
- Enable the critical-path heuristic in the scheduler. This heuristic favors instructions on the critical path. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
- -fsched-spec-insn-heuristic
- Enable the speculative instruction heuristic in the scheduler. This heuristic favors speculative instructions with greater dependency weakness. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
- -fsched-rank-heuristic
- Enable the rank heuristic in the scheduler. This heuristic favors the instruction belonging to a basic block with greater size or frequency. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
- -fsched-last-insn-heuristic
- Enable the last-instruction heuristic in the scheduler. This heuristic favors the instruction that is less dependent on the last instruction scheduled. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
- -fsched-dep-count-heuristic
- Enable the dependent-count heuristic in the scheduler. This heuristic favors the instruction that has more instructions depending on it. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
- -freschedule-modulo-scheduled-loops
- Modulo scheduling is performed before traditional scheduling. If a loop is modulo scheduled, later scheduling passes may change its schedule. Use this option to control that behavior.
- -fselective-scheduling
- Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass.
- -fselective-scheduling2
- Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass.
- -fsel-sched-pipelining
- Enable software pipelining of innermost loops during selective scheduling. This option has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned on.
- -fsel-sched-pipelining-outer-loops
- When pipelining loops during selective scheduling, also pipeline outer loops. This option has no effect unless -fsel-sched-pipelining is turned on.
- -fsemantic-interposition
- Some object formats, like ELF, allow interposing of symbols by the dynamic linker. This means that for symbols exported from the DSO, the compiler cannot perform interprocedural propagation, inlining and other optimizations in anticipation that the function or variable in question may change. While this feature is useful, for example, to rewrite memory allocation functions by a debugging implementation, it is expensive in the terms of code quality. With -fno-semantic-interposition the compiler assumes that if interposition happens for functions the overwriting function will have precisely the same semantics (and side effects). Similarly if interposition happens for variables, the constructor of the variable will be the same. The flag has no effect for functions explicitly declared inline (where it is never allowed for interposition to change semantics) and for symbols explicitly declared weak.
- -fshrink-wrap
- Emit function prologues only before parts of the function that need it, rather than at the top of the function. This flag is enabled by default at -O and higher.
- -fshrink-wrap-separate
- Shrink-wrap separate parts of the prologue and epilogue separately, so that those parts are only executed when needed. This option is on by default, but has no effect unless -fshrink-wrap is also turned on and the target supports this.
- -fcaller-saves
- Enable allocation of values to registers that are clobbered by function
calls, by emitting extra instructions to save and restore the registers
around such calls. Such allocation is done only when it seems to result in
better code.
This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
- -fcombine-stack-adjustments
- Tracks stack adjustments (pushes and pops) and stack memory references and
then tries to find ways to combine them.
Enabled by default at -O1 and higher.
- -fipa-ra
- Use caller save registers for allocation if those registers are not used
by any called function. In that case it is not necessary to save and
restore them around calls. This is only possible if called functions are
part of same compilation unit as current function and they are compiled
before it.
Enabled at levels -O2, -O3, -Os, however the option is disabled if generated code will be instrumented for profiling (-p, or -pg) or if callee's register usage cannot be known exactly (this happens on targets that do not expose prologues and epilogues in RTL).
- -fconserve-stack
- Attempt to minimize stack usage. The compiler attempts to use less stack space, even if that makes the program slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400.
- -ftree-reassoc
- Perform reassociation on trees. This flag is enabled by default at -O and higher.
- -fcode-hoisting
- Perform code hoisting. Code hoisting tries to move the evaluation of expressions executed on all paths to the function exit as early as possible. This is especially useful as a code size optimization, but it often helps for code speed as well. This flag is enabled by default at -O2 and higher.
- -ftree-pre
- Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3.
- -ftree-partial-pre
- Make partial redundancy elimination (PRE) more aggressive. This flag is enabled by default at -O3.
- -ftree-forwprop
- Perform forward propagation on trees. This flag is enabled by default at -O and higher.
- -ftree-fre
- Perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and higher.
- -ftree-phiprop
- Perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O and higher.
- -fhoist-adjacent-loads
- Speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. This flag is enabled by default at -O2 and higher.
- -ftree-copy-prop
- Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at -O and higher.
- -fipa-pure-const
- Discover which functions are pure or constant. Enabled by default at -O and higher.
- -fipa-reference
- Discover which static variables do not escape the compilation unit. Enabled by default at -O and higher.
- -fipa-pta
- Perform interprocedural pointer analysis and interprocedural modification and reference analysis. This option can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization level.
- -fipa-profile
- Perform interprocedural profile propagation. The functions called only from cold functions are marked as cold. Also functions executed once (such as "cold", "noreturn", static constructors or destructors) are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at -O and higher.
- -fipa-cp
- Perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions. This flag is enabled by default at -O2, -Os and -O3.
- -fipa-cp-clone
- Perform function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation performs function cloning when externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size (see --param ipcp-unit-growth=value). This flag is enabled by default at -O3.
- -fipa-bit-cp
- When enabled, perform interprocedural bitwise constant propagation. This flag is enabled by default at -O2. It requires that -fipa-cp is enabled.
- -fipa-vrp
- When enabled, perform interprocedural propagation of value ranges. This flag is enabled by default at -O2. It requires that -fipa-cp is enabled.
- -fipa-icf
- Perform Identical Code Folding for functions and read-only variables. The
optimization reduces code size and may disturb unwind stacks by replacing
a function by equivalent one with a different name. The optimization works
more effectively with link-time optimization enabled.
Nevertheless the behavior is similar to Gold Linker ICF optimization, GCC ICF works on different levels and thus the optimizations are not same - there are equivalences that are found only by GCC and equivalences found only by Gold.
This flag is enabled by default at -O2 and -Os.
- -flive-patching=level
- Control GCC's optimizations to produce output suitable for live-patching.
If the compiler's optimization uses a function's body or information extracted from its body to optimize/change another function, the latter is called an impacted function of the former. If a function is patched, its impacted functions should be patched too.
The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is impacted when inlining a function into its caller, cloning a function and changing its caller to call this new clone, or extracting a function's pureness/constness information to optimize its direct or indirect callers, etc.
Usually, the more IPA optimizations enabled, the larger the number of impacted functions for each function. In order to control the number of impacted functions and more easily compute the list of impacted function, IPA optimizations can be partially enabled at two different levels.
The level argument should be one of the following:
- inline-clone
- Only enable inlining and cloning optimizations, which includes inlining,
cloning, interprocedural scalar replacement of aggregates and partial
inlining. As a result, when patching a function, all its callers and its
clones' callers are impacted, therefore need to be patched as well.
-flive-patching=inline-clone disables the following optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment
- inline-only-static
- Only enable inlining of static functions. As a result, when patching a
static function, all its callers are impacted and so need to be patched as
well.
In addition to all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static disables the following additional optimization flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp
When -flive-patching is specified without any value, the default value is inline-clone.
This flag is disabled by default.
Note that -flive-patching is not supported with link-time optimization (-flto).
- -fisolate-erroneous-paths-dereference
- Detect paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap. This flag is enabled by default at -O2 and higher and depends on -fdelete-null-pointer-checks also being enabled.
- -fisolate-erroneous-paths-attribute
- Detect paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by a "returns_nonnull" or "nonnull" attribute. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap. This is not currently enabled, but may be enabled by -O2 in the future.
- -ftree-sink
- Perform forward store motion on trees. This flag is enabled by default at -O and higher.
- -ftree-bit-ccp
- Perform sparse conditional bit constant propagation on trees and propagate pointer alignment information. This pass only operates on local scalar variables and is enabled by default at -O and higher. It requires that -ftree-ccp is enabled.
- -ftree-ccp
- Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at -O and higher.
- -fssa-backprop
- Propagate information about uses of a value up the definition chain in order to simplify the definitions. For example, this pass strips sign operations if the sign of a value never matters. The flag is enabled by default at -O and higher.
- -fssa-phiopt
- Perform pattern matching on SSA PHI nodes to optimize conditional code. This pass is enabled by default at -O and higher.
- -ftree-switch-conversion
- Perform conversion of simple initializations in a switch to initializations from a scalar array. This flag is enabled by default at -O2 and higher.
- -ftree-tail-merge
- Look for identical code sequences. When found, replace one with a jump to the other. This optimization is known as tail merging or cross jumping. This flag is enabled by default at -O2 and higher. The compilation time in this pass can be limited using max-tail-merge-comparisons parameter and max-tail-merge-iterations parameter.
- -ftree-dce
- Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher.
- -ftree-builtin-call-dce
- Perform conditional dead code elimination (DCE) for calls to built-in functions that may set "errno" but are otherwise free of side effects. This flag is enabled by default at -O2 and higher if -Os is not also specified.
- -ftree-dominator-opts
- Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal. This also performs jump threading (to reduce jumps to jumps). This flag is enabled by default at -O and higher.
- -ftree-dse
- Perform dead store elimination (DSE) on trees. A dead store is a store into a memory location that is later overwritten by another store without any intervening loads. In this case the earlier store can be deleted. This flag is enabled by default at -O and higher.
- -ftree-ch
- Perform loop header copying on trees. This is beneficial since it increases effectiveness of code motion optimizations. It also saves one jump. This flag is enabled by default at -O and higher. It is not enabled for -Os, since it usually increases code size.
- -ftree-loop-optimize
- Perform loop optimizations on trees. This flag is enabled by default at -O and higher.
- -ftree-loop-linear
- -floop-strip-mine
- -floop-block
- Perform loop nest optimizations. Same as -floop-nest-optimize. To use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure.
- -fgraphite-identity
- Enable the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple. Using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops.
- -floop-nest-optimize
- Enable the isl based loop nest optimizer. This is a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism. This option is experimental.
- -floop-parallelize-all
- Use the Graphite data dependence analysis to identify loops that can be parallelized. Parallelize all the loops that can be analyzed to not contain loop carried dependences without checking that it is profitable to parallelize the loops.
- -ftree-coalesce-vars
- While transforming the program out of the SSA representation, attempt to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries. This may severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments. In the negated form, this flag prevents SSA coalescing of user variables. This option is enabled by default if optimization is enabled, and it does very little otherwise.
- -ftree-loop-if-convert
- Attempt to transform conditional jumps in the innermost loops to branch-less equivalents. The intent is to remove control-flow from the innermost loops in order to improve the ability of the vectorization pass to handle these loops. This is enabled by default if vectorization is enabled.
- -ftree-loop-distribution
- Perform loop distribution. This flag can improve cache performance on big
loop bodies and allow further loop optimizations, like parallelization or
vectorization, to take place. For example, the loop
DO I = 1, N A(I) = B(I) + C D(I) = E(I) * F ENDDO
is transformed to
DO I = 1, N A(I) = B(I) + C ENDDO DO I = 1, N D(I) = E(I) * F ENDDO
- -ftree-loop-distribute-patterns
- Perform loop distribution of patterns that can be code generated with
calls to a library. This flag is enabled by default at -O3.
This pass distributes the initialization loops and generates a call to memset zero. For example, the loop
DO I = 1, N A(I) = 0 B(I) = A(I) + I ENDDO
is transformed to
DO I = 1, N A(I) = 0 ENDDO DO I = 1, N B(I) = A(I) + I ENDDO
and the initialization loop is transformed into a call to memset zero.
- -floop-interchange
- Perform loop interchange outside of graphite. This flag can improve cache
performance on loop nest and allow further loop optimizations, like
vectorization, to take place. For example, the loop
for (int i = 0; i < N; i++) for (int j = 0; j < N; j++) for (int k = 0; k < N; k++) c[i][j] = c[i][j] + a[i][k]*b[k][j];
is transformed to
for (int i = 0; i < N; i++) for (int k = 0; k < N; k++) for (int j = 0; j < N; j++) c[i][j] = c[i][j] + a[i][k]*b[k][j];
This flag is enabled by default at -O3.
- -floop-unroll-and-jam
- Apply unroll and jam transformations on feasible loops. In a loop nest this unrolls the outer loop by some factor and fuses the resulting multiple inner loops. This flag is enabled by default at -O3.
- -ftree-loop-im
- Perform loop invariant motion on trees. This pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion.
- -ftree-loop-ivcanon
- Create a canonical counter for number of iterations in loops for which determining number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling.
- -fivopts
- Perform induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees.
- -ftree-parallelize-loops=n
- Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies -pthread, and thus is only supported on targets that have support for -pthread.
- -ftree-pta
- Perform function-local points-to analysis on trees. This flag is enabled by default at -O and higher.
- -ftree-sra
- Perform scalar replacement of aggregates. This pass replaces structure references with scalars to prevent committing structures to memory too early. This flag is enabled by default at -O and higher.
- -fstore-merging
- Perform merging of narrow stores to consecutive memory addresses. This pass merges contiguous stores of immediate values narrower than a word into fewer wider stores to reduce the number of instructions. This is enabled by default at -O2 and higher as well as -Os.
- -ftree-ter
- Perform temporary expression replacement during the SSA->normal phase. Single use/single def temporaries are replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on resulting in better RTL generation. This is enabled by default at -O and higher.
- -ftree-slsr
- Perform straight-line strength reduction on trees. This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. This is enabled by default at -O and higher.
- -ftree-vectorize
- Perform vectorization on trees. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly specified.
- -ftree-loop-vectorize
- Perform loop vectorization on trees. This flag is enabled by default at -O3 and when -ftree-vectorize is enabled.
- -ftree-slp-vectorize
- Perform basic block vectorization on trees. This flag is enabled by default at -O3 and when -ftree-vectorize is enabled.
- -fvect-cost-model=model
- Alter the cost model used for vectorization. The model argument should be one of unlimited, dynamic or cheap. With the unlimited model the vectorized code-path is assumed to be profitable while with the dynamic model a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop. The cheap model disables vectorization of loops where doing so would be cost prohibitive for example due to required runtime checks for data dependence or alignment but otherwise is equal to the dynamic model. The default cost model depends on other optimization flags and is either dynamic or cheap.
- -fsimd-cost-model=model
- Alter the cost model used for vectorization of loops marked with the OpenMP simd directive. The model argument should be one of unlimited, dynamic, cheap. All values of model have the same meaning as described in -fvect-cost-model and by default a cost model defined with -fvect-cost-model is used.
- -ftree-vrp
- Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks. This is enabled by default at -O2 and higher. Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled.
- -fsplit-paths
- Split paths leading to loop backedges. This can improve dead code elimination and common subexpression elimination. This is enabled by default at -O2 and above.
- -fsplit-ivs-in-unroller
- Enables expression of values of induction variables in later iterations of
the unrolled loop using the value in the first iteration. This breaks long
dependency chains, thus improving efficiency of the scheduling passes.
A combination of -fweb and CSE is often sufficient to obtain the same effect. However, that is not reliable in cases where the loop body is more complicated than a single basic block. It also does not work at all on some architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
- -fvariable-expansion-in-unroller
- With this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code.
- -fpartial-inlining
- Inline parts of functions. This option has any effect only when inlining
itself is turned on by the -finline-functions or
-finline-small-functions options.
Enabled at levels -O2, -O3, -Os.
- -fpredictive-commoning
- Perform predictive commoning optimization, i.e., reusing computations
(especially memory loads and stores) performed in previous iterations of
loops.
This option is enabled at level -O3.
- -fprefetch-loop-arrays
- If supported by the target machine, generate instructions to prefetch
memory to improve the performance of loops that access large arrays.
This option may generate better or worse code; results are highly dependent on the structure of loops within the source code.
Disabled at level -Os.
- -fno-printf-return-value
- Do not substitute constants for known return value of formatted output
functions such as "sprintf",
"snprintf",
"vsprintf", and
"vsnprintf" (but not
"printf" of
"fprintf"). This transformation allows
GCC to optimize or even eliminate branches based on the known return value
of these functions called with arguments that are either constant, or
whose values are known to be in a range that makes determining the exact
return value possible. For example, when -fprintf-return-value is
in effect, both the branch and the body of the
"if" statement (but not the call to
"snprint") can be optimized away when
"i" is a 32-bit or smaller integer
because the return value is guaranteed to be at most 8.
char buf[9]; if (snprintf (buf, "%08x", i) >= sizeof buf) ...
The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. The -fprintf-return-value option is enabled by default.
- -fno-peephole
- -fno-peephole2
- Disable any machine-specific peephole optimizations. The difference
between -fno-peephole and -fno-peephole2 is in how they are
implemented in the compiler; some targets use one, some use the other, a
few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
- -fno-guess-branch-probability
- Do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by "__builtin_expect", then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the "__builtin_expect" info into account. The interactions between the heuristics and "__builtin_expect" can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of "__builtin_expect" are easier to understand.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
- -freorder-blocks
- Reorder basic blocks in the compiled function in order to reduce number of
taken branches and improve code locality.
Enabled at levels -O, -O2, -O3, -Os.
- -freorder-blocks-algorithm=algorithm
- Use the specified algorithm for basic block reordering. The
algorithm argument can be simple, which does not increase
code size (except sometimes due to secondary effects like alignment), or
stc, the "software trace cache" algorithm, which tries to
put all often executed code together, minimizing the number of branches
executed by making extra copies of code.
The default is simple at levels -O, -Os, and stc at levels -O2, -O3.
- -freorder-blocks-and-partition
- In addition to reordering basic blocks in the compiled function, in order
to reduce number of taken branches, partitions hot and cold basic blocks
into separate sections of the assembly and .o files, to improve
paging and cache locality performance.
This optimization is automatically turned off in the presence of exception handling or unwind tables (on targets using setjump/longjump or target specific scheme), for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections. When -fsplit-stack is used this option is not enabled by default (to avoid linker errors), but may be enabled explicitly (if using a working linker).
Enabled for x86 at levels -O2, -O3, -Os.
- -freorder-functions
- Reorder functions in the object file in order to improve code locality.
This is implemented by using special subsections
".text.hot" for most frequently executed
functions and ".text.unlikely" for
unlikely executed functions. Reordering is done by the linker so object
file format must support named sections and linker must place them in a
reasonable way.
Also profile feedback must be available to make this option effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
- -fstrict-aliasing
- Allow the compiler to assume the strictest aliasing rules applicable to
the language being compiled. For C (and C++), this activates optimizations
based on the type of expressions. In particular, an object of one type is
assumed never to reside at the same address as an object of a different
type, unless the types are almost the same. For example, an
"unsigned int" can alias an
"int", but not a
"void*" or a
"double". A character type may alias any
other type.
Pay special attention to code like this:
union a_union { int i; double d; }; int f() { union a_union t; t.d = 3.0; return t.i; }
The practice of reading from a different union member than the one most recently written to (called "type-punning") is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So, the code above works as expected. However, this code might not:
int f() { union a_union t; int* ip; t.d = 3.0; ip = &t.i; return *ip; }
Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:
int f() { double d = 3.0; return ((union a_union *) &d)->i; }
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
- -falign-functions
- -falign-functions=n
- Align the start of functions to the next power-of-two greater than
n, skipping up to n bytes. For instance,
-falign-functions=32 aligns functions to the next 32-byte boundary,
but -falign-functions=24 aligns to the next 32-byte boundary only
if this can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
- -flimit-function-alignment
- If this option is enabled, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by -falign-functions, but not to skip more bytes than the size of the function.
- -falign-labels
- -falign-labels=n
- Align all branch targets to a power-of-two boundary, skipping up to
n bytes like -falign-functions. This option can easily make
code slower, because it must insert dummy operations for when the branch
target is reached in the usual flow of the code.
-fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
- -falign-loops
- -falign-loops=n
- Align loops to a power-of-two boundary, skipping up to n bytes like
-falign-functions. If the loops are executed many times, this makes
up for any execution of the dummy operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not aligned. The maximum allowed n option value is 65536.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
- -falign-jumps
- -falign-jumps=n
- Align branch targets to a power-of-two boundary, for branch targets where
the targets can only be reached by jumping, skipping up to n bytes
like -falign-functions. In this case, no dummy operations need be
executed.
-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
- -funit-at-a-time
- This option is left for compatibility reasons. -funit-at-a-time has
no effect, while -fno-unit-at-a-time implies
-fno-toplevel-reorder and -fno-section-anchors.
Enabled by default.
- -fno-toplevel-reorder
- Do not reorder top-level functions, variables, and
"asm" statements. Output them in the
same order that they appear in the input file. When this option is used,
unreferenced static variables are not removed. This option is intended to
support existing code that relies on a particular ordering. For new code,
it is better to use attributes when possible.
Enabled at level -O0. When disabled explicitly, it also implies -fno-section-anchors, which is otherwise enabled at -O0 on some targets.
- -fweb
- Constructs webs as commonly used for register allocation purposes and
assign each web individual pseudo register. This allows the register
allocation pass to operate on pseudos directly, but also strengthens
several other optimization passes, such as CSE, loop optimizer and trivial
dead code remover. It can, however, make debugging impossible, since
variables no longer stay in a "home register".
Enabled by default with -funroll-loops.
- -fwhole-program
- Assume that the current compilation unit represents the whole program
being compiled. All public functions and variables with the exception of
"main" and those merged by attribute
"externally_visible" become static
functions and in effect are optimized more aggressively by interprocedural
optimizers.
This option should not be used in combination with -flto. Instead relying on a linker plugin should provide safer and more precise information.
- -flto[=n]
- This option runs the standard link-time optimizer. When invoked with
source code, it generates GIMPLE (one of GCC's internal representations)
and writes it to special ELF sections in the object file. When the object
files are linked together, all the function bodies are read from these ELF
sections and instantiated as if they had been part of the same translation
unit.
To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the final link. It is recommended that you compile all the files participating in the same link with the same options and also specify those options at link time. For example:
gcc -c -O2 -flto foo.c gcc -c -O2 -flto bar.c gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog.
The only important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link step. GCC then automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You generally should specify the optimization options to be used for link-time optimization though GCC tries to be clever at guessing an optimization level to use from the options used at compile time if you fail to specify one at link time. You can always override the automatic decision to do link-time optimization by passing -fno-lto to the link command.
To make whole program optimization effective, it is necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin is not available, -fwhole-program should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions.
When -fuse-linker-plugin is not enabled, when a file is compiled with -flto, the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects. This means that object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster but you cannot perform a regular, non-LTO link on them.
Additionally, the optimization flags used to compile individual files are not necessarily related to those used at link time. For instance,
gcc -c -O0 -ffat-lto-objects -flto foo.c gcc -c -O0 -ffat-lto-objects -flto bar.c gcc -o myprog -O3 foo.o bar.o
This produces individual object files with unoptimized assembler code, but the resulting binary myprog is optimized at -O3. If, instead, the final binary is generated with -fno-lto, then myprog is not optimized.
When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing.
There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link stage. Generally options specified at link time override those specified at compile time.
If you do not specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files.
Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fPIC, -fpic, -fpie, -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.
Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time.
If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages.
Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages:
gcc -c -flto foo.c g++ -c -flto bar.cc gfortran -c -flto baz.f90 g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, it is possible to extract and use them in an LTO link if you are using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects.
Link-time optimizations do not require the presence of the whole program to operate. If the program does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when linker plugin is active (see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC.
Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF.
If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used. The default value for n is 1.
You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make.
- -flto-partition=alg
- Specify the partitioning algorithm used by the link-time optimizer. The value is either 1to1 to specify a partitioning mirroring the original source files or balanced to specify partitioning into equally sized chunks (whenever possible) or max to create new partition for every symbol where possible. Specifying none as an algorithm disables partitioning and streaming completely. The default value is balanced. While 1to1 can be used as an workaround for various code ordering issues, the max partitioning is intended for internal testing only. The value one specifies that exactly one partition should be used while the value none bypasses partitioning and executes the link-time optimization step directly from the WPA phase.
- -flto-odr-type-merging
- Enable streaming of mangled types names of C++ types and their unification at link time. This increases size of LTO object files, but enables diagnostics about One Definition Rule violations.
- -flto-compression-level=n
- This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode (-flto). Valid values are 0 (no compression) to 9 (maximum compression). Values outside this range are clamped to either 0 or 9. If the option is not given, a default balanced compression setting is used.
- -fuse-linker-plugin
- Enables the use of a linker plugin during link-time optimization. This
option relies on plugin support in the linker, which is available in gold
or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it.
This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold).
- -ffat-lto-objects
- Fat LTO objects are object files that contain both the intermediate
language and the object code. This makes them usable for both LTO linking
and normal linking. This option is effective only when compiling with
-flto and is ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO makefiles need to be modified to use them.
Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into $libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-ranlib).
The default is -fno-fat-lto-objects on targets with linker plugin support.
- -fcompare-elim
- After register allocation and post-register allocation instruction
splitting, identify arithmetic instructions that compute processor flags
similar to a comparison operation based on that arithmetic. If possible,
eliminate the explicit comparison operation.
This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.
Enabled at levels -O, -O2, -O3, -Os.
- -fcprop-registers
- After register allocation and post-register allocation instruction
splitting, perform a copy-propagation pass to try to reduce scheduling
dependencies and occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
- -fprofile-correction
- Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected.
- -fprofile-use
- -fprofile-use=path
- Enable profile feedback-directed optimizations, and the following
optimizations which are generally profitable only with profile feedback
available: -fbranch-probabilities, -fvpt,
-funroll-loops, -fpeel-loops, -ftracer,
-ftree-vectorize, and ftree-loop-distribute-patterns.
Before you can use this option, you must first generate profiling information.
By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using -Wcoverage-mismatch. Note this may result in poorly optimized code.
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
- -fauto-profile
- -fauto-profile=path
- Enable sampling-based feedback-directed optimizations, and the following
optimizations which are generally profitable only with profile feedback
available: -fbranch-probabilities, -fvpt,
-funroll-loops, -fpeel-loops, -ftracer,
-ftree-vectorize, -finline-functions, -fipa-cp,
-fipa-cp-clone, -fpredictive-commoning,
-funswitch-loops, -fgcse-after-reload, and
-ftree-loop-distribute-patterns.
path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory.
Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system. For more information, see <https://perf.wiki.kernel.org/>.
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \ -- your_program
Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC. You must also supply the unstripped binary for your program to this tool. See <https://github.com/google/autofdo>.
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \ --gcov=profile.afdo
The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled.
- -ffloat-store
- Do not store floating-point variables in registers, and inhibit other
options that might change whether a floating-point value is taken from a
register or memory.
This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a "double" is supposed to have. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables.
- -fexcess-precision=style
- This option allows further control over excess precision on machines where
floating-point operations occur in a format with more precision or range
than the IEEE standard and interchange floating-point types. By default,
-fexcess-precision=fast is in effect; this means that operations
may be carried out in a wider precision than the types specified in the
source if that would result in faster code, and it is unpredictable when
rounding to the types specified in the source code takes place. When
compiling C, if -fexcess-precision=standard is specified then
excess precision follows the rules specified in ISO C99; in particular,
both casts and assignments cause values to be rounded to their semantic
types (whereas -ffloat-store only affects assignments). This option
is enabled by default for C if a strict conformance option such as
-std=c99 is used. -ffast-math enables
-fexcess-precision=fast by default regardless of whether a strict
conformance option is used.
-fexcess-precision=standard is not implemented for languages other than C. On the x86, it has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable.
- -ffast-math
- Sets the options -fno-math-errno,
-funsafe-math-optimizations, -ffinite-math-only,
-fno-rounding-math, -fno-signaling-nans,
-fcx-limited-range and -fexcess-precision=fast.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
- -fno-math-errno
- Do not set "errno" after calling math
functions that are executed with a single instruction, e.g.,
"sqrt". A program that relies on IEEE
exceptions for math error handling may want to use this flag for speed
while maintaining IEEE arithmetic compatibility.
This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is therefore no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default.
- -funsafe-math-optimizations
- Allow optimizations for floating-point arithmetic that (a) assume that
arguments and results are valid and (b) may violate IEEE or ANSI
standards. When used at link time, it may include libraries or startup
files that change the default FPU control word or other similar
optimizations.
This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. Enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.
The default is -fno-unsafe-math-optimizations.
- -fassociative-math
- Allow re-association of operands in series of floating-point operations.
This violates the ISO C and C++ language standard by possibly changing
computation result. NOTE: re-ordering may change the sign of zero as well
as ignore NaNs and inhibit or create underflow or overflow (and thus
cannot be used on code that relies on rounding behavior like
"(x + 2**52) - 2**52". May also reorder
floating-point comparisons and thus may not be used when ordered
comparisons are required. This option requires that both
-fno-signed-zeros and -fno-trapping-math be in effect.
Moreover, it doesn't make much sense with -frounding-math. For
Fortran the option is automatically enabled when both
-fno-signed-zeros and -fno-trapping-math are in effect.
The default is -fno-associative-math.
- -freciprocal-math
- Allow the reciprocal of a value to be used instead of dividing by the
value if this enables optimizations. For example "x
/ y" can be replaced with "x *
(1/y)", which is useful if
"(1/y)" is subject to common
subexpression elimination. Note that this loses precision and increases
the number of flops operating on the value.
The default is -fno-reciprocal-math.
- -ffinite-math-only
- Allow optimizations for floating-point arithmetic that assume that
arguments and results are not NaNs or +-Infs.
This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
The default is -fno-finite-math-only.
- -fno-signed-zeros
- Allow optimizations for floating-point arithmetic that ignore the
signedness of zero. IEEE arithmetic specifies the behavior of distinct
+0.0 and -0.0 values, which then prohibits simplification of expressions
such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option
implies that the sign of a zero result isn't significant.
The default is -fsigned-zeros.
- -fno-trapping-math
- Compile code assuming that floating-point operations cannot generate
user-visible traps. These traps include division by zero, overflow,
underflow, inexact result and invalid operation. This option requires that
-fno-signaling-nans be in effect. Setting this option may allow
faster code if one relies on "non-stop" IEEE arithmetic, for
example.
This option should never be turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -ftrapping-math.
- -frounding-math
- Disable transformations and optimizations that assume default
floating-point rounding behavior. This is round-to-zero for all floating
point to integer conversions, and round-to-nearest for all other
arithmetic truncations. This option should be specified for programs that
change the FP rounding mode dynamically, or that may be executed with a
non-default rounding mode. This option disables constant folding of
floating-point expressions at compile time (which may be affected by
rounding mode) and arithmetic transformations that are unsafe in the
presence of sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's "FENV_ACCESS" pragma. This command-line option will be used to specify the default state for "FENV_ACCESS".
- -fsignaling-nans
- Compile code assuming that IEEE signaling NaNs may generate user-visible
traps during floating-point operations. Setting this option disables
optimizations that may change the number of exceptions visible with
signaling NaNs. This option implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.
- -fno-fp-int-builtin-inexact
- Do not allow the built-in functions
"ceil",
"floor",
"round" and
"trunc", and their
"float" and
"long double"
variants, to generate code that raises the "inexact"
floating-point exception for noninteger arguments. ISO C99 and C11 allow
these functions to raise the "inexact" exception, but ISO/IEC TS
18661-1:2014, the C bindings to IEEE 754-2008, does not allow these
functions to do so.
The default is -ffp-int-builtin-inexact, allowing the exception to be raised. This option does nothing unless -ftrapping-math is in effect.
Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function then the "inexact" exception may be raised if the library implementation does not follow TS 18661.
- -fsingle-precision-constant
- Treat floating-point constants as single precision instead of implicitly converting them to double-precision constants.
- -fcx-limited-range
- When enabled, this option states that a range reduction step is not needed
when performing complex division. Also, there is no checking whether the
result of a complex multiplication or division is
"NaN + I*NaN",
with an attempt to rescue the situation in that case. The default is
-fno-cx-limited-range, but is enabled by -ffast-math.
This option controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. Nevertheless, the option applies to all languages.
- -fcx-fortran-rules
- Complex multiplication and division follow Fortran rules. Range reduction
is done as part of complex division, but there is no checking whether the
result of a complex multiplication or division is
"NaN + I*NaN",
with an attempt to rescue the situation in that case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code.
- -fbranch-probabilities
- After running a program compiled with -fprofile-arcs, you can
compile it a second time using -fbranch-probabilities, to improve
optimizations based on the number of times each branch was taken. When a
program compiled with -fprofile-arcs exits, it saves arc execution
counts to a file called sourcename.gcda for each
source file. The information in this data file is very dependent on the
structure of the generated code, so you must use the same source code and
the same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization. Currently, they are only used in one place: in reorg.c, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more often.
- -fprofile-values
- If combined with -fprofile-arcs, it adds code so that some data
about values of expressions in the program is gathered.
With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
- -fprofile-reorder-functions
- Function reordering based on profile instrumentation collects first time
of execution of a function and orders these functions in ascending order.
Enabled with -fprofile-use.
- -fvpt
- If combined with -fprofile-arcs, this option instructs the compiler
to add code to gather information about values of expressions.
With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator.
- -frename-registers
- Attempt to avoid false dependencies in scheduled code by making use of
registers left over after register allocation. This optimization most
benefits processors with lots of registers. Depending on the debug
information format adopted by the target, however, it can make debugging
impossible, since variables no longer stay in a "home register".
Enabled by default with -funroll-loops.
- -fschedule-fusion
- Performs a target dependent pass over the instruction stream to schedule
instructions of same type together because target machine can execute them
more efficiently if they are adjacent to each other in the instruction
flow.
Enabled at levels -O2, -O3, -Os.
- -ftracer
- Perform tail duplication to enlarge superblock size. This transformation
simplifies the control flow of the function allowing other optimizations
to do a better job.
Enabled with -fprofile-use.
- -funroll-loops
- Unroll loops whose number of iterations can be determined at compile time
or upon entry to the loop. -funroll-loops implies
-frerun-cse-after-loop, -fweb and -frename-registers.
It also turns on complete loop peeling (i.e. complete removal of loops
with a small constant number of iterations). This option makes code
larger, and may or may not make it run faster.
Enabled with -fprofile-use.
- -funroll-all-loops
- Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops.
- -fpeel-loops
- Peels loops for which there is enough information that they do not roll
much (from profile feedback or static analysis). It also turns on complete
loop peeling (i.e. complete removal of loops with small constant number of
iterations).
Enabled with -O3 and/or -fprofile-use.
- -fmove-loop-invariants
- Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1
- -fsplit-loops
- Split a loop into two if it contains a condition that's always true for one side of the iteration space and false for the other.
- -funswitch-loops
- Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition).
- -ffunction-sections
- -fdata-sections
- Place each function or data item into its own section in the output file
if the target supports arbitrary sections. The name of the function or the
name of the data item determines the section's name in the output file.
Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.
Together with a linker garbage collection (linker --gc-sections option) these options may lead to smaller statically-linked executables (after stripping).
On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats.
Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions.
- -fbranch-target-load-optimize
- Perform branch target register load optimization before prologue / epilogue threading. The use of target registers can typically be exposed only during reload, thus hoisting loads out of loops and doing inter-block scheduling needs a separate optimization pass.
- -fbranch-target-load-optimize2
- Perform branch target register load optimization after prologue / epilogue threading.
- -fbtr-bb-exclusive
- When performing branch target register load optimization, don't reuse branch target registers within any basic block.
- -fstdarg-opt
- Optimize the prologue of variadic argument functions with respect to usage of those arguments.
- -fsection-anchors
- Try to reduce the number of symbolic address calculations by using shared
"anchor" symbols to address nearby objects. This transformation
can help to reduce the number of GOT entries and GOT accesses on some
targets.
For example, the implementation of the following function "foo":
static int a, b, c; int foo (void) { return a + b + c; }
usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't valid C):
int foo (void) { register int *xr = &x; return xr[&a - &x] + xr[&b - &x] + xr[&c - &x]; }
Not all targets support this option.
- --param name=value
- In some places, GCC uses various constants to control the amount of
optimization that is done. For example, GCC does not inline functions that
contain more than a certain number of instructions. You can control some
of these constants on the command line using the --param option.
The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases.
In each case, the value is an integer. The allowable choices for name are:
- predictable-branch-outcome
- When branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable. The default is 10.
- max-rtl-if-conversion-insns
- RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions. This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The default is 10, though the compiler will also use other heuristics to decide whether if-conversion is likely to be profitable.
- max-rtl-if-conversion-predictable-cost
- max-rtl-if-conversion-unpredictable-cost
- RTL if-conversion will try to remove conditional branches around a block and replace them with conditionally executed instructions. These parameters give the maximum permissible cost for the sequence that would be generated by if-conversion depending on whether the branch is statically determined to be predictable or not. The units for this parameter are the same as those for the GCC internal seq_cost metric. The compiler will try to provide a reasonable default for this parameter using the BRANCH_COST target macro.
- max-crossjump-edges
- The maximum number of incoming edges to consider for cross-jumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size.
- min-crossjump-insns
- The minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched. The default value is 5.
- max-grow-copy-bb-insns
- The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction. The default value is 8.
- max-goto-duplication-insns
- The maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored. The default value is 8.
- max-delay-slot-insn-search
- The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time.
- max-delay-slot-live-search
- When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.
- max-gcse-memory
- The approximate maximum amount of memory that can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done.
- max-gcse-insertion-ratio
- If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream. The default value is 20.
- max-pending-list-length
- The maximum number of pending dependencies scheduling allows before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources.
- max-modulo-backtrack-attempts
- The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time.
- max-inline-insns-single
- Several parameters control the tree inliner used in GCC. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++). The default value is 400.
- max-inline-insns-auto
- When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered for inlining by the compiler are investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied. The default value is 30.
- inline-min-speedup
- When estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can be inlined regardless of the limit on --param max-inline-insns-single and --param max-inline-insns-auto. The default value is 15.
- large-function-insns
- The limit specifying really large functions. For functions larger than this limit after inlining, inlining is constrained by --param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end. The default value is 2700.
- large-function-growth
- Specifies maximal growth of large function caused by inlining in percents. The default value is 100 which limits large function growth to 2.0 times the original size.
- large-unit-insns
- The limit specifying large translation unit. Growth caused by inlining of units larger than this limit is limited by --param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of unit is 300\% and yet such inlining is very sane. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth. The default is 10000.
- inline-unit-growth
- Specifies maximal overall growth of the compilation unit caused by inlining. The default value is 20 which limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size.
- ipcp-unit-growth
- Specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. The default value is 10 which limits unit growth to 1.1 times the original size.
- large-stack-frame
- The limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too much. The default value is 256 bytes.
- large-stack-frame-growth
- Specifies maximal growth of large stack frames caused by inlining in percents. The default value is 1000 which limits large stack frame growth to 11 times the original size.
- max-inline-insns-recursive
- max-inline-insns-recursive-auto
- Specifies the maximum number of instructions an out-of-line copy of a
self-recursive inline function can grow into by performing recursive
inlining.
--param max-inline-insns-recursive applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-insns-recursive-auto applies instead. The default value is 450.
- max-inline-recursive-depth
- max-inline-recursive-depth-auto
- Specifies the maximum recursion depth used for recursive inlining.
--param max-inline-recursive-depth applies to functions declared inline. For functions not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-recursive-depth-auto applies instead. The default value is 8.
- min-inline-recursive-probability
- Recursive inlining is profitable only for function having deep recursion
in average and can hurt for function having little recursion depth by
increasing the prologue size or complexity of function body to other
optimizers.
When profile feedback is available (see -fprofile-generate) the actual recursion depth can be guessed from the probability that function recurses via a given call expression. This parameter limits inlining only to call expressions whose probability exceeds the given threshold (in percents). The default value is 10.
- early-inlining-insns
- Specify growth that the early inliner can make. In effect it increases the amount of inlining for code having a large abstraction penalty. The default value is 14.
- max-early-inliner-iterations
- Limit of iterations of the early inliner. This basically bounds the number of nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining.
- comdat-sharing-probability
- Probability (in percent) that C++ inline function with comdat visibility are shared across multiple compilation units. The default value is 20.
- profile-func-internal-id
- A parameter to control whether to use function internal id in profile database lookup. If the value is 0, the compiler uses an id that is based on function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering etc. The default value is 0.
- min-vect-loop-bound
- The minimum number of iterations under which loops are not vectorized when -ftree-vectorize is used. The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization. The default value is 0.
- gcse-cost-distance-ratio
- Scaling factor in calculation of maximum distance an expression can be moved by GCSE optimizations. This is currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting is with simple expressions, i.e., the expressions that have cost less than gcse-unrestricted-cost. Specifying 0 disables hoisting of simple expressions. The default value is 10.
- gcse-unrestricted-cost
- Cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel. This is currently supported only in the code hoisting pass. The lesser the cost, the more aggressive code hoisting is. Specifying 0 allows all expressions to travel unrestricted distances. The default value is 3.
- max-hoist-depth
- The depth of search in the dominator tree for expressions to hoist. This is used to avoid quadratic behavior in hoisting algorithm. The value of 0 does not limit on the search, but may slow down compilation of huge functions. The default value is 30.
- max-tail-merge-comparisons
- The maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior in tree tail merging. The default value is 10.
- max-tail-merge-iterations
- The maximum amount of iterations of the pass over the function. This is used to limit compilation time in tree tail merging. The default value is 2.
- store-merging-allow-unaligned
- Allow the store merging pass to introduce unaligned stores if it is legal to do so. The default value is 1.
- max-stores-to-merge
- The maximum number of stores to attempt to merge into wider stores in the store merging pass. The minimum value is 2 and the default is 64.
- max-unrolled-insns
- The maximum number of instructions that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.
- max-average-unrolled-insns
- The maximum number of instructions biased by probabilities of their execution that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.
- max-unroll-times
- The maximum number of unrollings of a single loop.
- max-peeled-insns
- The maximum number of instructions that a loop may have to be peeled. If a loop is peeled, this parameter also determines how many times the loop code is peeled.
- max-peel-times
- The maximum number of peelings of a single loop.
- max-peel-branches
- The maximum number of branches on the hot path through the peeled sequence.
- max-completely-peeled-insns
- The maximum number of insns of a completely peeled loop.
- max-completely-peel-times
- The maximum number of iterations of a loop to be suitable for complete peeling.
- max-completely-peel-loop-nest-depth
- The maximum depth of a loop nest suitable for complete peeling.
- max-unswitch-insns
- The maximum number of insns of an unswitched loop.
- max-unswitch-level
- The maximum number of branches unswitched in a single loop.
- max-loop-headers-insns
- The maximum number of insns in loop header duplicated by the copy loop headers pass.
- lim-expensive
- The minimum cost of an expensive expression in the loop invariant motion.
- iv-consider-all-candidates-bound
- Bound on number of candidates for induction variables, below which all candidates are considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones are considered to avoid quadratic time complexity.
- iv-max-considered-uses
- The induction variable optimizations give up on loops that contain more induction variable uses.
- iv-always-prune-cand-set-bound
- If the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one.
- avg-loop-niter
- Average number of iterations of a loop.
- dse-max-object-size
- Maximum size (in bytes) of objects tracked bytewise by dead store elimination. Larger values may result in larger compilation times.
- scev-max-expr-size
- Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer.
- scev-max-expr-complexity
- Bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer.
- max-tree-if-conversion-phi-args
- Maximum number of arguments in a PHI supported by TREE if conversion unless the loop is marked with simd pragma.
- vect-max-version-for-alignment-checks
- The maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer.
- vect-max-version-for-alias-checks
- The maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.
- vect-max-peeling-for-alignment
- The maximum number of loop peels to enhance access alignment for vectorizer. Value -1 means no limit.
- max-iterations-to-track
- The maximum number of iterations of a loop the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate.
- hot-bb-count-ws-permille
- A basic block profile count is considered hot if it contributes to the given permillage (i.e. 0...1000) of the entire profiled execution.
- hot-bb-frequency-fraction
- Select fraction of the entry block frequency of executions of basic block in function given basic block needs to have to be considered hot.
- max-predicted-iterations
- The maximum number of loop iterations we predict statically. This is useful in cases where a function contains a single loop with known bound and another loop with unknown bound. The known number of iterations is predicted correctly, while the unknown number of iterations average to roughly 10. This means that the loop without bounds appears artificially cold relative to the other one.
- builtin-expect-probability
- Control the probability of the expression having the specified value. This parameter takes a percentage (i.e. 0 ... 100) as input. The default probability of 90 is obtained empirically.
- align-threshold
- Select fraction of the maximal frequency of executions of a basic block in a function to align the basic block.
- align-loop-iterations
- A loop expected to iterate at least the selected number of iterations is aligned.
- tracer-dynamic-coverage
- tracer-dynamic-coverage-feedback
- This value is used to limit superblock formation once the given percentage
of executed instructions is covered. This limits unnecessary code size
expansion.
The tracer-dynamic-coverage-feedback parameter is used only when profile feedback is available. The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value.
- tracer-max-code-growth
- Stop tail duplication once code growth has reached given percentage. This is a rather artificial limit, as most of the duplicates are eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth.
- tracer-min-branch-ratio
- Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).
- tracer-min-branch-probability
- tracer-min-branch-probability-feedback
- Stop forward growth if the best edge has probability lower than this
threshold.
Similarly to tracer-dynamic-coverage two parameters are provided. tracer-min-branch-probability-feedback is used for compilation with profile feedback and tracer-min-branch-probability compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.
- stack-clash-protection-guard-size
- Specify the size of the operating system provided stack guard as 2 raised to num bytes. The default value is 12 (4096 bytes). Acceptable values are between 12 and 30. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks.
- stack-clash-protection-probe-interval
- Stack clash protection involves probing stack space as it is allocated. This param controls the maximum distance between probes into the stack as 2 raised to num bytes. Acceptable values are between 10 and 16 and defaults to 12. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks.
- max-cse-path-length
- The maximum number of basic blocks on path that CSE considers. The default is 10.
- max-cse-insns
- The maximum number of instructions CSE processes before flushing. The default is 1000.
- ggc-min-expand
- GCC uses a garbage collector to manage its own memory allocation. This
parameter specifies the minimum percentage by which the garbage
collector's heap should be allowed to expand between collections. Tuning
this may improve compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If "getrlimit" is available, the notion of "RAM" is the smallest of actual RAM and "RLIMIT_DATA" or "RLIMIT_AS". If GCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. This is extremely slow, but can be useful for debugging.
- ggc-min-heapsize
- Minimum size of the garbage collector's heap before it begins bothering to
collect garbage. The first collection occurs after the heap expands by
ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this
may improve compilation speed, and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC is not able to calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity.
- max-reload-search-insns
- The maximum number of instruction reload should look backward for equivalent register. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance. The default value is 100.
- max-cselib-memory-locations
- The maximum number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance. The default value is 500.
- max-sched-ready-insns
- The maximum number of instructions ready to be issued the scheduler should consider at any given time during the first scheduling pass. Increasing values mean more thorough searches, making the compilation time increase with probably little benefit. The default value is 100.
- max-sched-region-blocks
- The maximum number of blocks in a region to be considered for interblock scheduling. The default value is 10.
- max-pipeline-region-blocks
- The maximum number of blocks in a region to be considered for pipelining in the selective scheduler. The default value is 15.
- max-sched-region-insns
- The maximum number of insns in a region to be considered for interblock scheduling. The default value is 100.
- max-pipeline-region-insns
- The maximum number of insns in a region to be considered for pipelining in the selective scheduler. The default value is 200.
- min-spec-prob
- The minimum probability (in percents) of reaching a source block for interblock speculative scheduling. The default value is 40.
- max-sched-extend-regions-iters
- The maximum number of iterations through CFG to extend regions. A value of 0 (the default) disables region extensions.
- max-sched-insn-conflict-delay
- The maximum conflict delay for an insn to be considered for speculative motion. The default value is 3.
- sched-spec-prob-cutoff
- The minimal probability of speculation success (in percents), so that speculative insns are scheduled. The default value is 40.
- sched-state-edge-prob-cutoff
- The minimum probability an edge must have for the scheduler to save its state across it. The default value is 10.
- sched-mem-true-dep-cost
- Minimal distance (in CPU cycles) between store and load targeting same memory locations. The default value is 1.
- selsched-max-lookahead
- The maximum size of the lookahead window of selective scheduling. It is a depth of search for available instructions. The default value is 50.
- selsched-max-sched-times
- The maximum number of times that an instruction is scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined. The default value is 2.
- selsched-insns-to-rename
- The maximum number of best instructions in the ready list that are considered for renaming in the selective scheduler. The default value is 2.
- sms-min-sc
- The minimum value of stage count that swing modulo scheduler generates. The default value is 2.
- max-last-value-rtl
- The maximum size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo register as last known value of that register. The default is 10000.
- max-combine-insns
- The maximum number of instructions the RTL combiner tries to combine. The default value is 2 at -Og and 4 otherwise.
- Small integer constants can use a shared data structure, reducing the compiler's memory usage and increasing its speed. This sets the maximum value of a shared integer constant. The default value is 256.
- ssp-buffer-size
- The minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-protection is used.
- min-size-for-stack-sharing
- The minimum size of variables taking part in stack slot sharing when not optimizing. The default value is 32.
- max-jump-thread-duplication-stmts
- Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
- max-fields-for-field-sensitive
- Maximum number of fields in a structure treated in a field sensitive manner during pointer analysis. The default is zero for -O0 and -O1, and 100 for -Os, -O2, and -O3.
- prefetch-latency
- Estimate on average number of instructions that are executed before prefetch finishes. The distance prefetched ahead is proportional to this constant. Increasing this number may also lead to less streams being prefetched (see simultaneous-prefetches).
- simultaneous-prefetches
- Maximum number of prefetches that can run at the same time.
- l1-cache-line-size
- The size of cache line in L1 cache, in bytes.
- l1-cache-size
- The size of L1 cache, in kilobytes.
- l2-cache-size
- The size of L2 cache, in kilobytes.
- loop-interchange-max-num-stmts
- The maximum number of stmts in a loop to be interchanged.
- loop-interchange-stride-ratio
- The minimum ratio between stride of two loops for interchange to be profitable.
- min-insn-to-prefetch-ratio
- The minimum ratio between the number of instructions and the number of prefetches to enable prefetching in a loop.
- prefetch-min-insn-to-mem-ratio
- The minimum ratio between the number of instructions and the number of memory references to enable prefetching in a loop.
- use-canonical-types
- Whether the compiler should use the "canonical" type system. By default, this should always be 1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. However, if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types.
- switch-conversion-max-branch-ratio
- Switch initialization conversion refuses to create arrays that are bigger than switch-conversion-max-branch-ratio times the number of branches in the switch.
- max-partial-antic-length
- Maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above. For some sorts of source code the enhanced partial redundancy elimination optimization can run away, consuming all of the memory available on the host machine. This parameter sets a limit on the length of the sets that are computed, which prevents the runaway behavior. Setting a value of 0 for this parameter allows an unlimited set length.
- sccvn-max-scc-size
- Maximum size of a strongly connected component (SCC) during SCCVN processing. If this limit is hit, SCCVN processing for the whole function is not done and optimizations depending on it are disabled. The default maximum SCC size is 10000.
- sccvn-max-alias-queries-per-access
- Maximum number of alias-oracle queries we perform when looking for redundancies for loads and stores. If this limit is hit the search is aborted and the load or store is not considered redundant. The number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry. The default maximum number of queries is 1000.
- ira-max-loops-num
- IRA uses regional register allocation by default. If a function contains more loops than the number given by this parameter, only at most the given number of the most frequently-executed loops form regions for regional register allocation. The default value of the parameter is 100.
- ira-max-conflict-table-size
- Although IRA uses a sophisticated algorithm to compress the conflict table, the table can still require excessive amounts of memory for huge functions. If the conflict table for a function could be more than the size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that does not require building a pseudo-register conflict table. The default value of the parameter is 2000.
- ira-loop-reserved-regs
- IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop invariants (see -O3). The number of available registers reserved for some other purposes is given by this parameter. The default value of the parameter is 2, which is the minimal number of registers needed by typical instructions. This value is the best found from numerous experiments.
- lra-inheritance-ebb-probability-cutoff
- LRA tries to reuse values reloaded in registers in subsequent insns. This optimization is called inheritance. EBB is used as a region to do this optimization. The parameter defines a minimal fall-through edge probability in percentage used to add BB to inheritance EBB in LRA. The default value of the parameter is 40. The value was chosen from numerous runs of SPEC2000 on x86-64.
- loop-invariant-max-bbs-in-loop
- Loop invariant motion can be very expensive, both in compilation time and in amount of needed compile-time memory, with very large loops. Loops with more basic blocks than this parameter won't have loop invariant motion optimization performed on them. The default value of the parameter is 1000 for -O1 and 10000 for -O2 and above.
- loop-max-datarefs-for-datadeps
- Building data dependencies is expensive for very large loops. This parameter limits the number of data references in loops that are considered for data dependence analysis. These large loops are no handled by the optimizations using loop data dependencies. The default value is 1000.
- max-vartrack-size
- Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. If this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function. If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function. Setting the parameter to zero makes it unlimited.
- max-vartrack-expr-depth
- Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value expressions. This trades compilation time for more complete debug information. If this is set too low, value expressions that are available and could be represented in debug information may end up not being used; setting this higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow. The default is 12.
- max-debug-marker-count
- Sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid complexity explosion at inlining or expanding to RTL. If a function has more such gimple stmts than the set limit, such stmts will be dropped from the inlined copy of a function, and from its RTL expansion. The default is 100000.
- min-nondebug-insn-uid
- Use uids starting at this parameter for nondebug insns. The range below the parameter is reserved exclusively for debug insns created by -fvar-tracking-assignments, but debug insns may get (non-overlapping) uids above it if the reserved range is exhausted.
- ipa-sra-ptr-growth-factor
- IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter.
- sra-max-scalarization-size-Ospeed
- sra-max-scalarization-size-Osize
- The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) aim to replace scalar parts of aggregates with uses of independent scalar variables. These parameters control the maximum size, in storage units, of aggregate which is considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or size (sra-max-scalarization-size-Osize) respectively.
- sra-max-propagations
- The maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, in order to facilitate copy propagation.
- tm-max-aggregate-size
- When making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables are saved with the logging functions as opposed to save/restore code sequence pairs. This option only applies when using -fgnu-tm.
- graphite-max-nb-scop-params
- To avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. The default value is 10 parameters, a value of zero can be used to lift the bound. A variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP.
- loop-block-tile-size
- Loop blocking or strip mining transforms, enabled with -floop-block or -floop-strip-mine, strip mine each loop in the loop nest by a given number of iterations. The strip length can be changed using the loop-block-tile-size parameter. The default value is 51 iterations.
- loop-unroll-jam-size
- Specify the unroll factor for the -floop-unroll-and-jam option. The default value is 4.
- loop-unroll-jam-depth
- Specify the dimension to be unrolled (counting from the most inner loop) for the -floop-unroll-and-jam. The default value is 2.
- ipa-cp-value-list-size
- IPA-CP attempts to track all possible values and types passed to a function's parameter in order to propagate them and perform devirtualization. ipa-cp-value-list-size is the maximum number of values and types it stores per one formal parameter of a function.
- ipa-cp-eval-threshold
- IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities with scores that exceed ipa-cp-eval-threshold.
- ipa-cp-recursion-penalty
- Percentage penalty the recursive functions will receive when they are evaluated for cloning.
- ipa-cp-single-call-penalty
- Percentage penalty functions containing a single call to another function will receive when they are evaluated for cloning.
- ipa-max-agg-items
- IPA-CP is also capable to propagate a number of scalar values passed in an aggregate. ipa-max-agg-items controls the maximum number of such values per one parameter.
- ipa-cp-loop-hint-bonus
- When IPA-CP determines that a cloning candidate would make the number of iterations of a loop known, it adds a bonus of ipa-cp-loop-hint-bonus to the profitability score of the candidate.
- ipa-cp-array-index-hint-bonus
- When IPA-CP determines that a cloning candidate would make the index of an array access known, it adds a bonus of ipa-cp-array-index-hint-bonus to the profitability score of the candidate.
- ipa-max-aa-steps
- During its analysis of function bodies, IPA-CP employs alias analysis in order to track values pointed to by function parameters. In order not spend too much time analyzing huge functions, it gives up and consider all memory clobbered after examining ipa-max-aa-steps statements modifying memory.
- lto-partitions
- Specify desired number of partitions produced during WHOPR compilation. The number of partitions should exceed the number of CPUs used for compilation. The default value is 32.
- lto-min-partition
- Size of minimal partition for WHOPR (in estimated instructions). This prevents expenses of splitting very small programs into too many partitions.
- lto-max-partition
- Size of max partition for WHOPR (in estimated instructions). to provide an upper bound for individual size of partition. Meant to be used only with balanced partitioning.
- cxx-max-namespaces-for-diagnostic-help
- The maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. The default is 1000.
- sink-frequency-threshold
- The maximum relative execution frequency (in percents) of the target block relative to a statement's original block to allow statement sinking of a statement. Larger numbers result in more aggressive statement sinking. The default value is 75. A small positive adjustment is applied for statements with memory operands as those are even more profitable so sink.
- max-stores-to-sink
- The maximum number of conditional store pairs that can be sunk. Set to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. The default is 2.
- allow-store-data-races
- Allow optimizers to introduce new data races on stores. Set to 1 to allow, otherwise to 0. This option is enabled by default at optimization level -Ofast.
- case-values-threshold
- The smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches. If the value is 0, use the default for the machine. The default is 0.
- tree-reassoc-width
- Set the maximum number of instructions executed in parallel in reassociated tree. This parameter overrides target dependent heuristics used by default if has non zero value.
- sched-pressure-algorithm
- Choose between the two available implementations of
-fsched-pressure. Algorithm 1 is the original implementation and is
the more likely to prevent instructions from being reordered. Algorithm 2
was designed to be a compromise between the relatively conservative
approach taken by algorithm 1 and the rather aggressive approach taken by
the default scheduler. It relies more heavily on having a regular register
file and accurate register pressure classes. See haifa-sched.c in
the GCC sources for more details.
The default choice depends on the target.
- max-slsr-cand-scan
- Set the maximum number of existing candidates that are considered when seeking a basis for a new straight-line strength reduction candidate.
- asan-globals
- Enable buffer overflow detection for global objects. This kind of protection is enabled by default if you are using -fsanitize=address option. To disable global objects protection use --param asan-globals=0.
- asan-stack
- Enable buffer overflow detection for stack objects. This kind of protection is enabled by default when using -fsanitize=address. To disable stack protection use --param asan-stack=0 option.
- asan-instrument-reads
- Enable buffer overflow detection for memory reads. This kind of protection is enabled by default when using -fsanitize=address. To disable memory reads protection use --param asan-instrument-reads=0.
- asan-instrument-writes
- Enable buffer overflow detection for memory writes. This kind of protection is enabled by default when using -fsanitize=address. To disable memory writes protection use --param asan-instrument-writes=0 option.
- asan-memintrin
- Enable detection for built-in functions. This kind of protection is enabled by default when using -fsanitize=address. To disable built-in functions protection use --param asan-memintrin=0.
- asan-use-after-return
- Enable detection of use-after-return. This kind of protection is enabled
by default when using the -fsanitize=address option. To disable it
use --param asan-use-after-return=0.
Note: By default the check is disabled at run time. To enable it, add "detect_stack_use_after_return=1" to the environment variable ASAN_OPTIONS.
- asan-instrumentation-with-call-threshold
- If number of memory accesses in function being instrumented is greater or equal to this number, use callbacks instead of inline checks. E.g. to disable inline code use --param asan-instrumentation-with-call-threshold=0.
- use-after-scope-direct-emission-threshold
- If the size of a local variable in bytes is smaller or equal to this number, directly poison (or unpoison) shadow memory instead of using run-time callbacks. The default value is 256.
- chkp-max-ctor-size
- Static constructors generated by Pointer Bounds Checker may become very large and significantly increase compile time at optimization level -O1 and higher. This parameter is a maximum number of statements in a single generated constructor. Default value is 5000.
- max-fsm-thread-path-insns
- Maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path. The default is 100.
- max-fsm-thread-length
- Maximum number of basic blocks on a finite state automaton jump thread path. The default is 10.
- max-fsm-thread-paths
- Maximum number of new jump thread paths to create for a finite state automaton. The default is 50.
- parloops-chunk-size
- Chunk size of omp schedule for loops parallelized by parloops. The default is 0.
- parloops-schedule
- Schedule type of omp schedule for loops parallelized by parloops (static, dynamic, guided, auto, runtime). The default is static.
- parloops-min-per-thread
- The minimum number of iterations per thread of an innermost parallelized loop for which the parallelized variant is prefered over the single threaded one. The default is 100. Note that for a parallelized loop nest the minimum number of iterations of the outermost loop per thread is two.
- max-ssa-name-query-depth
- Maximum depth of recursion when querying properties of SSA names in things like fold routines. One level of recursion corresponds to following a use-def chain.
- hsa-gen-debug-stores
- Enable emission of special debug stores within HSA kernels which are then read and reported by libgomp plugin. Generation of these stores is disabled by default, use --param hsa-gen-debug-stores=1 to enable it.
- max-speculative-devirt-maydefs
- The maximum number of may-defs we analyze when looking for a must-def specifying the dynamic type of an object that invokes a virtual call we may be able to devirtualize speculatively.
- max-vrp-switch-assertions
- The maximum number of assertions to add along the default edge of a switch statement during VRP. The default is 10.
- unroll-jam-min-percent
- The minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to be considered profitable.
- unroll-jam-max-unroll
- The maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation.
Program Instrumentation Options¶
GCC supports a number of command-line options that control adding run-time instrumentation to the code it normally generates. For example, one purpose of instrumentation is collect profiling statistics for use in finding program hot spots, code coverage analysis, or profile-guided optimizations. Another class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses, as well as deliberately hostile attacks such as stack smashing or C++ vtable hijacking. There is also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes.
- -p
- Generate extra code to write profile information suitable for the analysis program prof. You must use this option when compiling the source files you want data about, and you must also use it when linking.
- -pg
- Generate extra code to write profile information suitable for the analysis program gprof. You must use this option when compiling the source files you want data about, and you must also use it when linking.
- -fprofile-arcs
- Add code so that program flow arcs are instrumented. During
execution the program records how many times each branch and call is
executed and how many times it is taken or returns. On targets that
support constructors with priority support, profiling properly handles
constructors, destructors and C++ constructors (and destructors) of
classes which are used as a type of a global variable.
When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o).
- --coverage
- This option is used to compile and link code instrumented for coverage analysis. The option is a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details.
- Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program.
- Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories.
- Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
- Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect, "fork" calls are detected and correctly handled without double counting.
- For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities.
- For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.
- -ftest-coverage
- Produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of auxname and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize.
- -fprofile-abs-path
- Automatically convert relative source file names to absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories.
- -fprofile-dir=path
- Set the directory to search for the profile data files in to path. This option affects only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file.
- -fprofile-generate
- -fprofile-generate=path
- Enable options usually used for instrumenting application to produce
profile useful for later recompilation with profile feedback based
optimization. You must use -fprofile-generate both when compiling
and when linking your program.
The following options are enabled: -fprofile-arcs, -fprofile-values, -fvpt.
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
To optimize the program based on the collected profile information, use -fprofile-use.
- -fprofile-update=method
- Alter the update method for an application instrumented for profile
feedback based optimization. The method argument should be one of
single, atomic or prefer-atomic. The first one is
useful for single-threaded applications, while the second one prevents
profile corruption by emitting thread-safe code.
Warning: When an application does not properly join all threads (or creates an detached thread), a profile file can be still corrupted.
Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is present in the command line.
- -fsanitize=address
- Enable AddressSanitizer, a fast memory error detector. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables -fsanitize-address-use-after-scope. See <https://github.com/google/sanitizers/wiki/AddressSanitizer> for more details. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable. When set to "help=1", the available options are shown at startup of the instrumented program. See <https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags> for a list of supported options. The option cannot be combined with -fsanitize=thread and/or -fcheck-pointer-bounds.
- -fsanitize=kernel-address
- Enable AddressSanitizer for Linux kernel. See <https://github.com/google/kasan/wiki> for more details. The option cannot be combined with -fcheck-pointer-bounds.
- -fsanitize=pointer-compare
- Instrument comparison operation (<, <=, >, >=) with pointer operands. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address The option cannot be combined with -fsanitize=thread and/or -fcheck-pointer-bounds. Note: By default the check is disabled at run time. To enable it, add "detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers are non-null.
- -fsanitize=pointer-subtract
- Instrument subtraction with pointer operands. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address The option cannot be combined with -fsanitize=thread and/or -fcheck-pointer-bounds. Note: By default the check is disabled at run time. To enable it, add "detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers are non-null.
- -fsanitize=thread
- Enable ThreadSanitizer, a fast data race detector. Memory access
instructions are instrumented to detect data race bugs. See
<https://github.com/google/sanitizers/wiki#threadsanitizer>
for more details. The run-time behavior can be influenced using the
TSAN_OPTIONS environment variable; see
<https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags>
for a list of supported options. The option cannot be combined with
-fsanitize=address, -fsanitize=leak and/or
-fcheck-pointer-bounds.
Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions).
- -fsanitize=leak
- Enable LeakSanitizer, a memory leak detector. This option only matters for linking of executables and the executable is linked against a library that overrides "malloc" and other allocator functions. See <https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer> for more details. The run-time behavior can be influenced using the LSAN_OPTIONS environment variable. The option cannot be combined with -fsanitize=thread.
- -fsanitize=undefined
- Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations are instrumented to detect undefined behavior at runtime. Current suboptions are:
- -fsanitize=shift
- This option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This option has two suboptions, -fsanitize=shift-base and -fsanitize=shift-exponent.
- -fsanitize=shift-exponent
- This option enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument.
- -fsanitize=shift-base
- If the second argument of a shift operation is within range, check that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc.
- -fsanitize=integer-divide-by-zero
- Detect integer division by zero as well as "INT_MIN / -1" division.
- -fsanitize=unreachable
- With this option, the compiler turns the "__builtin_unreachable" call into a diagnostics message call instead. When reaching the "__builtin_unreachable" call, the behavior is undefined.
- -fsanitize=vla-bound
- This option instructs the compiler to check that the size of a variable length array is positive.
- -fsanitize=null
- This option enables pointer checking. Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed by a NULL pointer.
- -fsanitize=return
- This option enables return statement checking. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only.
- -fsanitize=signed-integer-overflow
- This option enables signed integer overflow checking. We check that the
result of "+",
"*", and both unary and binary
"-" does not overflow in the signed
arithmetics. Note, integer promotion rules must be taken into account.
That is, the following is not an overflow:
signed char a = SCHAR_MAX; a++;
- -fsanitize=bounds
- This option enables instrumentation of array bounds. Various out of bounds accesses are detected. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented. The option cannot be combined with -fcheck-pointer-bounds.
- -fsanitize=bounds-strict
- This option enables strict instrumentation of array bounds. Most out of bounds accesses are detected, including flexible array members and flexible array member-like arrays. Initializers of variables with static storage are not instrumented. The option cannot be combined with -fcheck-pointer-bounds.
- -fsanitize=alignment
- This option enables checking of alignment of pointers when they are dereferenced, or when a reference is bound to insufficiently aligned target, or when a method or constructor is invoked on insufficiently aligned object.
- -fsanitize=object-size
- This option enables instrumentation of memory references using the "__builtin_object_size" function. Various out of bounds pointer accesses are detected.
- -fsanitize=float-divide-by-zero
- Detect floating-point division by zero. Unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs.
- -fsanitize=float-cast-overflow
- This option enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. Unlike other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. This option does not work well with "FE_INVALID" exceptions enabled.
- -fsanitize=nonnull-attribute
- This option enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the "nonnull" function attribute.
- -fsanitize=returns-nonnull-attribute
- This option enables instrumentation of return statements in functions marked with "returns_nonnull" function attribute, to detect returning of null values from such functions.
- -fsanitize=bool
- This option enables instrumentation of loads from bool. If a value other than 0/1 is loaded, a run-time error is issued.
- -fsanitize=enum
- This option enables instrumentation of loads from an enum type. If a value outside the range of values for the enum type is loaded, a run-time error is issued.
- -fsanitize=vptr
- This option enables instrumentation of C++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type.
- -fsanitize=pointer-overflow
- This option enables instrumentation of pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued.
- -fsanitize=builtin
- This option enables instrumentation of arguments to selected builtin functions. If an invalid value is passed to such arguments, a run-time error is issued. E.g. passing 0 as the argument to "__builtin_ctz" or "__builtin_clz" invokes undefined behavior and is diagnosed by this option.
While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message. This currently works only for the C family of languages.
- -fno-sanitize=all
- This option disables all previously enabled sanitizers. -fsanitize=all is not allowed, as some sanitizers cannot be used together.
- -fasan-shadow-offset=number
- This option forces GCC to use custom shadow offset in AddressSanitizer checks. It is useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer.
- -fsanitize-sections=s1,s2,...
- Sanitize global variables in selected user-defined sections. si may contain wildcards.
- -fsanitize-recover[=opts]
- -fsanitize-recover= controls error recovery mode for sanitizers
mentioned in comma-separated list of opts. Enabling this option for
a sanitizer component causes it to attempt to continue running the program
as if no error happened. This means multiple runtime errors can be
reported in a single program run, and the exit code of the program may
indicate success even when errors have been reported. The
-fno-sanitize-recover= option can be used to alter this behavior:
only the first detected error is reported and program then exits with a
non-zero exit code.
Currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and -fsanitize=address. For these sanitizers error recovery is turned on by default, except -fsanitize=address, for which this feature is experimental. -fsanitize-recover=all and -fno-sanitize-recover=all is also accepted, the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.
Even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to "halt_on_error=0" for ThreadSanitizer and UndefinedBehaviorSanitizer, while default value for AddressSanitizer is "halt_on_error=1". This can be overridden through setting the "halt_on_error" flag in the corresponding environment variable.
Syntax without an explicit opts parameter is deprecated. It is equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
- -fsanitize-address-use-after-scope
- Enable sanitization of local variables to detect use-after-scope bugs. The option sets -fstack-reuse to none.
- -fsanitize-undefined-trap-on-error
- The -fsanitize-undefined-trap-on-error option instructs the compiler to report undefined behavior using "__builtin_trap" rather than a "libubsan" library routine. The advantage of this is that the "libubsan" library is not needed and is not linked in, so this is usable even in freestanding environments.
- -fsanitize-coverage=trace-pc
- Enable coverage-guided fuzzing code instrumentation. Inserts a call to "__sanitizer_cov_trace_pc" into every basic block.
- -fsanitize-coverage=trace-cmp
- Enable dataflow guided fuzzing code instrumentation. Inserts a call to "__sanitizer_cov_trace_cmp1", "__sanitizer_cov_trace_cmp2", "__sanitizer_cov_trace_cmp4" or "__sanitizer_cov_trace_cmp8" for integral comparison with both operands variable or "__sanitizer_cov_trace_const_cmp1", "__sanitizer_cov_trace_const_cmp2", "__sanitizer_cov_trace_const_cmp4" or "__sanitizer_cov_trace_const_cmp8" for integral comparison with one operand constant, "__sanitizer_cov_trace_cmpf" or "__sanitizer_cov_trace_cmpd" for float or double comparisons and "__sanitizer_cov_trace_switch" for switch statements.
- -fbounds-check
- For front ends that support it, generate additional code to check that indices used to access arrays are within the declared range. This is currently only supported by the Fortran front end, where this option defaults to false.
- -fcheck-pointer-bounds
- Enable Pointer Bounds Checker instrumentation. Each memory reference is
instrumented with checks of the pointer used for memory access against
bounds associated with that pointer.
Currently there is only an implementation for Intel MPX available, thus x86 GNU/Linux target and -mmpx are required to enable this feature. MPX-based instrumentation requires a runtime library to enable MPX in hardware and handle bounds violation signals. By default when -fcheck-pointer-bounds and -mmpx options are used to link a program, the GCC driver links against the libmpx and libmpxwrappers libraries. Bounds checking on calls to dynamic libraries requires a linker with -z bndplt support; if GCC was configured with a linker without support for this option (including the Gold linker and older versions of ld), a warning is given if you link with -mmpx without also specifying -static, since the overall effectiveness of the bounds checking protection is reduced. See also -static-libmpxwrappers.
MPX-based instrumentation may be used for debugging and also may be included in production code to increase program security. Depending on usage, you may have different requirements for the runtime library. The current version of the MPX runtime library is more oriented for use as a debugging tool. MPX runtime library usage implies -lpthread. See also -static-libmpx. The runtime library behavior can be influenced using various CHKP_RT_* environment variables. See <https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler> for more details.
Generated instrumentation may be controlled by various -fchkp-* options and by the "bnd_variable_size" structure field attribute and "bnd_legacy", and "bnd_instrument" function attributes. GCC also provides a number of built-in functions for controlling the Pointer Bounds Checker.
- -fchkp-check-incomplete-type
- Generate pointer bounds checks for variables with incomplete type. Enabled by default.
- -fchkp-narrow-bounds
- Controls bounds used by Pointer Bounds Checker for pointers to object fields. If narrowing is enabled then field bounds are used. Otherwise object bounds are used. See also -fchkp-narrow-to-innermost-array and -fchkp-first-field-has-own-bounds. Enabled by default.
- -fchkp-first-field-has-own-bounds
- Forces Pointer Bounds Checker to use narrowed bounds for the address of the first field in the structure. By default a pointer to the first field has the same bounds as a pointer to the whole structure.
- -fchkp-flexible-struct-trailing-arrays
- Forces Pointer Bounds Checker to treat all trailing arrays in structures as possibly flexible. By default only array fields with zero length or that are marked with attribute bnd_variable_size are treated as flexible.
- -fchkp-narrow-to-innermost-array
- Forces Pointer Bounds Checker to use bounds of the innermost arrays in case of nested static array access. By default this option is disabled and bounds of the outermost array are used.
- -fchkp-optimize
- Enables Pointer Bounds Checker optimizations. Enabled by default at optimization levels -O, -O2, -O3.
- -fchkp-use-fast-string-functions
- Enables use of *_nobnd versions of string functions (not copying bounds) by Pointer Bounds Checker. Disabled by default.
- -fchkp-use-nochk-string-functions
- Enables use of *_nochk versions of string functions (not checking bounds) by Pointer Bounds Checker. Disabled by default.
- -fchkp-use-static-bounds
- Allow Pointer Bounds Checker to generate static bounds holding bounds of static variables. Enabled by default.
- -fchkp-use-static-const-bounds
- Use statically-initialized bounds for constant bounds instead of generating them each time they are required. By default enabled when -fchkp-use-static-bounds is enabled.
- -fchkp-treat-zero-dynamic-size-as-infinite
- With this option, objects with incomplete type whose dynamically-obtained size is zero are treated as having infinite size instead by Pointer Bounds Checker. This option may be helpful if a program is linked with a library missing size information for some symbols. Disabled by default.
- -fchkp-check-read
- Instructs Pointer Bounds Checker to generate checks for all read accesses to memory. Enabled by default.
- -fchkp-check-write
- Instructs Pointer Bounds Checker to generate checks for all write accesses to memory. Enabled by default.
- -fchkp-store-bounds
- Instructs Pointer Bounds Checker to generate bounds stores for pointer writes. Enabled by default.
- -fchkp-instrument-calls
- Instructs Pointer Bounds Checker to pass pointer bounds to calls. Enabled by default.
- -fchkp-instrument-marked-only
- Instructs Pointer Bounds Checker to instrument only functions marked with the "bnd_instrument" attribute. Disabled by default.
- -fchkp-use-wrappers
- Allows Pointer Bounds Checker to replace calls to built-in functions with calls to wrapper functions. When -fchkp-use-wrappers is used to link a program, the GCC driver automatically links against libmpxwrappers. See also -static-libmpxwrappers. Enabled by default.
- -fcf-protection=[full|branch|return|none]
- Enable code instrumentation of control-flow transfers to increase program
security by checking that target addresses of control-flow transfer
instructions (such as indirect function call, function return, indirect
jump) are valid. This prevents diverting the flow of control to an
unexpected target. This is intended to protect against such threats as
Return-oriented Programming (ROP), and similarly call/jmp-oriented
programming (COP/JOP).
The value "branch" tells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e. call/jmp instructions. The value "return" implements checking of validity at the point of returning from a function. The value "full" is an alias for specifying both "branch" and "return". The value "none" turns off instrumentation.
The macro "__CET__" is defined when -fcf-protection is used. The first bit of "__CET__" is set to 1 for the value "branch" and the second bit of "__CET__" is set to 1 for the "return".
You can also use the "nocf_check" attribute to identify which functions and calls should be skipped from instrumentation.
Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET).
- -fstack-protector
- Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a guard variable to functions with vulnerable objects. This includes functions that call "alloca", and functions with buffers larger than 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits.
- -fstack-protector-all
- Like -fstack-protector except that all functions are protected.
- -fstack-protector-strong
- Like -fstack-protector but includes additional functions to be protected --- those that have local array definitions, or have references to local frame addresses.
- -fstack-protector-explicit
- Like -fstack-protector but only protects those functions which have the "stack_protect" attribute.
- -fstack-check
- Generate code to verify that you do not go beyond the boundary of the
stack. You should specify this flag if you are running in an environment
with multiple threads, but you only rarely need to specify it in a
single-threaded environment since stack overflow is automatically detected
on nearly all systems if there is only one stack.
Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended.
You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, specific means use the best checking method and is equivalent to bare -fstack-check.
Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks:
- 1.
- Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code.
- 2.
- Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
- 3.
- Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered.
Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.
-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code. It is not generally sufficient to protect against stack-clash attacks. To protect against those you want -fstack-clash-protection.
- -fstack-clash-protection
- Generate code to prevent stack clash style attacks. When this option is
enabled, the compiler will only allocate one page of stack space at a time
and each page is accessed immediately after allocation. Thus, it prevents
allocations from jumping over any stack guard page provided by the
operating system.
Most targets do not fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. -fstack-clash-protection may also provide limited protection for static stack allocations if the target supports -fstack-check=specific.
- -fstack-limit-register=reg
- -fstack-limit-symbol=sym
- -fno-stack-limit
- Generate code to ensure that the stack does not grow beyond a certain
value, either the value of a register or the address of a symbol. If a
larger stack is required, a signal is raised at run time. For most
targets, the signal is raised before the stack overruns the boundary, so
it is possible to catch the signal without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.
You can locally override stack limit checking by using the "no_stack_limit" function attribute.
- -fsplit-stack
- Generate code to automatically split the stack before it overflows. The
resulting program has a discontiguous stack which can only overflow if the
program is unable to allocate any more memory. This is most useful when
running threaded programs, as it is no longer necessary to calculate a
good stack size to use for each thread. This is currently only implemented
for the x86 targets running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run. If compiling all code, including library code, with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later.
- -fvtable-verify=[std|preinit|none]
- This option is only available when compiling C++ code. It turns on (or
off, if using -fvtable-verify=none) the security feature that
verifies at run time, for every virtual call, that the vtable pointer
through which the call is made is valid for the type of the object, and
has not been corrupted or overwritten. If an invalid vtable pointer is
detected at run time, an error is reported and execution of the program is
immediately halted.
This option causes run-time data structures to be built at program startup, which are used for verifying the vtable pointers. The options std and preinit control the timing of when these data structures are built. In both cases the data structures are built before execution reaches "main". Using -fvtable-verify=std causes the data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have been loaded and initialized.
If this option appears multiple times in the command line with different values specified, none takes highest priority over both std and preinit; preinit takes priority over std.
- -fvtv-debug
- When used in conjunction with -fvtable-verify=std or
-fvtable-verify=preinit, causes debug versions of the runtime
functions for the vtable verification feature to be called. This flag also
causes the compiler to log information about which vtable pointers it
finds for each class. This information is written to a file named
vtv_set_ptr_data.log in the directory named by the environment
variable VTV_LOGS_DIR if that is defined or the current working
directory otherwise.
Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one.
- -fvtv-counts
- This is a debugging flag. When used in conjunction with
-fvtable-verify=std or -fvtable-verify=preinit, this causes
the compiler to keep track of the total number of virtual calls it
encounters and the number of verifications it inserts. It also counts the
number of calls to certain run-time library functions that it inserts and
logs this information for each compilation unit. The compiler writes this
information to a file named vtv_count_data.log in the directory
named by the environment variable VTV_LOGS_DIR if that is defined
or the current working directory otherwise. It also counts the size of the
vtable pointer sets for each class, and writes this information to
vtv_class_set_sizes.log in the same directory.
Note: This feature appends data to the log files. To get fresh log files, be sure to delete any existing ones.
- -finstrument-functions
- Generate instrumentation calls for entry and exit to functions. Just after
function entry and just before function exit, the following profiling
functions are called with the address of the current function and its call
site. (On some platforms,
"__builtin_return_address" does not work
beyond the current function, so the call site information may not be
available to the profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn, void *call_site); void __cyg_profile_func_exit (void *this_fn, void *call_site);
The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use "extern inline" in your C code, an addressable version of such functions must be provided. (This is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)
A function may be given the attribute "no_instrument_function", in which case this instrumentation is not done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory).
- -finstrument-functions-exclude-file-list=file,file,...
- Set the list of functions that are excluded from instrumentation (see the
description of -finstrument-functions). If the file that contains a
function definition matches with one of file, then that function is
not instrumented. The match is done on substrings: if the file
parameter is a substring of the file name, it is considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.
If, for some reason, you want to include letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option).
- -finstrument-functions-exclude-function-list=sym,sym,...
- This is similar to -finstrument-functions-exclude-file-list, but this option sets the list of function names to be excluded from instrumentation. The function name to be matched is its user-visible name, such as "vector<int> blah(const vector<int> &)", not the internal mangled name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). The match is done on substrings: if the sym parameter is a substring of the function name, it is considered to be a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names.
- -fpatchable-function-entry=N[,M]
- Generate N NOPs right at the beginning of each function, with the
function entry point before the Mth NOP. If M is omitted, it
defaults to 0 so the function entry points to the
address just at the first NOP. The NOP instructions reserve extra space
which can be used to patch in any desired instrumentation at run time,
provided that the code segment is writable. The amount of space is
controllable indirectly via the number of NOPs; the NOP instruction used
corresponds to the instruction emitted by the internal GCC back-end
interface "gen_nop". This behavior is
target-specific and may also depend on the architecture variant and/or
other compilation options.
For run-time identification, the starting addresses of these areas, which correspond to their respective function entries minus M, are additionally collected in the "__patchable_function_entries" section of the resulting binary.
Note that the value of "__attribute__ ((patchable_function_entry (N,M)))" takes precedence over command-line option -fpatchable-function-entry=N,M. This can be used to increase the area size or to remove it completely on a single function. If "N=0", no pad location is recorded.
The NOP instructions are inserted at---and maybe before, depending on M---the function entry address, even before the prologue.
Options Controlling the Preprocessor¶
These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E because they cause the preprocessor output to be unsuitable for actual compilation.
In addition to the options listed here, there are a number of options to control search paths for include files documented in Directory Options. Options to control preprocessor diagnostics are listed in Warning Options.
- -D name
- Predefine name as a macro, with definition 1.
- -D name=definition
- The contents of definition are tokenized and processed as if they
appeared during translation phase three in a #define directive. In
particular, the definition is truncated by embedded newline characters.
If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you should quote the option. With sh and csh, -D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are processed after all -D and -U options.
- -U name
- Cancel any previous definition of name, either built in or provided with a -D option.
- -include file
- Process file as if "#include
"file"" appeared as the first line of the primary
source file. However, the first directory searched for file is the
preprocessor's working directory instead of the directory
containing the main source file. If not found there, it is searched for in
the remainder of the "#include
"..."" search chain as normal.
If multiple -include options are given, the files are included in the order they appear on the command line.
- -imacros file
- Exactly like -include, except that any output produced by scanning
file is thrown away. Macros it defines remain defined. This allows
you to acquire all the macros from a header without also processing its
declarations.
All files specified by -imacros are processed before all files specified by -include.
- -undef
- Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.
- -pthread
- Define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.
- -M
- Instead of outputting the result of preprocessing, output a rule suitable
for make describing the dependencies of the main source file. The
preprocessor outputs one make rule containing the object file name
for that source file, a colon, and the names of all the included files,
including those coming from -include or -imacros
command-line options.
Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files then the rule is split into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.
- -MM
- Like -M but do not mention header files that are found in system
header directories, nor header files that are included, directly or
indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header appears in -MM dependency output.
- -MF file
- When used with -M or -MM, specifies a file to write the
dependencies to. If no -MF switch is given the preprocessor sends
the rules to the same place it would send preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
If file is -, then the dependencies are written to stdout.
- -MG
- In conjunction with an option such as -M requesting dependency
generation, -MG assumes missing header files are generated files
and adds them to the dependency list without raising an error. The
dependency filename is taken directly from the
"#include" directive without prepending
any path. -MG also suppresses preprocessed output, as a missing
header file renders this useless.
This feature is used in automatic updating of makefiles.
- -MP
- This option instructs CPP to add a phony target for each dependency other
than the main file, causing each to depend on nothing. These dummy rules
work around errors make gives if you remove header files without
updating the Makefile to match.
This is typical output:
test.o: test.c test.h test.h:
- -MT target
- Change the target of the rule emitted by dependency generation. By default
CPP takes the name of the main input file, deletes any directory
components and any file suffix such as .c, and appends the
platform's usual object suffix. The result is the target.
An -MT option sets the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
- -MQ target
- Same as -MT, but it quotes any characters which are special to
Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with -MQ.
- -MD
- -MD is equivalent to -M -MF file, except that
-E is not implied. The driver determines file based on
whether an -o option is given. If it is, the driver uses its
argument but with a suffix of .d, otherwise it takes the name of
the input file, removes any directory components and suffix, and applies a
.d suffix.
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file, but if used without -E, each -o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process.
- -MMD
- Like -MD except mention only user header files, not system header files.
- -fpreprocessed
- Indicate to the preprocessor that the input file has already been
preprocessed. This suppresses things like macro expansion, trigraph
conversion, escaped newline splicing, and processing of most directives.
The preprocessor still recognizes and removes comments, so that you can
pass a file preprocessed with -C to the compiler without problems.
In this mode the integrated preprocessor is little more than a tokenizer
for the front ends.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the extensions that GCC uses for preprocessed files created by -save-temps.
- -fdirectives-only
- When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options.
With -E, preprocessing is limited to the handling of directives such as "#define", "#ifdef", and "#error". Other preprocessor operations, such as macro expansion and trigraph conversion are not performed. In addition, the -dD option is implicitly enabled.
With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as "__LINE__", which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with "-E -fdirectives-only".
With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full preprocessing of files previously preprocessed with "-E -fdirectives-only".
- -fdollars-in-identifiers
- Accept $ in identifiers.
- -fextended-identifiers
- Accept universal character names in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++.
- -fno-canonical-system-headers
- When preprocessing, do not shorten system header paths with canonicalization.
- -ftabstop=width
- Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8.
- -ftrack-macro-expansion[=level]
- Track locations of tokens across macro expansions. This allows the
compiler to emit diagnostic about the current macro expansion stack when a
compilation error occurs in a macro expansion. Using this option makes the
preprocessor and the compiler consume more memory. The level
parameter can be used to choose the level of precision of token location
tracking thus decreasing the memory consumption if necessary. Value
0 of level de-activates this option. Value 1 tracks
tokens locations in a degraded mode for the sake of minimal memory
overhead. In this mode all tokens resulting from the expansion of an
argument of a function-like macro have the same location. Value 2
tracks tokens locations completely. This value is the most memory hungry.
When this option is given no argument, the default parameter value is
2.
Note that "-ftrack-macro-expansion=2" is activated by default.
- -fmacro-prefix-map=old=new
- When preprocessing files residing in directory old, expand the "__FILE__" and "__BASE_FILE__" macros as if the files resided in directory new instead. This can be used to change an absolute path to a relative path by using . for new which can result in more reproducible builds that are location independent. This option also affects "__builtin_FILE()" during compilation. See also -ffile-prefix-map.
- -fexec-charset=charset
- Set the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system's "iconv" library routine.
- -fwide-exec-charset=charset
- Set the wide execution character set, used for wide string and character constants. The default is UTF-32 or UTF-16, whichever corresponds to the width of "wchar_t". As with -fexec-charset, charset can be any encoding supported by the system's "iconv" library routine; however, you will have problems with encodings that do not fit exactly in "wchar_t".
- -finput-charset=charset
- Set the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default is UTF-8. This can be overridden by either the locale or this command-line option. Currently the command-line option takes precedence if there's a conflict. charset can be any encoding supported by the system's "iconv" library routine.
- -fpch-deps
- When using precompiled headers, this flag causes the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified, only the precompiled header are listed and not the files that were used to create it, because those files are not consulted when a precompiled header is used.
- -fpch-preprocess
- This option allows use of a precompiled header together with -E. It
inserts a special "#pragma",
"#pragma GCC pch_preprocess
"filename""
in the output to mark the place where the precompiled header was found,
and its filename. When -fpreprocessed is in use, GCC
recognizes this "#pragma" and loads the
PCH.
This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by -save-temps.
You should not write this "#pragma" in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory.
- -fworking-directory
- Enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no "#line" directives are emitted whatsoever.
- -A predicate=answer
- Make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A predicate(answer), which is still supported, because it does not use shell special characters.
- -A -predicate=answer
- Cancel an assertion with the predicate predicate and answer answer.
- -C
- Do not discard comments. All comments are passed through to the output
file, except for comments in processed directives, which are deleted along
with the directive.
You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #.
- -CC
- Do not discard comments, including during macro expansion. This is like
-C, except that comments contained within macros are also passed
through to the output file where the macro is expanded.
In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
- -P
- Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.
- -traditional
- -traditional-cpp
- Try to imitate the behavior of pre-standard C preprocessors, as opposed to
ISO C preprocessors. See the GNU CPP manual for details.
Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly.
- -trigraphs
- Support ISO C trigraphs. These are three-character sequences, all starting
with ??, that are defined by ISO C to stand for single characters.
For example, ??/ stands for \, so '??/n' is a
character constant for a newline.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??- Replacement: [ ] { } # \ ^ | ~
By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options.
- -remap
- Enable special code to work around file systems which only permit very short file names, such as MS-DOS.
- -H
- Print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the #include stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! .
- -dletters
- Says to make debugging dumps during compilation as specified by letters. The flags documented here are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined.
- -dM
- Instead of the normal output, generate a list of #define directives
for all the macros defined during the execution of the preprocessor,
including predefined macros. This gives you a way of finding out what is
predefined in your version of the preprocessor. Assuming you have no file
foo.h, the command
touch foo.h; cpp -dM foo.h
shows all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach.
- -dD
- Like -dM except in two respects: it does not include the predefined macros, and it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file.
- -dN
- Like -dD, but emit only the macro names, not their expansions.
- -dI
- Output #include directives in addition to the result of preprocessing.
- -dU
- Like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and #undef directives are also output for macros tested but undefined at the time.
- -fdebug-cpp
- This option is only useful for debugging GCC. When used from CPP or with
-E, it dumps debugging information about location maps. Every token
in the output is preceded by the dump of the map its location belongs to.
When used from GCC without -E, this option has no effect.
- -Wp,option
- You can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead.
- -Xpreprocessor option
- Pass option as an option to the preprocessor. You can use this to
supply system-specific preprocessor options that GCC does not recognize.
If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the option and once for the argument.
- -no-integrated-cpp
- Perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice, once for preprocessing only and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation.
Passing Options to the Assembler¶
You can pass options to the assembler.
- -Wa,option
- Pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas.
- -Xassembler option
- Pass option as an option to the assembler. You can use this to
supply system-specific assembler options that GCC does not recognize.
If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option and once for the argument.
Options for Linking¶
These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.
- object-file-name
- A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker.
- -c
- -S
- -E
- If any of these options is used, then the linker is not run, and object file names should not be used as arguments.
- -fuse-ld=bfd
- Use the bfd linker instead of the default linker.
- -fuse-ld=gold
- Use the gold linker instead of the default linker.
- -fuse-ld=lld
- Use the LLVM lld linker instead of the default linker.
- -llibrary
- -l library
- Search the library named library when linking. (The second
alternative with the library as a separate argument is only for POSIX
compliance and is not recommended.)
It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded.
The linker searches a standard list of directories for the library, which is actually a file named liblibrary.a. The linker then uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories plus any that you specify with -L.
Normally the files found this way are library files---archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fashion. The only difference between using an -l option and specifying a file name is that -l surrounds library with lib and .a and searches several directories.
- -lobjc
- You need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
- -nostartfiles
- Do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib or -nodefaultlibs is used.
- -nodefaultlibs
- Do not use the standard system libraries when linking. Only the libraries
you specify are passed to the linker, and options specifying linkage of
the system libraries, such as -static-libgcc or
-shared-libgcc, are ignored. The standard startup files are used
normally, unless -nostartfiles is used.
The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove". These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.
- -nostdlib
- Do not use the standard system startup files or libraries when linking. No
startup files and only the libraries you specify are passed to the linker,
and options specifying linkage of the system libraries, such as
-static-libgcc or -shared-libgcc, are ignored.
The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove". These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages.
In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is "__main", used to ensure C++ constructors are called.)
- -pie
- Produce a dynamically linked position independent executable on targets that support it. For predictable results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option.
- -no-pie
- Don't produce a dynamically linked position independent executable.
- -static-pie
- Produce a static position independent executable on targets that support it. A static position independent executable is similar to a static executable, but can be loaded at any address without a dynamic linker. For predictable results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option.
- -pthread
- Link with the POSIX threads library. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. On some targets this option also sets flags for the preprocessor, so it should be used consistently for both compilation and linking.
- -rdynamic
- Pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of "dlopen" or to allow obtaining backtraces from within a program.
- -s
- Remove all symbol table and relocation information from the executable.
- -static
- On systems that support dynamic linking, this overrides -pie and prevents linking with the shared libraries. On other systems, this option has no effect.
- Produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options used for compilation (-fpic, -fPIC, or model suboptions) when you specify this linker option.[1]
- -static-libgcc
- On systems that provide libgcc as a shared library, these options
force the use of either the shared or static version, respectively. If no
shared version of libgcc was built when the compiler was
configured, these options have no effect.
There are several situations in which an application should use the shared libgcc instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc.
Therefore, the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option -shared-libgcc, such that it is linked with the shared libgcc.
- -static-libasan
- When the -fsanitize=address option is used to link a program, the GCC driver automatically links against libasan. If libasan is available as a shared library, and the -static option is not used, then this links against the shared version of libasan. The -static-libasan option directs the GCC driver to link libasan statically, without necessarily linking other libraries statically.
- -static-libtsan
- When the -fsanitize=thread option is used to link a program, the GCC driver automatically links against libtsan. If libtsan is available as a shared library, and the -static option is not used, then this links against the shared version of libtsan. The -static-libtsan option directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically.
- -static-liblsan
- When the -fsanitize=leak option is used to link a program, the GCC driver automatically links against liblsan. If liblsan is available as a shared library, and the -static option is not used, then this links against the shared version of liblsan. The -static-liblsan option directs the GCC driver to link liblsan statically, without necessarily linking other libraries statically.
- -static-libubsan
- When the -fsanitize=undefined option is used to link a program, the GCC driver automatically links against libubsan. If libubsan is available as a shared library, and the -static option is not used, then this links against the shared version of libubsan. The -static-libubsan option directs the GCC driver to link libubsan statically, without necessarily linking other libraries statically.
- -static-libmpx
- When the -fcheck-pointer bounds and -mmpx options are used to link a program, the GCC driver automatically links against libmpx. If libmpx is available as a shared library, and the -static option is not used, then this links against the shared version of libmpx. The -static-libmpx option directs the GCC driver to link libmpx statically, without necessarily linking other libraries statically.
- -static-libmpxwrappers
- When the -fcheck-pointer bounds and -mmpx options are used to link a program without also using -fno-chkp-use-wrappers, the GCC driver automatically links against libmpxwrappers. If libmpxwrappers is available as a shared library, and the -static option is not used, then this links against the shared version of libmpxwrappers. The -static-libmpxwrappers option directs the GCC driver to link libmpxwrappers statically, without necessarily linking other libraries statically.
- -static-libstdc++
- When the g++ program is used to link a C++ program, it normally automatically links against libstdc++. If libstdc++ is available as a shared library, and the -static option is not used, then this links against the shared version of libstdc++. That is normally fine. However, it is sometimes useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link. The -static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically.
- -symbolic
- Bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option.
- -T script
- Use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the -T option may be required when linking to avoid references to undefined symbols.
- -Xlinker option
- Pass option as an option to the linker. You can use this to supply
system-specific linker options that GCC does not recognize.
If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects.
When using the GNU linker, it is usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments. For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this syntax for command-line options.
- -Wl,option
- Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map output.map to the linker. When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map.
- -u symbol
- Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u multiple times with different symbols to force loading of additional library modules.
- -z keyword
- -z is passed directly on to the linker along with the keyword keyword. See the section in the documentation of your linker for permitted values and their meanings.
Options for Directory Search¶
These options specify directories to search for header files, for libraries and for parts of the compiler:
- -I dir
- -iquote dir
- -isystem dir
- -idirafter dir
- Add the directory dir to the list of directories to be searched for
header files during preprocessing. If dir begins with = or
$SYSROOT, then the = or
$SYSROOT is replaced by the sysroot prefix; see
--sysroot and -isysroot.
Directories specified with -iquote apply only to the quote form of the directive, "#include "file"". Directories specified with -I, -isystem, or -idirafter apply to lookup for both the "#include "file"" and "#include <file>" directives.
You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:
- 1.
- For the quote form of the include directive, the directory of the current file is searched first.
- 2.
- For the quote form of the include directive, the directories specified by -iquote options are searched in left-to-right order, as they appear on the command line.
- 3.
- Directories specified with -I options are scanned in left-to-right order.
- 4.
- Directories specified with -isystem options are scanned in left-to-right order.
- 5.
- Standard system directories are scanned.
- 6.
- Directories specified with -idirafter options are scanned in left-to-right order.
You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files; use -isystem for that.
The -isystem and -idirafter options also mark the directory as a system directory, so that it gets the same special treatment that is applied to the standard system directories.
If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the "#include_next" directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options.
- -I-
- Split the include path. This option has been deprecated. Please use
-iquote instead for -I directories before the -I- and
remove the -I- option.
Any directories specified with -I options before -I- are searched only for headers requested with "#include "file""; they are not searched for "#include <file>". If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for "#include "file"". There is no way to override this effect of -I-.
- -iprefix prefix
- Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /.
- -iwithprefix dir
- -iwithprefixbefore dir
- Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would.
- -isysroot dir
- This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information.
- -imultilib dir
- Use dir as a subdirectory of the directory containing target-specific C++ headers.
- -nostdinc
- Do not search the standard system directories for header files. Only the directories explicitly specified with -I, -iquote, -isystem, and/or -idirafter options (and the directory of the current file, if appropriate) are searched.
- -nostdinc++
- Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.)
- -iplugindir=dir
- Set the directory to search for plugins that are passed by -fplugin=name instead of -fplugin=path/name.so. This option is not meant to be used by the user, but only passed by the driver.
- -Ldir
- Add directory dir to the list of directories to be searched for -l.
- -Bprefix
- This option specifies where to find the executables, libraries, include
files, and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/ for the corresponding target machine and compiler version.
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.
The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include. This is to help with boot-strapping the compiler.
- -no-canonical-prefixes
- Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix.
- --sysroot=dir
- Use dir as the logical root directory for headers and libraries.
For example, if the compiler normally searches for headers in
/usr/include and libraries in /usr/lib, it instead searches
dir/usr/include and dir/usr/lib.
If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not.
- --no-sysroot-suffix
- For some targets, a suffix is added to the root directory specified with --sysroot, depending on the other options used, so that headers may for example be found in dir/suffix/usr/include instead of dir/usr/include. This option disables the addition of such a suffix.
Options for Code Generation Conventions¶
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one that is not the default. You can figure out the other form by either removing no- or adding it.
- -fstack-reuse=reuse-level
- This option controls stack space reuse for user declared local/auto
variables and compiler generated temporaries. reuse_level can be
all, named_vars, or none. all enables stack
reuse for all local variables and temporaries, named_vars enables
the reuse only for user defined local variables with names, and
none disables stack reuse completely. The default value is
all. The option is needed when the program extends the lifetime of
a scoped local variable or a compiler generated temporary beyond the end
point defined by the language. When a lifetime of a variable ends, and if
the variable lives in memory, the optimizing compiler has the freedom to
reuse its stack space with other temporaries or scoped local variables
whose live range does not overlap with it. Legacy code extending local
lifetime is likely to break with the stack reuse optimization.
For example,
int *p; { int local1; p = &local1; local1 = 10; .... } { int local2; local2 = 20; ... } if (*p == 10) // out of scope use of local1 { }
Another example:
struct A { A(int k) : i(k), j(k) { } int i; int j; }; A *ap; void foo(const A& ar) { ap = &ar; } void bar() { foo(A(10)); // temp object's lifetime ends when foo returns { A a(20); .... } ap->i+= 10; // ap references out of scope temp whose space // is reused with a. What is the value of ap->i? }
The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization.
- -ftrapv
- This option generates traps for signed overflow on addition, subtraction, multiplication operations. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective.
- -fwrapv
- This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective.
- -fwrapv-pointer
- This option instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. This flag disables some optimizations which assume pointer overflow is invalid.
- -fstrict-overflow
- This option implies -fno-wrapv -fno-wrapv-pointer and when negated implies -fwrapv -fwrapv-pointer.
- -fexceptions
- Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCC generates frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution. If you do not specify this option, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that do not normally require it. However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling.
- -fnon-call-exceptions
- Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as "SIGALRM".
- -fdelete-dead-exceptions
- Consider that instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away. This option is enabled by default for the Ada front end, as permitted by the Ada language specification. Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels.
- -funwind-tables
- Similar to -fexceptions, except that it just generates any needed static data, but does not affect the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf.
- -fasynchronous-unwind-tables
- Generate unwind table in DWARF format, if supported by target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector).
- -fno-gnu-unique
- On systems with recent GNU assembler and C library, the C++ compiler uses the "STB_GNU_UNIQUE" binding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence of "RTLD_LOCAL"; this is necessary to avoid problems with a library used by two different "RTLD_LOCAL" plugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol. But this causes "dlclose" to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via "dlclose" and "dlopen", you can use -fno-gnu-unique.
- -fpcc-struct-return
- Return "short" "struct" and
"union" values in memory like longer
ones, rather than in registers. This convention is less efficient, but it
has the advantage of allowing intercallability between GCC-compiled files
and files compiled with other compilers, particularly the Portable C
Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface.
- -freg-struct-return
- Return "struct" and
"union" values in registers when
possible. This is more efficient for small structures than
-fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface.
- -fshort-enums
- Allocate to an "enum" type only as many
bytes as it needs for the declared range of possible values. Specifically,
the "enum" type is equivalent to the
smallest integer type that has enough room.
Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
- -fshort-wchar
- Override the underlying type for
"wchar_t" to be
"short unsigned
int" instead of the default for the target. This option is
useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
- -fno-common
- In C code, this option controls the placement of global variables defined
without an initializer, known as tentative definitions in the C
standard. Tentative definitions are distinct from declarations of a
variable with the "extern" keyword,
which do not allocate storage.
Unix C compilers have traditionally allocated storage for uninitialized global variables in a common block. This allows the linker to resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. This is the behavior specified by -fcommon, and is the default for GCC on most targets. On the other hand, this behavior is not required by ISO C, and on some targets may carry a speed or code size penalty on variable references.
The -fno-common option specifies that the compiler should instead place uninitialized global variables in the data section of the object file. This inhibits the merging of tentative definitions by the linker so you get a multiple-definition error if the same variable is defined in more than one compilation unit. Compiling with -fno-common is useful on targets for which it provides better performance, or if you wish to verify that the program will work on other systems that always treat uninitialized variable definitions this way.
- -fno-ident
- Ignore the "#ident" directive.
- -finhibit-size-directive
- Don't output a ".size" assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling crtstuff.c; you should not need to use it for anything else.
- -fverbose-asm
- Put extra commentary information in the generated assembly code to make it
more readable. This option is generally only of use to those who actually
need to read the generated assembly code (perhaps while debugging the
compiler itself).
-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files.
The added comments include:
- information on the compiler version and command-line options,
- the source code lines associated with the assembly instructions, in the form FILENAME:LINENUMBER:CONTENT OF LINE,
- hints on which high-level expressions correspond to the various assembly instruction operands.
For example, given this C source file:
int test (int n) { int i; int total = 0; for (i = 0; i < n; i++) total += i * i; return total; }
compiling to (x86_64) assembly via -S and emitting the result direct to stdout via -o -
gcc -S test.c -fverbose-asm -Os -o -
gives output similar to this:
.file "test.c" # GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu) [...snip...] # options passed: [...snip...] .text .globl test .type test, @function test: .LFB0: .cfi_startproc # test.c:4: int total = 0; xorl %eax, %eax # <retval> # test.c:6: for (i = 0; i < n; i++) xorl %edx, %edx # i .L2: # test.c:6: for (i = 0; i < n; i++) cmpl %edi, %edx # n, i jge .L5 #, # test.c:7: total += i * i; movl %edx, %ecx # i, tmp92 imull %edx, %ecx # i, tmp92 # test.c:6: for (i = 0; i < n; i++) incl %edx # i # test.c:7: total += i * i; addl %ecx, %eax # tmp92, <retval> jmp .L2 # .L5: # test.c:10: } ret .cfi_endproc .LFE0: .size test, .-test .ident "GCC: (GNU) 7.0.0 20160809 (experimental)" .section .note.GNU-stack,"",@progbits
The comments are intended for humans rather than machines and hence the precise format of the comments is subject to change.
- -frecord-gcc-switches
- This switch causes the command line used to invoke the compiler to be recorded into the object file that is being created. This switch is only implemented on some targets and the exact format of the recording is target and binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is related to the -fverbose-asm switch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also -grecord-gcc-switches for another way of storing compiler options into the object file.
- -fpic
- Generate position-independent code (PIC) suitable for use in a shared
library, if supported for the target machine. Such code accesses all
constant addresses through a global offset table (GOT). The dynamic loader
resolves the GOT entries when the program starts (the dynamic loader is
not part of GCC; it is part of the operating system). If the GOT size for
the linked executable exceeds a machine-specific maximum size, you get an
error message from the linker indicating that -fpic does not work;
in that case, recompile with -fPIC instead. (These maximums are 8k
on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has
no such limit.)
Position-independent code requires special support, and therefore works only on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.
When this flag is set, the macros "__pic__" and "__PIC__" are defined to 1.
- -fPIC
- If supported for the target machine, emit position-independent code,
suitable for dynamic linking and avoiding any limit on the size of the
global offset table. This option makes a difference on AArch64, m68k,
PowerPC and SPARC.
Position-independent code requires special support, and therefore works only on certain machines.
When this flag is set, the macros "__pic__" and "__PIC__" are defined to 2.
- -fpie
- -fPIE
- These options are similar to -fpic and -fPIC, but generated
position independent code can be only linked into executables. Usually
these options are used when -pie GCC option is used during linking.
-fpie and -fPIE both define the macros "__pie__" and "__PIE__". The macros have the value 1 for -fpie and 2 for -fPIE.
- -fno-plt
- Do not use the PLT for external function calls in position-independent
code. Instead, load the callee address at call sites from the GOT and
branch to it. This leads to more efficient code by eliminating PLT stubs
and exposing GOT loads to optimizations. On architectures such as 32-bit
x86 where PLT stubs expect the GOT pointer in a specific register, this
gives more register allocation freedom to the compiler. Lazy binding
requires use of the PLT; with -fno-plt all external symbols are
resolved at load time.
Alternatively, the function attribute "noplt" can be used to avoid calls through the PLT for specific external functions.
In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.
- -fno-jump-tables
- Do not use jump tables for switch statements even where it would be more efficient than other code generation strategies. This option is of use in conjunction with -fpic or -fPIC for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed.
- -ffixed-reg
- Treat the register named reg as a fixed register; generated code
should never refer to it (except perhaps as a stack pointer, frame pointer
or in some other fixed role).
reg must be the name of a register. The register names accepted are machine-specific and are defined in the "REGISTER_NAMES" macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice.
- -fcall-used-reg
- Treat the register named reg as an allocable register that is
clobbered by function calls. It may be allocated for temporaries or
variables that do not live across a call. Functions compiled this way do
not save and restore the register reg.
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.
This flag does not have a negative form, because it specifies a three-way choice.
- -fcall-saved-reg
- Treat the register named reg as an allocable register saved by
functions. It may be allocated even for temporaries or variables that live
across a call. Functions compiled this way save and restore the register
reg if they use it.
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.
A different sort of disaster results from the use of this flag for a register in which function values may be returned.
This flag does not have a negative form, because it specifies a three-way choice.
- -fpack-struct[=n]
- Without a value specified, pack all structure members together without
holes. When a value is specified (which must be a small power of two),
pack structure members according to this value, representing the maximum
alignment (that is, objects with default alignment requirements larger
than this are output potentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.
- -fleading-underscore
- This option and its counterpart, -fno-leading-underscore, forcibly
change the way C symbols are represented in the object file. One use is to
help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.
- -ftls-model=model
- Alter the thread-local storage model to be used. The model argument
should be one of global-dynamic, local-dynamic,
initial-exec or local-exec. Note that the choice is subject
to optimization: the compiler may use a more efficient model for symbols
not visible outside of the translation unit, or if -fpic is not
given on the command line.
The default without -fpic is initial-exec; with -fpic the default is global-dynamic.
- -ftrampolines
- For targets that normally need trampolines for nested functions, always
generate them instead of using descriptors. Otherwise, for targets that do
not need them, like for example HP-PA or IA-64, do nothing.
A trampoline is a small piece of code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. Therefore, it requires the stack to be made executable in order for the program to work properly.
-fno-trampolines is enabled by default on a language by language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada.
Moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if nested functions are present. This option must therefore be used on a program-wide basis and be manipulated with extreme care.
- -fvisibility=[default|internal|hidden|protected]
- Set the default ELF image symbol visibility to the specified option---all
symbols are marked with this unless overridden within the code. Using this
feature can very substantially improve linking and load times of shared
object libraries, produce more optimized code, provide near-perfect API
export and prevent symbol clashes. It is strongly recommended that
you use this in any shared objects you distribute.
Despite the nomenclature, default always means public; i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage so the only other commonly used option is hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public.
A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by "How To Write Shared Libraries" by Ulrich Drepper (which can be found at <https://www.akkadia.org/drepper/>)---however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLLs on Windows and with -fvisibility=hidden and "__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)" you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects.
For those adding visibility support to existing code, you may find "#pragma GCC visibility" of use. This works by you enclosing the declarations you wish to set visibility for with (for example) "#pragma GCC visibility push(hidden)" and "#pragma GCC visibility pop". Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads---making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, "operator new" and "operator delete" must always be of default visibility.
Be aware that headers from outside your project, in particular system headers and headers from any other library you use, may not be expecting to be compiled with visibility other than the default. You may need to explicitly say "#pragma GCC visibility push(default)" before including any such headers.
"extern" declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden with no modifications. However, this means that calls to "extern" functions with no explicit visibility use the PLT, so it is more effective to use "__attribute ((visibility))" and/or "#pragma GCC visibility" to tell the compiler which "extern" declarations should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception class that is be thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes are unified between the DSOs.
An overview of these techniques, their benefits and how to use them is at <http://gcc.gnu.org/wiki/Visibility>.
- -fstrict-volatile-bitfields
- This option should be used if accesses to volatile bit-fields (or other
structure fields, although the compiler usually honors those types anyway)
should use a single access of the width of the field's type, aligned to a
natural alignment if possible. For example, targets with memory-mapped
peripheral registers might require all such accesses to be 16 bits wide;
with this flag you can declare all peripheral bit-fields as
"unsigned short" (assuming short is 16
bits on these targets) to force GCC to use 16-bit accesses instead of,
perhaps, a more efficient 32-bit access.
If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated.
In some cases, such as when the "packed" attribute is applied to a structure field, it may not be possible to access the field with a single read or write that is correctly aligned for the target machine. In this case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time.
Note: Due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. It is therefore recommended to define all bits of the field's type as bit-field members.
The default value of this option is determined by the application binary interface for the target processor.
- -fsync-libcalls
- This option controls whether any out-of-line instance of the
"__sync" family of functions may be used
to implement the C++11 "__atomic" family
of functions.
The default value of this option is enabled, thus the only useful form of the option is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library.
GCC Developer Options¶
This section describes command-line options that are primarily of interest to GCC developers, including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. This includes options that produce debug dumps at various points in the compilation; that print statistics such as memory use and execution time; and that print information about GCC's configuration, such as where it searches for libraries. You should rarely need to use any of these options for ordinary compilation and linking tasks.
- -dletters
- -fdump-rtl-pass
- -fdump-rtl-pass=filename
- Says to make debugging dumps during compilation at times specified by
letters. This is used for debugging the RTL-based passes of the
compiler. The file names for most of the dumps are made by appending a
pass number and a word to the dumpname, and the files are created
in the directory of the output file. In case of =filename
option, the dump is output on the given file instead of the pass numbered
dump files. Note that the pass number is assigned as passes are registered
into the pass manager. Most passes are registered in the order that they
will execute and for these passes the number corresponds to the pass
execution order. However, passes registered by plugins, passes specific to
compilation targets, or passes that are otherwise registered after all the
other passes are numbered higher than a pass named "final", even
if they are executed earlier. dumpname is generated from the name
of the output file if explicitly specified and not an executable,
otherwise it is the basename of the source file.
Some -dletters switches have different meaning when -E is used for preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for use in pass and letters, and their meanings:
- -fdump-rtl-alignments
- Dump after branch alignments have been computed.
- -fdump-rtl-asmcons
- Dump after fixing rtl statements that have unsatisfied in/out constraints.
- -fdump-rtl-auto_inc_dec
- Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.
- -fdump-rtl-barriers
- Dump after cleaning up the barrier instructions.
- -fdump-rtl-bbpart
- Dump after partitioning hot and cold basic blocks.
- -fdump-rtl-bbro
- Dump after block reordering.
- -fdump-rtl-btl1
- -fdump-rtl-btl2
- -fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.
- -fdump-rtl-bypass
- Dump after jump bypassing and control flow optimizations.
- -fdump-rtl-combine
- Dump after the RTL instruction combination pass.
- -fdump-rtl-compgotos
- Dump after duplicating the computed gotos.
- -fdump-rtl-ce1
- -fdump-rtl-ce2
- -fdump-rtl-ce3
- -fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.
- -fdump-rtl-cprop_hardreg
- Dump after hard register copy propagation.
- -fdump-rtl-csa
- Dump after combining stack adjustments.
- -fdump-rtl-cse1
- -fdump-rtl-cse2
- -fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes.
- -fdump-rtl-dce
- Dump after the standalone dead code elimination passes.
- -fdump-rtl-dbr
- Dump after delayed branch scheduling.
- -fdump-rtl-dce1
- -fdump-rtl-dce2
- -fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
- -fdump-rtl-eh
- Dump after finalization of EH handling code.
- -fdump-rtl-eh_ranges
- Dump after conversion of EH handling range regions.
- -fdump-rtl-expand
- Dump after RTL generation.
- -fdump-rtl-fwprop1
- -fdump-rtl-fwprop2
- -fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
- -fdump-rtl-gcse1
- -fdump-rtl-gcse2
- -fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
- -fdump-rtl-init-regs
- Dump after the initialization of the registers.
- -fdump-rtl-initvals
- Dump after the computation of the initial value sets.
- -fdump-rtl-into_cfglayout
- Dump after converting to cfglayout mode.
- -fdump-rtl-ira
- Dump after iterated register allocation.
- -fdump-rtl-jump
- Dump after the second jump optimization.
- -fdump-rtl-loop2
- -fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
- -fdump-rtl-mach
- Dump after performing the machine dependent reorganization pass, if that pass exists.
- -fdump-rtl-mode_sw
- Dump after removing redundant mode switches.
- -fdump-rtl-rnreg
- Dump after register renumbering.
- -fdump-rtl-outof_cfglayout
- Dump after converting from cfglayout mode.
- -fdump-rtl-peephole2
- Dump after the peephole pass.
- -fdump-rtl-postreload
- Dump after post-reload optimizations.
- -fdump-rtl-pro_and_epilogue
- Dump after generating the function prologues and epilogues.
- -fdump-rtl-sched1
- -fdump-rtl-sched2
- -fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.
- -fdump-rtl-ree
- Dump after sign/zero extension elimination.
- -fdump-rtl-seqabstr
- Dump after common sequence discovery.
- -fdump-rtl-shorten
- Dump after shortening branches.
- -fdump-rtl-sibling
- Dump after sibling call optimizations.
- -fdump-rtl-split1
- -fdump-rtl-split2
- -fdump-rtl-split3
- -fdump-rtl-split4
- -fdump-rtl-split5
- These options enable dumping after five rounds of instruction splitting.
- -fdump-rtl-sms
- Dump after modulo scheduling. This pass is only run on some architectures.
- -fdump-rtl-stack
- Dump after conversion from GCC's "flat register file" registers to the x87's stack-like registers. This pass is only run on x86 variants.
- -fdump-rtl-subreg1
- -fdump-rtl-subreg2
- -fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes.
- -fdump-rtl-unshare
- Dump after all rtl has been unshared.
- -fdump-rtl-vartrack
- Dump after variable tracking.
- -fdump-rtl-vregs
- Dump after converting virtual registers to hard registers.
- -fdump-rtl-web
- Dump after live range splitting.
- -fdump-rtl-regclass
- -fdump-rtl-subregs_of_mode_init
- -fdump-rtl-subregs_of_mode_finish
- -fdump-rtl-dfinit
- -fdump-rtl-dfinish
- These dumps are defined but always produce empty files.
- -da
- -fdump-rtl-all
- Produce all the dumps listed above.
- -dA
- Annotate the assembler output with miscellaneous debugging information.
- -dD
- Dump all macro definitions, at the end of preprocessing, in addition to normal output.
- -dH
- Produce a core dump whenever an error occurs.
- -dp
- Annotate the assembler output with a comment indicating which pattern and alternative is used. The length and cost of each instruction are also printed.
- -dP
- Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.
- -dx
- Just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand.
- -fdump-noaddr
- When doing debugging dumps, suppress address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations.
- -freport-bug
- Collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs.
- -fdump-unnumbered
- When doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g.
- -fdump-unnumbered-links
- When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence.
- -fdump-ipa-switch
- Control the dumping at various stages of inter-procedural analysis language tree to a file. The file name is generated by appending a switch specific suffix to the source file name, and the file is created in the same directory as the output file. The following dumps are possible:
- -fdump-lang-all
- -fdump-lang-switch
- -fdump-lang-switch-options
- -fdump-lang-switch-options=filename
- Control the dumping of language-specific information. The options and filename portions behave as described in the -fdump-tree option. The following switch values are accepted:
- -fdump-passes
- Print on stderr the list of optimization passes that are turned on and off by the current command-line options.
- -fdump-statistics-option
- Enable and control dumping of pass statistics in a separate file. The file name is generated by appending a suffix ending in .statistics to the source file name, and the file is created in the same directory as the output file. If the -option form is used, -stats causes counters to be summed over the whole compilation unit while -details dumps every event as the passes generate them. The default with no option is to sum counters for each function compiled.
- -fdump-tree-all
- -fdump-tree-switch
- -fdump-tree-switch-options
- -fdump-tree-switch-options=filename
- Control the dumping at various stages of processing the intermediate language tree to a file. The file name is generated by appending a switch-specific suffix to the source file name, and the file is created in the same directory as the output file. In case of =filename option, the dump is output on the given file instead of the auto named dump files. If the -options form is used, options is a list of - separated options which control the details of the dump. Not all options are applicable to all dumps; those that are not meaningful are ignored. The following options are available
- address
- Print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment.
- asmname
- If "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead of "DECL_NAME". Its primary use is ease of use working backward from mangled names in the assembly file.
- slim
- When dumping front-end intermediate representations, inhibit dumping of
members of a scope or body of a function merely because that scope has
been reached. Only dump such items when they are directly reachable by
some other path.
When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.
When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation.
- raw
- Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.
- details
- Enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes.
- stats
- Enable dumping various statistics about the pass (not honored by every dump option).
- blocks
- Enable showing basic block boundaries (disabled in raw dumps).
- graph
- For each of the other indicated dump files
(-fdump-rtl-pass), dump a representation of the control flow
graph suitable for viewing with GraphViz to
file.passid.pass.dot.
Each function in the file is pretty-printed as a subgraph, so that
GraphViz can render them all in a single plot.
This option currently only works for RTL dumps, and the RTL is always dumped in slim form.
- vops
- Enable showing virtual operands for every statement.
- lineno
- Enable showing line numbers for statements.
- uid
- Enable showing the unique ID ("DECL_UID") for each variable.
- verbose
- Enable showing the tree dump for each statement.
- eh
- Enable showing the EH region number holding each statement.
- scev
- Enable showing scalar evolution analysis details.
- optimized
- Enable showing optimization information (only available in certain passes).
- missed
- Enable showing missed optimization information (only available in certain passes).
- note
- Enable other detailed optimization information (only available in certain passes).
- =filename
- Instead of an auto named dump file, output into the given file name. The
file names stdout and stderr are treated specially and are
considered already open standard streams. For example,
gcc -O2 -ftree-vectorize -fdump-tree-vect-blocks=foo.dump -fdump-tree-pre=/dev/stderr file.c
outputs vectorizer dump into foo.dump, while the PRE dump is output on to stderr. If two conflicting dump filenames are given for the same pass, then the latter option overrides the earlier one.
- all
- Turn on all options, except raw, slim, verbose and lineno.
- optall
- Turn on all optimization options, i.e., optimized, missed, and note.
To determine what tree dumps are available or find the dump for a pass of interest follow the steps below.
- 1.
- Invoke GCC with -fdump-passes and in the stderr output look for a code that corresponds to the pass you are interested in. For example, the codes "tree-evrp", "tree-vrp1", and "tree-vrp2" correspond to the three Value Range Propagation passes. The number at the end distinguishes distinct invocations of the same pass.
- 2.
- To enable the creation of the dump file, append the pass code to the -fdump- option prefix and invoke GCC with it. For example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the -fdump-tree-evrp option. Optionally, you may specify the name of the dump file. If you don't specify one, GCC creates as described below.
- 3.
- Find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter t for tree passes (and the letter r for RTL passes), and finally the pass code. For example, the Early VRP pass dump might be in a file named myfile.c.038t.evrp in the current working directory. Note that the numeric codes are not stable and may change from one version of GCC to another.
- -fopt-info
- -fopt-info-options
- -fopt-info-options=filename
- Controls optimization dumps from various optimization passes. If the
-options form is used, options is a list of -
separated option keywords to select the dump details and optimizations.
The options can be divided into two groups: options describing the verbosity of the dump, and options describing which optimizations should be included. The options from both the groups can be freely mixed as they are non-overlapping. However, in case of any conflicts, the later options override the earlier options on the command line.
The following options control the dump verbosity:
- optimized
- Print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized.
- missed
- Print information about missed optimizations. Individual passes control which information to include in the output.
- note
- Print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.
- all
- Print detailed optimization information. This includes optimized, missed, and note.
One or more of the following option keywords can be used to describe a group of optimizations:
- ipa
- Enable dumps from all interprocedural optimizations.
- loop
- Enable dumps from all loop optimizations.
- inline
- Enable dumps from all inlining optimizations.
- omp
- Enable dumps from all OMP (Offloading and Multi Processing) optimizations.
- vec
- Enable dumps from all vectorization optimizations.
- optall
- Enable dumps from all optimizations. This is a superset of the optimization groups listed above.
If options is omitted, it defaults to optimized-optall, which means to dump all info about successful optimizations from all the passes.
If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored.
Note that the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead.
In the following example, the optimization info is output to stderr:
gcc -O3 -fopt-info
This example:
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
prints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types listed after -fopt-info does not matter.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
outputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is produced which contains dumps from the vectorizer about missed opportunities.
- -fsched-verbose=n
- On targets that use instruction scheduling, this option controls the
amount of debugging output the scheduler prints to the dump files.
For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info.
- -fenable-kind-pass
- -fdisable-kind-pass=range-list
- This is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead.
- -fdisable-ipa-pass
- Disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
- -fdisable-rtl-pass
- -fdisable-rtl-pass=range-list
- Disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes.
- -fdisable-tree-pass
- -fdisable-tree-pass=range-list
- Disable tree pass pass. See -fdisable-rtl for the description of option arguments.
- -fenable-ipa-pass
- Enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
- -fenable-rtl-pass
- -fenable-rtl-pass=range-list
- Enable RTL pass pass. See -fdisable-rtl for option argument description and examples.
- -fenable-tree-pass
- -fenable-tree-pass=range-list
- Enable tree pass pass. See -fdisable-rtl for the description of option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions -fdisable-tree-ccp1 # disable complete unroll for function whose cgraph node uid is 1 -fenable-tree-cunroll=1 # disable gcse2 for functions at the following ranges [1,1], # [300,400], and [400,1000] # disable gcse2 for functions foo and foo2 -fdisable-rtl-gcse2=foo,foo2 # disable early inlining -fdisable-tree-einline # disable ipa inlining -fdisable-ipa-inline # enable tree full unroll -fenable-tree-unroll
- -fchecking
- -fchecking=n
- Enable internal consistency checking. The default depends on the compiler configuration. -fchecking=2 enables further internal consistency checking that might affect code generation.
- -frandom-seed=string
- This option provides a seed that GCC uses in place of random numbers in
generating certain symbol names that have to be different in every
compiled file. It is also used to place unique stamps in coverage data
files and the object files that produce them. You can use the
-frandom-seed option to produce reproducibly identical object
files.
The string can either be a number (decimal, octal or hex) or an arbitrary string (in which case it's converted to a number by computing CRC32).
The string should be different for every file you compile.
- -save-temps
- -save-temps=cwd
- Store the usual "temporary" intermediate files permanently;
place them in the current directory and name them based on the source
file. Thus, compiling foo.c with -c -save-temps produces
files foo.i and foo.s, as well as foo.o. This creates
a preprocessed foo.i output file even though the compiler now
normally uses an integrated preprocessor.
When used in combination with the -x command-line option, -save-temps is sensible enough to avoid over writing an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using -save-temps.
If you invoke GCC in parallel, compiling several different source files that share a common base name in different subdirectories or the same source file compiled for multiple output destinations, it is likely that the different parallel compilers will interfere with each other, and overwrite the temporary files. For instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c& gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may result in foo.i and foo.o being written to simultaneously by both compilers.
- -save-temps=obj
- Store the usual "temporary" intermediate files permanently. If
the -o option is used, the temporary files are based on the object
file. If the -o option is not used, the -save-temps=obj
switch behaves like -save-temps.
For example:
gcc -save-temps=obj -c foo.c gcc -save-temps=obj -c bar.c -o dir/xbar.o gcc -save-temps=obj foobar.c -o dir2/yfoobar
creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s, and dir2/yfoobar.o.
- -time[=file]
- Report the CPU time taken by each subprocess in the compilation sequence.
For C source files, this is the compiler proper and assembler (plus the
linker if linking is done).
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01 # as 0.00 0.01
The first number on each line is the "user time", that is time spent executing the program itself. The second number is "system time", time spent executing operating system routines on behalf of the program. Both numbers are in seconds.
With the specification of an output file, the output is appended to the named file, and it looks like this:
0.12 0.01 cc1 <options> 0.00 0.01 as <options>
The "user time" and the "system time" are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options.
- -fdump-final-insns[=file]
- Dump the final internal representation (RTL) to file. If the optional argument is omitted (or if file is "."), the name of the dump file is determined by appending ".gkd" to the compilation output file name.
- -fcompare-debug[=opts]
- If no error occurs during compilation, run the compiler a second time,
adding opts and -fcompare-debug-second to the arguments
passed to the second compilation. Dump the final internal representation
in both compilations, and print an error if they differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables -fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used for opts, otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do.
- -fcompare-debug-second
- This option is implicitly passed to the compiler for the second
compilation requested by -fcompare-debug, along with options to
silence warnings, and omitting other options that would cause the compiler
to produce output to files or to standard output as a side effect. Dump
files and preserved temporary files are renamed so as to contain the
".gk" additional extension during the
second compilation, to avoid overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper.
- -gtoggle
- Turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it is given. This is mainly intended to be used with -fcompare-debug.
- -fvar-tracking-assignments-toggle
- Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.
- -Q
- Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.
- -ftime-report
- Makes the compiler print some statistics about the time consumed by each pass when it finishes.
- -ftime-report-details
- Record the time consumed by infrastructure parts separately for each pass.
- -fira-verbose=n
- Control the verbosity of the dump file for the integrated register allocator. The default value is 5. If the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10.
- -flto-report
- Prints a report with internal details on the workings of the link-time
optimizer. The contents of this report vary from version to version. It is
meant to be useful to GCC developers when processing object files in LTO
mode (via -flto).
Disabled by default.
- -flto-report-wpa
- Like -flto-report, but only print for the WPA phase of Link Time Optimization.
- -fmem-report
- Makes the compiler print some statistics about permanent memory allocation when it finishes.
- -fmem-report-wpa
- Makes the compiler print some statistics about permanent memory allocation for the WPA phase only.
- -fpre-ipa-mem-report
- -fpost-ipa-mem-report
- Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.
- -fprofile-report
- Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes.
- -fstack-usage
- Makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. An entry is made up of three fields:
- The name of the function.
- A number of bytes.
- One or more qualifiers: "static", "dynamic", "bounded".
The qualifier "static" means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.
The qualifier "dynamic" means that the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier "bounded" is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part.
- -fstats
- Emit statistics about front-end processing at the end of the compilation. This option is supported only by the C++ front end, and the information is generally only useful to the G++ development team.
- -fdbg-cnt-list
- Print the name and the counter upper bound for all debug counters.
- -fdbg-cnt=counter-value-list
- Set the internal debug counter upper bound. counter-value-list is a comma-separated list of name:value pairs which sets the upper bound of each debug counter name to value. All debug counters have the initial upper bound of "UINT_MAX"; thus "dbg_cnt" returns true always unless the upper bound is set by this option. For example, with -fdbg-cnt=dce:10,tail_call:0, "dbg_cnt(dce)" returns true only for first 10 invocations.
- -print-file-name=library
- Print the full absolute name of the library file library that would be used when linking---and don't do anything else. With this option, GCC does not compile or link anything; it just prints the file name.
- -print-multi-directory
- Print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
- -print-multi-lib
- Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ;, and each switch starts with an @ instead of the -, without spaces between multiple switches. This is supposed to ease shell processing.
- -print-multi-os-directory
- Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
- -print-multiarch
- Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
- -print-prog-name=program
- Like -print-file-name, but searches for a program such as cpp.
- -print-libgcc-file-name
- Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
- -print-search-dirs
- Print the name of the configured installation directory and a list of
program and library directories gcc searches---and don't do
anything else.
This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing /.
- -print-sysroot
- Print the target sysroot directory that is used during compilation. This is the target sysroot specified either at configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing.
- -print-sysroot-headers-suffix
- Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix---and don't do anything else.
- -dumpmachine
- Print the compiler's target machine (for example, i686-pc-linux-gnu)---and don't do anything else.
- -dumpversion
- Print the compiler version (for example, 3.0, 6.3.0 or 7)---and don't do anything else. This is the compiler version used in filesystem paths, specs, can be depending on how the compiler has been configured just a single number (major version), two numbers separated by dot (major and minor version) or three numbers separated by dots (major, minor and patchlevel version).
- -dumpfullversion
- Print the full compiler version, always 3 numbers separated by dots, major, minor and patchlevel version.
- -dumpspecs
- Print the compiler's built-in specs---and don't do anything else. (This is used when GCC itself is being built.)
Machine-Dependent Options¶
Each target machine supported by GCC can have its own options---for example, to allow you to compile for a particular processor variant or ABI, or to control optimizations specific to that machine. By convention, the names of machine-specific options start with -m.
Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform.
AArch64 Options¶
These options are defined for AArch64 implementations:
- -mabi=name
- Generate code for the specified data model. Permissible values are
ilp32 for SysV-like data model where int, long int and pointers are
32 bits, and lp64 for SysV-like data model where int is 32 bits,
but long int and pointers are 64 bits.
The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.
- -mbig-endian
- Generate big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
- -mgeneral-regs-only
- Generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler.
- -mlittle-endian
- Generate little-endian code. This is the default when GCC is configured for an aarch64-*-* but not an aarch64_be-*-* target.
- -mcmodel=tiny
- Generate code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked.
- -mcmodel=small
- Generate code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model.
- -mcmodel=large
- Generate code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only.
- -mstrict-align
- Avoid generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification.
- -momit-leaf-frame-pointer
- -mno-omit-leaf-frame-pointer
- Omit or keep the frame pointer in leaf functions. The former behavior is the default.
- -mtls-dialect=desc
- Use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.
- -mtls-dialect=traditional
- Use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.
- -mtls-size=size
- Specify bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. This option requires binutils 2.26 or newer.
- -mfix-cortex-a53-835769
- -mno-fix-cortex-a53-835769
- Enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions.
- -mfix-cortex-a53-843419
- -mno-fix-cortex-a53-843419
- Enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made at link time and this will only pass the corresponding flag to the linker.
- -mlow-precision-recip-sqrt
- -mno-low-precision-recip-sqrt
- Enable or disable the reciprocal square root approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision.
- -mlow-precision-sqrt
- -mno-low-precision-sqrt
- Enable or disable the square root approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of square root results to about 16 bits for single precision and to 32 bits for double precision. If enabled, it implies -mlow-precision-recip-sqrt.
- -mlow-precision-div
- -mno-low-precision-div
- Enable or disable the division approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision.
- -moutline-atomics
- -mno-outline-atomics
- Enable or disable calls to out-of-line helpers to implement atomic
operations. These helpers will, at runtime, determine if the LSE
instructions from ARMv8.1-A can be used; if not, they will use the
load/store-exclusive instructions that are present in the base ARMv8.0
ISA.
This option is only applicable when compiling for the base ARMv8.0 instruction set. If using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. The same applies when using -mcpu= when the selected cpu supports the lse feature.
- -march=name
- Specify the name of the target architecture and, optionally, one or more
feature modifiers. This option has the form
-march=arch{+[no]feature}*.
The permissible values for arch are armv8-a, armv8.1-a, armv8.2-a, armv8.3-a or armv8.4-a or native.
The value armv8.4-a implies armv8.3-a and enables compiler support for the ARMv8.4-A architecture extensions.
The value armv8.3-a implies armv8.2-a and enables compiler support for the ARMv8.3-A architecture extensions.
The value armv8.2-a implies armv8.1-a and enables compiler support for the ARMv8.2-A architecture extensions.
The value armv8.1-a implies armv8-a and enables compiler support for the ARMv8.1-A architecture extension. In particular, it enables the +crc, +lse, and +rdma features.
The value native is available on native AArch64 GNU/Linux and causes the compiler to pick the architecture of the host system. This option has no effect if the compiler is unable to recognize the architecture of the host system,
The permissible values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it can emit when generating assembly code. If -march is specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture.
- -mtune=name
- Specify the name of the target processor for which GCC should tune the
performance of the code. Permissible values for this option are:
generic, cortex-a35, cortex-a53, cortex-a55,
cortex-a57, cortex-a72, cortex-a73,
cortex-a75, cortex-a76, ares, neoverse-n1,
neoverse-n2, neoverse-v1, zeus,
neoverse-512tvb, exynos-m1, falkor, qdf24xx,
saphira, xgene1, vulcan, thunderx,
thunderxt88, thunderxt88p1, thunderxt81,
thunderxt83, thunderx2t99, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a73.cortex-a35,
cortex-a73.cortex-a53, cortex-a75.cortex-a55, native.
The values cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55 specify that GCC should tune for a big.LITTLE system.
The value neoverse-512tvb specifies that GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. In other words, the option tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). This is more general than tuning for a specific core like Neoverse V1 but is more specific than the default tuning described below.
Additionally on native AArch64 GNU/Linux systems the value native tunes performance to the host system. This option has no effect if the compiler is unable to recognize the processor of the host system.
Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.
This option cannot be suffixed by feature modifiers.
- -mcpu=name
- Specify the name of the target processor, optionally suffixed by one or
more feature modifiers. This option has the form
-mcpu=cpu{+[no]feature}*, where the
permissible values for cpu are the same as those available for
-mtune. The permissible values for feature are documented in
the sub-section on aarch64-feature-modifiers,,-march
and -mcpu Feature Modifiers. Where conflicting
feature modifiers are specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor for which to tune for performance (as if by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option.
-mcpu=neoverse-512tvb is special in that it does not refer to a specific core, but instead refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless overridden by -march, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with these properties. Unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb.
- -moverride=string
- Override tuning decisions made by the back-end in response to a
-mtune= switch. The syntax, semantics, and accepted values for
string in this option are not guaranteed to be consistent across
releases.
This option is only intended to be useful when developing GCC.
- -mverbose-cost-dump
- Enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler.
- -mpc-relative-literal-loads
- -mno-pc-relative-literal-loads
- Enable or disable PC-relative literal loads. With this option literal pools are accessed using a single instruction and emitted after each function. This limits the maximum size of functions to 1MB. This is enabled by default for -mcmodel=tiny.
- -msign-return-address=scope
- Select the function scope on which return address signing will be applied. Permissible values are none, which disables return address signing, non-leaf, which enables pointer signing for functions which are not leaf functions, and all, which enables pointer signing for all functions. The default value is none.
- -msve-vector-bits=bits
- Specify the number of bits in an SVE vector register. This option only has
an effect when SVE is enabled.
GCC supports two forms of SVE code generation: "vector-length agnostic" output that works with any size of vector register and "vector-length specific" output that allows GCC to make assumptions about the vector length when it is useful for optimization reasons. The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. Specifying scalable selects vector-length agnostic output. At present -msve-vector-bits=128 also generates vector-length agnostic output. All other values generate vector-length specific code. The behavior of these values may change in future releases and no value except scalable should be relied on for producing code that is portable across different hardware SVE vector lengths.
The default is -msve-vector-bits=scalable, which produces vector-length agnostic code.
-march and -mcpu Feature Modifiers
Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature:
- crc
- Enable CRC extension. This is on by default for -march=armv8.1-a.
- crypto
- Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.
- fp
- Enable floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
- simd
- Enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
- sve
- Enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.
- lse
- Enable Large System Extension instructions. This is on by default for -march=armv8.1-a.
- rdma
- Enable Round Double Multiply Accumulate instructions. This is on by default for -march=armv8.1-a.
- fp16
- Enable FP16 extension. This also enables floating-point instructions.
- fp16fml
- Enable FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. This option is enabled by default for -march=armv8.4-a. Use of this option with architectures prior to Armv8.2-A is not supported.
- rcpc
- Enable the RcPc extension. This does not change code generation from GCC, but is passed on to the assembler, enabling inline asm statements to use instructions from the RcPc extension.
- dotprod
- Enable the Dot Product extension. This also enables Advanced SIMD instructions.
- aes
- Enable the Armv8-a aes and pmull crypto extension. This also enables Advanced SIMD instructions.
- sha2
- Enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.
- sha3
- Enable the sha512 and sha3 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
- sm4
- Enable the sm3 and sm4 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2.
Adapteva Epiphany Options¶
These -m options are defined for Adapteva Epiphany:
- -mhalf-reg-file
- Don't allocate any register in the range "r32"..."r63". That allows code to run on hardware variants that lack these registers.
- -mprefer-short-insn-regs
- Preferentially allocate registers that allow short instruction generation. This can result in increased instruction count, so this may either reduce or increase overall code size.
- -mbranch-cost=num
- Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases.
- -mcmove
- Enable the generation of conditional moves.
- -mnops=num
- Emit num NOPs before every other generated instruction.
- -mno-soft-cmpsf
- For single-precision floating-point comparisons, emit an "fsub" instruction and test the flags. This is faster than a software comparison, but can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero. The default is -msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons.
- -mstack-offset=num
- Set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the eight bytes in the range "sp+0...sp+7" can be used by leaf functions without stack allocation. Values other than 8 or 16 are untested and unlikely to work. Note also that this option changes the ABI; compiling a program with a different stack offset than the libraries have been compiled with generally does not work. This option can be useful if you want to evaluate if a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=num option.
- -mno-round-nearest
- Make the scheduler assume that the rounding mode has been set to truncating. The default is -mround-nearest.
- -mlong-calls
- If not otherwise specified by an attribute, assume all calls might be beyond the offset range of the "b" / "bl" instructions, and therefore load the function address into a register before performing a (otherwise direct) call. This is the default.
- -mshort-calls
- If not otherwise specified by an attribute, assume all direct calls are in the range of the "b" / "bl" instructions, so use these instructions for direct calls. The default is -mlong-calls.
- -msmall16
- Assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which -mlong-calls semantics are in effect.
- -mfp-mode=mode
- Set the prevailing mode of the floating-point unit. This determines the
floating-point mode that is provided and expected at function call and
return time. Making this mode match the mode you predominantly need at
function start can make your programs smaller and faster by avoiding
unnecessary mode switches.
mode can be set to one the following values:
- caller
- Any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode.
- truncate
- This is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.
- round-nearest
- This is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
- int
- This is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.
The default is -mfp-mode=caller
- -mnosplit-lohi
- -mno-postinc
- -mno-postmodify
- Code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses. The defaults are msplit-lohi, -mpost-inc, and -mpost-modify.
- -mnovect-double
- Change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as preferred SIMD mode.
- -max-vect-align=num
- The maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types.
- -msplit-vecmove-early
- Split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case.
- -m1reg-reg
- Specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg are r43 and r63, which specify use of that register as a fixed register, and none, which means that no register is used for this purpose. The default is -m1reg-none.
ARC Options¶
The following options control the architecture variant for which code is being compiled:
- -mbarrel-shifter
- Generate instructions supported by barrel shifter. This is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
- -mjli-always
- Force to call a function using jli_s instruction. This option is valid only for ARCv2 architecture.
- -mcpu=cpu
- Set architecture type, register usage, and instruction scheduling parameters for cpu. There are also shortcut alias options available for backward compatibility and convenience. Supported values for cpu are
- arc600
- Compile for ARC600. Aliases: -mA6, -mARC600.
- arc601
- Compile for ARC601. Alias: -mARC601.
- arc700
- Compile for ARC700. Aliases: -mA7, -mARC700. This is the default when configured with --with-cpu=arc700.
- arcem
- Compile for ARC EM.
- archs
- Compile for ARC HS.
- em
- Compile for ARC EM CPU with no hardware extensions.
- em4
- Compile for ARC EM4 CPU.
- em4_dmips
- Compile for ARC EM4 DMIPS CPU.
- em4_fpus
- Compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
- em4_fpuda
- Compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.
- hs
- Compile for ARC HS CPU with no hardware extensions except the atomic instructions.
- hs34
- Compile for ARC HS34 CPU.
- hs38
- Compile for ARC HS38 CPU.
- hs38_linux
- Compile for ARC HS38 CPU with all hardware extensions on.
- arc600_norm
- Compile for ARC 600 CPU with "norm" instructions enabled.
- arc600_mul32x16
- Compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.
- arc600_mul64
- Compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.
- arc601_norm
- Compile for ARC 601 CPU with "norm" instructions enabled.
- arc601_mul32x16
- Compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.
- arc601_mul64
- Compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.
- nps400
- Compile for ARC 700 on NPS400 chip.
- em_mini
- Compile for ARC EM minimalist configuration featuring reduced register set.
- -mdpfp
- -mdpfp-compact
- Generate double-precision FPX instructions, tuned for the compact implementation.
- -mdpfp-fast
- Generate double-precision FPX instructions, tuned for the fast implementation.
- -mno-dpfp-lrsr
- Disable "lr" and "sr" instructions from using FPX extension aux registers.
- -mea
- Generate extended arithmetic instructions. Currently only "divaw", "adds", "subs", and "sat16" are supported. This is always enabled for -mcpu=ARC700.
- -mno-mpy
- Do not generate "mpy"-family instructions for ARC700. This option is deprecated.
- -mmul32x16
- Generate 32x16-bit multiply and multiply-accumulate instructions.
- -mmul64
- Generate "mul64" and "mulu64" instructions. Only valid for -mcpu=ARC600.
- -mnorm
- Generate "norm" instructions. This is the default if -mcpu=ARC700 is in effect.
- -mspfp
- -mspfp-compact
- Generate single-precision FPX instructions, tuned for the compact implementation.
- -mspfp-fast
- Generate single-precision FPX instructions, tuned for the fast implementation.
- -msimd
- Enable generation of ARC SIMD instructions via target-specific builtins. Only valid for -mcpu=ARC700.
- -msoft-float
- This option ignored; it is provided for compatibility purposes only. Software floating-point code is emitted by default, and this default can overridden by FPX options; -mspfp, -mspfp-compact, or -mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision.
- -mswap
- Generate "swap" instructions.
- -matomic
- This enables use of the locked load/store conditional extension to implement atomic memory built-in functions. Not available for ARC 6xx or ARC EM cores.
- -mdiv-rem
- Enable "div" and "rem" instructions for ARCv2 cores.
- -mcode-density
- Enable code density instructions for ARC EM. This option is on by default for ARC HS.
- -mll64
- Enable double load/store operations for ARC HS cores.
- -mtp-regno=regno
- Specify thread pointer register number.
- -mmpy-option=multo
- Compile ARCv2 code with a multiplier design option. You can specify the option using either a string or numeric value for multo. wlh1 is the default value. The recognized values are:
- 0
- none
- No multiplier available.
- 1
- w
- 16x16 multiplier, fully pipelined. The following instructions are enabled: "mpyw" and "mpyuw".
- 2
- wlh1
- 32x32 multiplier, fully pipelined (1 stage). The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
- 3
- wlh2
- 32x32 multiplier, fully pipelined (2 stages). The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
- 4
- wlh3
- Two 16x16 multipliers, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
- 5
- wlh4
- One 16x16 multiplier, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
- 6
- wlh5
- One 32x4 multiplier, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
- 7
- plus_dmpy
- ARC HS SIMD support.
- 8
- plus_macd
- ARC HS SIMD support.
- 9
- plus_qmacw
- ARC HS SIMD support.
This option is only available for ARCv2 cores.
- -mfpu=fpu
- Enables support for specific floating-point hardware extensions for ARCv2 cores. Supported values for fpu are:
- fpus
- Enables support for single-precision floating-point hardware extensions.
- fpud
- Enables support for double-precision floating-point hardware extensions. The single-precision floating-point extension is also enabled. Not available for ARC EM.
- fpuda
- Enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point extension is also enabled. This option is only available for ARC EM.
- fpuda_div
- Enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point, square-root, and divide extensions are also enabled. This option is only available for ARC EM.
- fpuda_fma
- Enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point and fused multiply and add hardware extensions are also enabled. This option is only available for ARC EM.
- fpuda_all
- Enables support for double-precision floating-point hardware extensions using double-precision assist instructions. All single-precision floating-point hardware extensions are also enabled. This option is only available for ARC EM.
- fpus_div
- Enables support for single-precision floating-point, square-root and divide hardware extensions.
- fpud_div
- Enables support for double-precision floating-point, square-root and divide hardware extensions. This option includes option fpus_div. Not available for ARC EM.
- fpus_fma
- Enables support for single-precision floating-point and fused multiply and add hardware extensions.
- fpud_fma
- Enables support for double-precision floating-point and fused multiply and add hardware extensions. This option includes option fpus_fma. Not available for ARC EM.
- fpus_all
- Enables support for all single-precision floating-point hardware extensions.
- fpud_all
- Enables support for all single- and double-precision floating-point hardware extensions. Not available for ARC EM.
- -mirq-ctrl-saved=register-range, blink, lp_count
- Specifies general-purposes registers that the processor automatically saves/restores on interrupt entry and exit. register-range is specified as two registers separated by a dash. The register range always starts with "r0", the upper limit is "fp" register. blink and lp_count are optional. This option is only valid for ARC EM and ARC HS cores.
- -mrgf-banked-regs=number
- Specifies the number of registers replicated in second register bank on entry to fast interrupt. Fast interrupts are interrupts with the highest priority level P0. These interrupts save only PC and STATUS32 registers to avoid memory transactions during interrupt entry and exit sequences. Use this option when you are using fast interrupts in an ARC V2 family processor. Permitted values are 4, 8, 16, and 32.
- -mlpc-width=width
- Specify the width of the "lp_count" register. Valid values for width are 8, 16, 20, 24, 28 and 32 bits. The default width is fixed to 32 bits. If the width is less than 32, the compiler does not attempt to transform loops in your program to use the zero-delay loop mechanism unless it is known that the "lp_count" register can hold the required loop-counter value. Depending on the width specified, the compiler and run-time library might continue to use the loop mechanism for various needs. This option defines macro "__ARC_LPC_WIDTH__" with the value of width.
- -mrf16
- This option instructs the compiler to generate code for a 16-entry register file. This option defines the "__ARC_RF16__" preprocessor macro.
The following options are passed through to the assembler, and also define preprocessor macro symbols.
- -mdsp-packa
- Passed down to the assembler to enable the DSP Pack A extensions. Also sets the preprocessor symbol "__Xdsp_packa". This option is deprecated.
- -mdvbf
- Passed down to the assembler to enable the dual Viterbi butterfly extension. Also sets the preprocessor symbol "__Xdvbf". This option is deprecated.
- -mlock
- Passed down to the assembler to enable the locked load/store conditional extension. Also sets the preprocessor symbol "__Xlock".
- -mmac-d16
- Passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_d16". This option is deprecated.
- -mmac-24
- Passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_24". This option is deprecated.
- -mrtsc
- Passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. Also sets the preprocessor symbol "__Xrtsc". This option is deprecated.
- -mswape
- Passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the preprocessor symbol "__Xswape".
- -mtelephony
- Passed down to the assembler to enable dual- and single-operand instructions for telephony. Also sets the preprocessor symbol "__Xtelephony". This option is deprecated.
- -mxy
- Passed down to the assembler to enable the XY memory extension. Also sets the preprocessor symbol "__Xxy".
The following options control how the assembly code is annotated:
- -misize
- Annotate assembler instructions with estimated addresses.
- -mannotate-align
- Explain what alignment considerations lead to the decision to make an instruction short or long.
The following options are passed through to the linker:
- -marclinux
- Passed through to the linker, to specify use of the "arclinux" emulation. This option is enabled by default in tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets when profiling is not requested.
- -marclinux_prof
- Passed through to the linker, to specify use of the "arclinux_prof" emulation. This option is enabled by default in tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets when profiling is requested.
The following options control the semantics of generated code:
- -mlong-calls
- Generate calls as register indirect calls, thus providing access to the full 32-bit address range.
- -mmedium-calls
- Don't use less than 25-bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction. Conditional execution of function calls is suppressed, to allow use of the 25-bit range, rather than the 21-bit range with conditional branch-and-link. This is the default for tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets.
- -G num
- Put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. The default value of num is 4 for any ARC configuration, or 8 when we have double load/store operations.
- -mno-sdata
- Do not generate sdata references. This is the default for tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets.
- -mvolatile-cache
- Use ordinarily cached memory accesses for volatile references. This is the default.
- -mno-volatile-cache
- Enable cache bypass for volatile references.
The following options fine tune code generation:
- -malign-call
- Do alignment optimizations for call instructions.
- -mauto-modify-reg
- Enable the use of pre/post modify with register displacement.
- -mbbit-peephole
- Enable bbit peephole2.
- -mno-brcc
- This option disables a target-specific pass in arc_reorg to generate compare-and-branch ("brcc") instructions. It has no effect on generation of these instructions driven by the combiner pass.
- -mcase-vector-pcrel
- Use PC-relative switch case tables to enable case table shortening. This is the default for -Os.
- -mcompact-casesi
- Enable compact "casesi" pattern. This is the default for -Os, and only available for ARCv1 cores.
- -mno-cond-exec
- Disable the ARCompact-specific pass to generate conditional execution
instructions.
Due to delay slot scheduling and interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the target-independent pass to generate conditional execution is often lacking, so the ARC port has kept a special pass around that tries to find more conditional execution generation opportunities after register allocation, branch shortening, and delay slot scheduling have been done. This pass generally, but not always, improves performance and code size, at the cost of extra compilation time, which is why there is an option to switch it off. If you have a problem with call instructions exceeding their allowable offset range because they are conditionalized, you should consider using -mmedium-calls instead.
- -mearly-cbranchsi
- Enable pre-reload use of the "cbranchsi" pattern.
- -mexpand-adddi
- Expand "adddi3" and "subdi3" at RTL generation time into "add.f", "adc" etc. This option is deprecated.
- -mindexed-loads
- Enable the use of indexed loads. This can be problematic because some optimizers then assume that indexed stores exist, which is not the case.
- -mlra
- Enable Local Register Allocation. This is still experimental for ARC, so by default the compiler uses standard reload (i.e. -mno-lra).
- -mlra-priority-none
- Don't indicate any priority for target registers.
- -mlra-priority-compact
- Indicate target register priority for r0..r3 / r12..r15.
- -mlra-priority-noncompact
- Reduce target register priority for r0..r3 / r12..r15.
- -mno-millicode
- When optimizing for size (using -Os), prologues and epilogues that have to save or restore a large number of registers are often shortened by using call to a special function in libgcc; this is referred to as a millicode call. As these calls can pose performance issues, and/or cause linking issues when linking in a nonstandard way, this option is provided to turn off millicode call generation.
- -mmixed-code
- Tweak register allocation to help 16-bit instruction generation. This generally has the effect of decreasing the average instruction size while increasing the instruction count.
- -mq-class
- Enable q instruction alternatives. This is the default for -Os.
- -mRcq
- Enable Rcq constraint handling. Most short code generation depends on this. This is the default.
- -mRcw
- Enable Rcw constraint handling. Most ccfsm condexec mostly depends on this. This is the default.
- -msize-level=level
- Fine-tune size optimization with regards to instruction lengths and alignment. The recognized values for level are:
- 0
- No size optimization. This level is deprecated and treated like 1.
- 1
- Short instructions are used opportunistically.
- 2
- In addition, alignment of loops and of code after barriers are dropped.
- 3
- In addition, optional data alignment is dropped, and the option Os is enabled.
This defaults to 3 when -Os is in effect. Otherwise, the behavior when this is not set is equivalent to level 1.
- -mtune=cpu
- Set instruction scheduling parameters for cpu, overriding any
implied by -mcpu=.
Supported values for cpu are
- -mmultcost=num
- Cost to assume for a multiply instruction, with 4 being equal to a normal instruction.
- -munalign-prob-threshold=probability
- Set probability threshold for unaligning branches. When tuning for ARC700 and optimizing for speed, branches without filled delay slot are preferably emitted unaligned and long, unless profiling indicates that the probability for the branch to be taken is below probability. The default is (REG_BR_PROB_BASE/2), i.e. 5000.
The following options are maintained for backward compatibility, but are now deprecated and will be removed in a future release:
- -margonaut
- Obsolete FPX.
- -mbig-endian
- -EB
- Compile code for big-endian targets. Use of these options is now deprecated. Big-endian code is supported by configuring GCC to build "arceb-elf32" and "arceb-linux-uclibc" targets, for which big endian is the default.
- -mlittle-endian
- -EL
- Compile code for little-endian targets. Use of these options is now deprecated. Little-endian code is supported by configuring GCC to build "arc-elf32" and "arc-linux-uclibc" targets, for which little endian is the default.
- -mbarrel_shifter
- Replaced by -mbarrel-shifter.
- -mdpfp_compact
- Replaced by -mdpfp-compact.
- -mdpfp_fast
- Replaced by -mdpfp-fast.
- -mdsp_packa
- Replaced by -mdsp-packa.
- -mEA
- Replaced by -mea.
- -mmac_24
- Replaced by -mmac-24.
- -mmac_d16
- Replaced by -mmac-d16.
- -mspfp_compact
- Replaced by -mspfp-compact.
- -mspfp_fast
- Replaced by -mspfp-fast.
- -mtune=cpu
- Values arc600, arc601, arc700 and arc700-xmac for cpu are replaced by ARC600, ARC601, ARC700 and ARC700-xmac respectively.
- -multcost=num
- Replaced by -mmultcost.
ARM Options¶
These -m options are defined for the ARM port:
- -mabi=name
- Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
- -mapcs-frame
- Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer with this option causes the stack frames not to be generated for leaf functions. The default is -mno-apcs-frame. This option is deprecated.
- -mapcs
- This is a synonym for -mapcs-frame and is deprecated.
- -mthumb-interwork
- Generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. The default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified. In AAPCS configurations this option is meaningless.
- -mno-sched-prolog
- Prevent the reordering of instructions in the function prologue, or the merging of those instruction with the instructions in the function's body. This means that all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start of functions inside an executable piece of code. The default is -msched-prolog.
- -mfloat-abi=name
- Specifies which floating-point ABI to use. Permissible values are:
soft, softfp and hard.
Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.
- -mlittle-endian
- Generate code for a processor running in little-endian mode. This is the default for all standard configurations.
- -mbig-endian
- Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.
- -mbe8
- -mbe32
- When linking a big-endian image select between BE8 and BE32 formats. The option has no effect for little-endian images and is ignored. The default is dependent on the selected target architecture. For ARMv6 and later architectures the default is BE8, for older architectures the default is BE32. BE32 format has been deprecated by ARM.
- -march=name[+extension...]
- This specifies the name of the target ARM architecture. GCC uses this name
to determine what kind of instructions it can emit when generating
assembly code. This option can be used in conjunction with or instead of
the -mcpu= option.
Permissible names are: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m, armv8-m.base, armv8-m.main, iwmmxt and iwmmxt2.
Additionally, the following architectures, which lack support for the Thumb execution state, are recognized but support is deprecated: armv2, armv2a, armv3, armv3m, armv4, armv5 and armv5e.
Many of the architectures support extensions. These can be added by appending +extension to the architecture name. Extension options are processed in order and capabilities accumulate. An extension will also enable any necessary base extensions upon which it depends. For example, the +crypto extension will always enable the +simd extension. The exception to the additive construction is for extensions that are prefixed with +no...: these extensions disable the specified option and any other extensions that may depend on the presence of that extension.
For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing -march=armv7-a+vfpv4 since the +simd option is entirely disabled by the +nofp option that follows it.
Most extension names are generically named, but have an effect that is dependent upon the architecture to which it is applied. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but will enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a.
The table below lists the supported extensions for each architecture. Architectures not mentioned do not support any extensions.
- +fp
- The VFPv2 floating-point instructions. The extension +vfpv2 can be used as an alias for this extension.
- +nofp
- Disable the floating-point instructions.
- armv7
- The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures.
- +fp
- The VFPv3 floating-point instructions, with 16 double-precision registers. The extension +vfpv3-d16 can be used as an alias for this extension. Note that floating-point is not supported by the base ARMv7-M architecture, but is compatible with both the ARMv7-A and ARMv7-R architectures.
- +nofp
- Disable the floating-point instructions.
- +mp
- The multiprocessing extension.
- +sec
- The security extension.
- +fp
- The VFPv3 floating-point instructions, with 16 double-precision registers. The extension +vfpv3-d16 can be used as an alias for this extension.
- +simd
- The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions +neon and +neon-vfpv3 can be used as aliases for this extension.
- +vfpv3
- The VFPv3 floating-point instructions, with 32 double-precision registers.
- +vfpv3-d16-fp16
- The VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.
- +vfpv3-fp16
- The VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.
- +vfpv4-d16
- The VFPv4 floating-point instructions, with 16 double-precision registers.
- +vfpv4
- The VFPv4 floating-point instructions, with 32 double-precision registers.
- +neon-fp16
- The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
- +neon-vfpv4
- The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.
- +nosimd
- Disable the Advanced SIMD instructions (does not disable floating point).
- +nofp
- Disable the floating-point and Advanced SIMD instructions.
- armv7ve
- The extended version of the ARMv7-A architecture with support for virtualization.
- +fp
- The VFPv4 floating-point instructions, with 16 double-precision registers. The extension +vfpv4-d16 can be used as an alias for this extension.
- +simd
- The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension +neon-vfpv4 can be used as an alias for this extension.
- +vfpv3-d16
- The VFPv3 floating-point instructions, with 16 double-precision registers.
- +vfpv3
- The VFPv3 floating-point instructions, with 32 double-precision registers.
- +vfpv3-d16-fp16
- The VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.
- +vfpv3-fp16
- The VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.
- +vfpv4-d16
- The VFPv4 floating-point instructions, with 16 double-precision registers.
- +vfpv4
- The VFPv4 floating-point instructions, with 32 double-precision registers.
- +neon
- The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extension +neon-vfpv3 can be used as an alias for this extension.
- +neon-fp16
- The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
- +nosimd
- Disable the Advanced SIMD instructions (does not disable floating point).
- +nofp
- Disable the floating-point and Advanced SIMD instructions.
- +crc
- The Cyclic Redundancy Check (CRC) instructions.
- +simd
- The ARMv8-A Advanced SIMD and floating-point instructions.
- +crypto
- The cryptographic instructions.
- +nocrypto
- Disable the cryptographic instructions.
- +nofp
- Disable the floating-point, Advanced SIMD and cryptographic instructions.
- +simd
- The ARMv8.1-A Advanced SIMD and floating-point instructions.
- +crypto
- The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.
- +nocrypto
- Disable the cryptographic instructions.
- +nofp
- Disable the floating-point, Advanced SIMD and cryptographic instructions.
- +fp16
- The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions.
- +fp16fml
- The half-precision floating-point fmla extension. This also enables the half-precision floating-point extension and Advanced SIMD and floating-point instructions.
- +simd
- The ARMv8.1-A Advanced SIMD and floating-point instructions.
- +crypto
- The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.
- +dotprod
- Enable the Dot Product extension. This also enables Advanced SIMD instructions.
- +nocrypto
- Disable the cryptographic extension.
- +nofp
- Disable the floating-point, Advanced SIMD and cryptographic instructions.
- +fp16
- The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.
- +simd
- The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
- +crypto
- The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
- +nocrypto
- Disable the cryptographic extension.
- +nofp
- Disable the floating-point, Advanced SIMD and cryptographic instructions.
- +fp.sp
- The single-precision VFPv3 floating-point instructions. The extension +vfpv3xd can be used as an alias for this extension.
- +fp
- The VFPv3 floating-point instructions with 16 double-precision registers. The extension +vfpv3-d16 can be used as an alias for this extension.
- +vfpv3xd-d16-fp16
- The single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
- +vfpv3-d16-fp16
- The VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.
- +nofp
- Disable the floating-point extension.
- +idiv
- The ARM-state integer division instructions.
- +noidiv
- Disable the ARM-state integer division extension.
- +fp
- The single-precision VFPv4 floating-point instructions.
- +fpv5
- The single-precision FPv5 floating-point instructions.
- +fp.dp
- The single- and double-precision FPv5 floating-point instructions.
- +nofp
- Disable the floating-point extensions.
- +dsp
- The DSP instructions.
- +nodsp
- Disable the DSP extension.
- +fp
- The single-precision floating-point instructions.
- +fp.dp
- The single- and double-precision floating-point instructions.
- +nofp
- Disable the floating-point extension.
- +crc
- The Cyclic Redundancy Check (CRC) instructions.
- +fp.sp
- The single-precision FPv5 floating-point instructions.
- +simd
- The ARMv8-A Advanced SIMD and floating-point instructions.
- +crypto
- The cryptographic instructions.
- +nocrypto
- Disable the cryptographic instructions.
- +nofp
- Disable the floating-point, Advanced SIMD and cryptographic instructions.
-march=native causes the compiler to auto-detect the architecture of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.
- -mtune=name
- This option specifies the name of the target ARM processor for which GCC
should tune the performance of the code. For some ARM implementations
better performance can be obtained by using this option. Permissible names
are: arm2, arm250, arm3, arm6, arm60,
arm600, arm610, arm620, arm7, arm7m,
arm7d, arm7dm, arm7di, arm7dmi, arm70,
arm700, arm700i, arm710, arm710c,
arm7100, arm720, arm7500, arm7500fe,
arm7tdmi, arm7tdmi-s, arm710t, arm720t,
arm740t, strongarm, strongarm110,
strongarm1100, strongarm1110, arm8, arm810,
arm9, arm9e, arm920, arm920t, arm922t,
arm946e-s, arm966e-s, arm968e-s, arm926ej-s,
arm940t, arm9tdmi, arm10tdmi, arm1020t,
arm1026ej-s, arm10e, arm1020e, arm1022e,
arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp,
arm1156t2-s, arm1156t2f-s, arm1176jz-s,
arm1176jzf-s, generic-armv7-a, cortex-a5,
cortex-a7, cortex-a8, cortex-a9, cortex-a12,
cortex-a15, cortex-a17, cortex-a32,
cortex-a35, cortex-a53, cortex-a55,
cortex-a57, cortex-a72, cortex-a73,
cortex-a75, neoverse-v1, neoverse-n2,
cortex-r4, cortex-r4f, cortex-r5, cortex-r7,
cortex-r8, cortex-r52, cortex-m33, cortex-m23,
cortex-m7, cortex-m4, cortex-m3, cortex-m1,
cortex-m0, cortex-m0plus, cortex-m1.small-multiply,
cortex-m0.small-multiply, cortex-m0plus.small-multiply,
exynos-m1, marvell-pj4, xscale, iwmmxt,
iwmmxt2, ep9312, fa526, fa626, fa606te,
fa626te, fmp626, fa726te, xgene1.
Additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system. Permissible names are: cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55.
-mtune=generic-arch specifies that GCC should tune the performance for a blend of processors within architecture arch. The aim is to generate code that run well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs. The effects of this option may change in future GCC versions as CPU models come and go.
-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the generated code.
-mtune=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.
- -mcpu=name[+extension...]
- This specifies the name of the target ARM processor. GCC uses this name to
derive the name of the target ARM architecture (as if specified by
-march) and the ARM processor type for which to tune for
performance (as if specified by -mtune). Where this option is used
in conjunction with -march or -mtune, those options take
precedence over the appropriate part of this option.
Many of the supported CPUs implement optional architectural extensions. Where this is so the architectural extensions are normally enabled by default. If implementations that lack the extension exist, then the extension syntax can be used to disable those extensions that have been omitted. For floating-point and Advanced SIMD (Neon) instructions, the settings of the options -mfloat-abi and -mfpu must also be considered: floating-point and Advanced SIMD instructions will only be used if -mfloat-abi is not set to soft; and any setting of -mfpu other than auto will override the available floating-point and SIMD extension instructions.
For example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit or with floating-point and Advanced SIMD. The default is to enable all the instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD or both the SIMD and floating-point instructions respectively.
Permissible names for this option are the same as those for -mtune.
The following extension options are common to the listed CPUs:
- +nodsp
- Disable the DSP instructions on cortex-m33.
- +nofp
- Disables the floating-point instructions on arm9e, arm946e-s, arm966e-s, arm968e-s, arm10e, arm1020e, arm1022e, arm926ej-s, arm1026ej-s, cortex-r5, cortex-r7, cortex-r8, cortex-m4, cortex-m7 and cortex-m33. Disables the floating-point and SIMD instructions on generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17, cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a32, cortex-a35, cortex-a53 and cortex-a55.
- +nofp.dp
- Disables the double-precision component of the floating-point instructions on cortex-r5, cortex-r7, cortex-r8, cortex-r52 and cortex-m7.
- +nosimd
- Disables the SIMD (but not floating-point) instructions on generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9.
- +crypto
- Enables the cryptographic instructions on cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, exynos-m1, xgene1, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53 and cortex-a75.cortex-a55.
Additionally the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers. It supports the following extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same as for the extensions to -march=armv7-a.
-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. See -mtune for more information.
-mcpu=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.
- -mfpu=name
- This specifies what floating-point hardware (or hardware emulation) is
available on the target. Permissible names are: auto, vfpv2,
vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16,
vfpv3xd, vfpv3xd-fp16, neon-vfpv3, neon-fp16,
vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4,
fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8
and crypto-neon-fp-armv8. Note that neon is an alias for
neon-vfpv3 and vfp is an alias for vfpv2.
The setting auto is the default and is special. It causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march.
If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), note that floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified. This is because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero), so the use of NEON instructions may lead to a loss of precision.
You can also set the fpu name at function level by using the "target("fpu=")" function attributes or pragmas.
- -mfp16-format=name
- Specify the format of the "__fp16" half-precision floating-point type. Permissible names are none, ieee, and alternative; the default is none, in which case the "__fp16" type is not defined.
- -mstructure-size-boundary=n
- The sizes of all structures and unions are rounded up to a multiple of the
number of bits set by this option. Permissible values are 8, 32 and 64.
The default value varies for different toolchains. For the COFF targeted
toolchain the default value is 8. A value of 64 is only allowed if the
underlying ABI supports it.
Specifying a larger number can produce faster, more efficient code, but can also increase the size of the program. Different values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions.
This option is deprecated.
- -mabort-on-noreturn
- Generate a call to the function "abort" at the end of a "noreturn" function. It is executed if the function tries to return.
- -mlong-calls
- -mno-long-calls
- Tells the compiler to perform function calls by first loading the address
of the function into a register and then performing a subroutine call on
this register. This switch is needed if the target function lies outside
of the 64-megabyte addressing range of the offset-based version of
subroutine call instruction.
Even if this switch is enabled, not all function calls are turned into long calls. The heuristic is that static functions, functions that have the "short_call" attribute, functions that are inside the scope of a "#pragma no_long_calls" directive, and functions whose definitions have already been compiled within the current compilation unit are not turned into long calls. The exceptions to this rule are that weak function definitions, functions with the "long_call" attribute or the "section" attribute, and functions that are within the scope of a "#pragma long_calls" directive are always turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior, as does placing the function calls within the scope of a "#pragma long_calls_off" directive. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.
- -msingle-pic-base
- Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins.
- -mpic-register=reg
- Specify the register to be used for PIC addressing. For standard PIC base case, the default is any suitable register determined by compiler. For single PIC base case, the default is R9 if target is EABI based or stack-checking is enabled, otherwise the default is R10.
- -mpic-data-is-text-relative
- Assume that the displacement between the text and data segments is fixed at static link time. This permits using PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When disabled on such targets, it will enable -msingle-pic-base by default.
- -mpoke-function-name
- Write the name of each function into the text section, directly preceding
the function prologue. The generated code is similar to this:
t0 .ascii "arm_poke_function_name", 0 .align t1 .word 0xff000000 + (t1 - t0) arm_poke_function_name mov ip, sp stmfd sp!, {fp, ip, lr, pc} sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of "pc" stored at "fp + 0". If the trace function then looks at location "pc - 12" and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length "((pc[-3]) & 0xff000000)".
- -mthumb
- -marm
- Select between generating code that executes in ARM and Thumb states. The
default for most configurations is to generate code that executes in ARM
state, but the default can be changed by configuring GCC with the
--with-mode=state configure option.
You can also override the ARM and Thumb mode for each function by using the "target("thumb")" and "target("arm")" function attributes or pragmas.
- -mflip-thumb
- Switch ARM/Thumb modes on alternating functions. This option is provided for regression testing of mixed Thumb/ARM code generation, and is not intended for ordinary use in compiling code.
- -mtpcs-frame
- Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame.
- -mtpcs-leaf-frame
- Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame.
- -mcallee-super-interworking
- Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default.
- -mcaller-super-interworking
- Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled. This option is not valid in AAPCS configurations because interworking is enabled by default.
- -mtp=name
- Specify the access model for the thread local storage pointer. The valid models are soft, which generates calls to "__aeabi_read_tp", cp15, which fetches the thread pointer from "cp15" directly (supported in the arm6k architecture), and auto, which uses the best available method for the selected processor. The default setting is auto.
- -mtls-dialect=dialect
- Specify the dialect to use for accessing thread local storage. Two dialects are supported---gnu and gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme.
- -mword-relocations
- Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified.
- -mfix-cortex-m3-ldrd
- Some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping destination and base registers are used. This option avoids generating these instructions. This option is enabled by default when -mcpu=cortex-m3 is specified.
- -munaligned-access
- -mno-unaligned-access
- Enables (or disables) reading and writing of 16- and 32- bit values from
addresses that are not 16- or 32- bit aligned. By default unaligned access
is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline
architectures, and enabled for all other architectures. If unaligned
access is not enabled then words in packed data structures are accessed a
byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" is set in the generated object file to either true or false, depending upon the setting of this option. If unaligned access is enabled then the preprocessor symbol "__ARM_FEATURE_UNALIGNED" is also defined.
- -mneon-for-64bits
- Enables using Neon to handle scalar 64-bits operations. This is disabled by default since the cost of moving data from core registers to Neon is high.
- -mslow-flash-data
- Assume loading data from flash is slower than fetching instruction. Therefore literal load is minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and off by default.
- -masm-syntax-unified
- Assume inline assembler is using unified asm syntax. The default is currently off which implies divided syntax. This option has no impact on Thumb2. However, this may change in future releases of GCC. Divided syntax should be considered deprecated.
- -mrestrict-it
- Restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8-A Thumb mode.
- -mprint-tune-info
- Print CPU tuning information as comment in assembler file. This is an option used only for regression testing of the compiler and not intended for ordinary use in compiling code. This option is disabled by default.
- -mverbose-cost-dump
- Enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler.
- -mpure-code
- Do not allow constant data to be placed in code sections. Additionally, when compiling for ELF object format give all text sections the ELF processor-specific section attribute "SHF_ARM_PURECODE". This option is only available when generating non-pic code for M-profile targets with the MOVT instruction.
- -mcmse
- Generate secure code as per the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification", which can be found on <https://developer.arm.com/documentation/ecm0359818/latest/>.
AVR Options¶
These options are defined for AVR implementations:
- -mmcu=mcu
- Specify Atmel AVR instruction set architectures (ISA) or MCU type.
The default for this option is avr2.
GCC supports the following AVR devices and ISAs:
- "avr2"
- "Classic" devices with up to 8 KiB of program memory. mcu = "attiny22", "attiny26", "at90s2313", "at90s2323", "at90s2333", "at90s2343", "at90s4414", "at90s4433", "at90s4434", "at90c8534", "at90s8515", "at90s8535".
- "avr25"
- "Classic" devices with up to 8 KiB of program memory and with the "MOVW" instruction. mcu = "attiny13", "attiny13a", "attiny24", "attiny24a", "attiny25", "attiny261", "attiny261a", "attiny2313", "attiny2313a", "attiny43u", "attiny44", "attiny44a", "attiny45", "attiny48", "attiny441", "attiny461", "attiny461a", "attiny4313", "attiny84", "attiny84a", "attiny85", "attiny87", "attiny88", "attiny828", "attiny841", "attiny861", "attiny861a", "ata5272", "ata6616c", "at86rf401".
- "avr3"
- "Classic" devices with 16 KiB up to 64 KiB of program memory. mcu = "at76c711", "at43usb355".
- "avr31"
- "Classic" devices with 128 KiB of program memory. mcu = "atmega103", "at43usb320".
- "avr35"
- "Classic" devices with 16 KiB up to 64 KiB of program memory and with the "MOVW" instruction. mcu = "attiny167", "attiny1634", "atmega8u2", "atmega16u2", "atmega32u2", "ata5505", "ata6617c", "ata664251", "at90usb82", "at90usb162".
- "avr4"
- "Enhanced" devices with up to 8 KiB of program memory. mcu = "atmega48", "atmega48a", "atmega48p", "atmega48pa", "atmega48pb", "atmega8", "atmega8a", "atmega8hva", "atmega88", "atmega88a", "atmega88p", "atmega88pa", "atmega88pb", "atmega8515", "atmega8535", "ata6285", "ata6286", "ata6289", "ata6612c", "at90pwm1", "at90pwm2", "at90pwm2b", "at90pwm3", "at90pwm3b", "at90pwm81".
- "avr5"
- "Enhanced" devices with 16 KiB up to 64 KiB of program memory. mcu = "atmega16", "atmega16a", "atmega16hva", "atmega16hva2", "atmega16hvb", "atmega16hvbrevb", "atmega16m1", "atmega16u4", "atmega161", "atmega162", "atmega163", "atmega164a", "atmega164p", "atmega164pa", "atmega165", "atmega165a", "atmega165p", "atmega165pa", "atmega168", "atmega168a", "atmega168p", "atmega168pa", "atmega168pb", "atmega169", "atmega169a", "atmega169p", "atmega169pa", "atmega32", "atmega32a", "atmega32c1", "atmega32hvb", "atmega32hvbrevb", "atmega32m1", "atmega32u4", "atmega32u6", "atmega323", "atmega324a", "atmega324p", "atmega324pa", "atmega325", "atmega325a", "atmega325p", "atmega325pa", "atmega328", "atmega328p", "atmega328pb", "atmega329", "atmega329a", "atmega329p", "atmega329pa", "atmega3250", "atmega3250a", "atmega3250p", "atmega3250pa", "atmega3290", "atmega3290a", "atmega3290p", "atmega3290pa", "atmega406", "atmega64", "atmega64a", "atmega64c1", "atmega64hve", "atmega64hve2", "atmega64m1", "atmega64rfr2", "atmega640", "atmega644", "atmega644a", "atmega644p", "atmega644pa", "atmega644rfr2", "atmega645", "atmega645a", "atmega645p", "atmega649", "atmega649a", "atmega649p", "atmega6450", "atmega6450a", "atmega6450p", "atmega6490", "atmega6490a", "atmega6490p", "ata5795", "ata5790", "ata5790n", "ata5791", "ata6613c", "ata6614q", "ata5782", "ata5831", "ata8210", "ata8510", "ata5702m322", "at90pwm161", "at90pwm216", "at90pwm316", "at90can32", "at90can64", "at90scr100", "at90usb646", "at90usb647", "at94k", "m3000".
- "avr51"
- "Enhanced" devices with 128 KiB of program memory. mcu = "atmega128", "atmega128a", "atmega128rfa1", "atmega128rfr2", "atmega1280", "atmega1281", "atmega1284", "atmega1284p", "atmega1284rfr2", "at90can128", "at90usb1286", "at90usb1287".
- "avr6"
- "Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB of program memory. mcu = "atmega256rfr2", "atmega2560", "atmega2561", "atmega2564rfr2".
- "avrxmega2"
- "XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. mcu = "atxmega8e5", "atxmega16a4", "atxmega16a4u", "atxmega16c4", "atxmega16d4", "atxmega16e5", "atxmega32a4", "atxmega32a4u", "atxmega32c3", "atxmega32c4", "atxmega32d3", "atxmega32d4", "atxmega32e5".
- "avrxmega3"
- "XMEGA" devices with up to 64 KiB of combined program memory and RAM, and with program memory visible in the RAM address space. mcu = "attiny202", "attiny204", "attiny212", "attiny214", "attiny402", "attiny404", "attiny406", "attiny412", "attiny414", "attiny416", "attiny417", "attiny804", "attiny806", "attiny807", "attiny814", "attiny816", "attiny817", "attiny1604", "attiny1606", "attiny1607", "attiny1614", "attiny1616", "attiny1617", "attiny3214", "attiny3216", "attiny3217", "atmega808", "atmega809", "atmega1608", "atmega1609", "atmega3208", "atmega3209", "atmega4808", "atmega4809".
- "avrxmega4"
- "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory. mcu = "atxmega64a3", "atxmega64a3u", "atxmega64a4u", "atxmega64b1", "atxmega64b3", "atxmega64c3", "atxmega64d3", "atxmega64d4".
- "avrxmega5"
- "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM. mcu = "atxmega64a1", "atxmega64a1u".
- "avrxmega6"
- "XMEGA" devices with more than 128 KiB of program memory. mcu = "atxmega128a3", "atxmega128a3u", "atxmega128b1", "atxmega128b3", "atxmega128c3", "atxmega128d3", "atxmega128d4", "atxmega192a3", "atxmega192a3u", "atxmega192c3", "atxmega192d3", "atxmega256a3", "atxmega256a3b", "atxmega256a3bu", "atxmega256a3u", "atxmega256c3", "atxmega256d3", "atxmega384c3", "atxmega384d3".
- "avrxmega7"
- "XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM. mcu = "atxmega128a1", "atxmega128a1u", "atxmega128a4u".
- "avrtiny"
- "TINY" Tiny core devices with 512 B up to 4 KiB of program memory. mcu = "attiny4", "attiny5", "attiny9", "attiny10", "attiny20", "attiny40".
- "avr1"
- This ISA is implemented by the minimal AVR core and supported for assembler only. mcu = "attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".
- -mabsdata
- Assume that all data in static storage can be accessed by LDS / STS instructions. This option has only an effect on reduced Tiny devices like ATtiny40. See also the "absdata" AVR Variable Attributes,variable attribute.
- -maccumulate-args
- Accumulate outgoing function arguments and acquire/release the needed
stack space for outgoing function arguments once in function
prologue/epilogue. Without this option, outgoing arguments are pushed
before calling a function and popped afterwards.
Popping the arguments after the function call can be expensive on AVR so that accumulating the stack space might lead to smaller executables because arguments need not be removed from the stack after such a function call.
This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack like calls to printf-like functions.
- -mbranch-cost=cost
- Set the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is 0.
- -mcall-prologues
- Functions prologues/epilogues are expanded as calls to appropriate subroutines. Code size is smaller.
- -mgas-isr-prologues
- Interrupt service routines (ISRs) may use the "__gcc_isr" pseudo instruction supported by GNU Binutils. If this option is on, the feature can still be disabled for individual ISRs by means of the AVR Function Attributes,,"no_gccisr" function attribute. This feature is activated per default if optimization is on (but not with -Og, @pxref{Optimize Options}), and if GNU Binutils support PR21683 ("https://sourceware.org/PR21683").
- -mint8
- Assume "int" to be 8-bit integer. This affects the sizes of all types: a "char" is 1 byte, an "int" is 1 byte, a "long" is 2 bytes, and "long long" is 4 bytes. Please note that this option does not conform to the C standards, but it results in smaller code size.
- -mmain-is-OS_task
- Do not save registers in "main". The effect is the same like attaching attribute AVR Function Attributes,,"OS_task" to "main". It is activated per default if optimization is on.
- -mn-flash=num
- Assume that the flash memory has a size of num times 64 KiB.
- -mno-interrupts
- Generated code is not compatible with hardware interrupts. Code size is smaller.
- -mrelax
- Try to replace "CALL" resp.
"JMP" instruction by the shorter
"RCALL" resp.
"RJMP" instruction if applicable.
Setting -mrelax just adds the --mlink-relax option to the
assembler's command line and the --relax option to the linker's
command line.
Jump relaxing is performed by the linker because jump offsets are not known before code is located. Therefore, the assembler code generated by the compiler is the same, but the instructions in the executable may differ from instructions in the assembler code.
Relaxing must be turned on if linker stubs are needed, see the section on "EIND" and linker stubs below.
- -mrmw
- Assume that the device supports the Read-Modify-Write instructions "XCH", "LAC", "LAS" and "LAT".
- -mshort-calls
- Assume that "RJMP" and
"RCALL" can target the whole program
memory.
This option is used internally for multilib selection. It is not an optimization option, and you don't need to set it by hand.
- -msp8
- Treat the stack pointer register as an 8-bit register, i.e. assume the
high byte of the stack pointer is zero. In general, you don't need to set
this option by hand.
This option is used internally by the compiler to select and build multilibs for architectures "avr2" and "avr25". These architectures mix devices with and without "SPH". For any setting other than -mmcu=avr2 or -mmcu=avr25 the compiler driver adds or removes this option from the compiler proper's command line, because the compiler then knows if the device or architecture has an 8-bit stack pointer and thus no "SPH" register or not.
- -mstrict-X
- Use address register "X" in a way
proposed by the hardware. This means that
"X" is only used in indirect,
post-increment or pre-decrement addressing.
Without this option, the "X" register may be used in the same way as "Y" or "Z" which then is emulated by additional instructions. For example, loading a value with "X+const" addressing with a small non-negative "const < 64" to a register Rn is performed as
adiw r26, const ; X += const ld <Rn>, X ; <Rn> = *X sbiw r26, const ; X -= const
- -mtiny-stack
- Only change the lower 8 bits of the stack pointer.
- -mfract-convert-truncate
- Allow to use truncation instead of rounding towards zero for fractional fixed-point types.
- -nodevicelib
- Don't link against AVR-LibC's device specific library "lib<mcu>.a".
- -nodevicespecs
- Don't add -specs=device-specs/specs-<mcu> to the compiler driver's command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options.
- -Waddr-space-convert
- Warn about conversions between address spaces in the case where the resulting address space is not contained in the incoming address space.
- -Wmisspelled-isr
- Warn if the ISR is misspelled, i.e. without __vector prefix. Enabled by default.
"EIND" and Devices with More Than 128 Ki Bytes of Flash
Pointers in the implementation are 16 bits wide. The address of a function or label is represented as word address so that indirect jumps and calls can target any code address in the range of 64 Ki words.
In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there is a special function register called "EIND" that serves as most significant part of the target address when "EICALL" or "EIJMP" instructions are used.
Indirect jumps and calls on these devices are handled as follows by the compiler and are subject to some limitations:
- The compiler never sets "EIND".
- The compiler uses "EIND" implicitly in "EICALL"/"EIJMP" instructions or might read "EIND" directly in order to emulate an indirect call/jump by means of a "RET" instruction.
- The compiler assumes that "EIND" never changes during the startup code or during the application. In particular, "EIND" is not saved/restored in function or interrupt service routine prologue/epilogue.
- For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads sometimes also called trampolines. Thus, the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address.
- Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option -mrelax and the linker option --relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message.
- The default linker script is arranged for code with "EIND = 0". If code is supposed to work for a setup with "EIND != 0", a custom linker script has to be used in order to place the sections whose name start with ".trampolines" into the segment where "EIND" points to.
- The startup code from libgcc never sets "EIND". Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on "EIND", see the AVR-LibC user manual ("http://nongnu.org/avr-libc/user-manual/").
- It is legitimate for user-specific startup code to set up
"EIND" early, for example by means of
initialization code located in section
".init3". Such code runs prior to
general startup code that initializes RAM and calls constructors, but
after the bit of startup code from AVR-LibC that sets
"EIND" to the segment where the vector
table is located.
#include <avr/io.h> static void __attribute__((section(".init3"),naked,used,no_instrument_function)) init3_set_eind (void) { __asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t" "out %i0,r24" :: "n" (&EIND) : "r24","memory"); }
The "__trampolines_start" symbol is defined in the linker script.
- Stubs are generated automatically by the linker if the following two conditions are met:
- -<The address of a label is taken by means of the "gs" modifier>
- (short for generate stubs) like so:
LDI r24, lo8(gs(<func>)) LDI r25, hi8(gs(<func>))
- -<The final location of that label is in a code segment>
- outside the segment where the stubs are located.
- *
- The compiler emits such "gs" modifiers for code labels in the following situations:
- -<Taking address of a function or code label.>
- -<Computed goto.>
- -<If prologue-save function is used, see -mcall-prologues>
- command-line option.
- -<Switch/case dispatch tables. If you do not want such dispatch>
- tables you can specify the -fno-jump-tables command-line option.
- -<C and C++ constructors/destructors called during startup/shutdown.>
- -<If the tools hit a "gs()" modifier explained above.>
- *
- Jumping to non-symbolic addresses like so is not supported:
int main (void) { /* Call function at word address 0x2 */ return ((int(*)(void)) 0x2)(); }
Instead, a stub has to be set up, i.e. the function has to be called through a symbol ("func_4" in the example):
int main (void) { extern int func_4 (void); /* Call function at byte address 0x4 */ return func_4(); }
and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, "func_4" can be defined in the linker script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function Registers
Some AVR devices support memories larger than the 64 KiB range that can be accessed with 16-bit pointers. To access memory locations outside this 64 KiB range, the content of a "RAMP" register is used as high part of the address: The "X", "Y", "Z" address register is concatenated with the "RAMPX", "RAMPY", "RAMPZ" special function register, respectively, to get a wide address. Similarly, "RAMPD" is used together with direct addressing.
- The startup code initializes the "RAMP" special function registers with zero.
- If a AVR Named Address Spaces,named address space other than generic or "__flash" is used, then "RAMPZ" is set as needed before the operation.
- If the device supports RAM larger than 64 KiB and the compiler needs to change "RAMPZ" to accomplish an operation, "RAMPZ" is reset to zero after the operation.
- If the device comes with a specific "RAMP" register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it.
- RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the "RAMP" registers, you must reset it to zero after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option.
For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.
- "__AVR_ARCH__"
- Build-in macro that resolves to a decimal number that identifies the
architecture and depends on the -mmcu=mcu option. Possible
values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51", "avr6",
respectively and
100, 102, 103, 104, 105, 106, 107
for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4", "avrxmega5", "avrxmega6", "avrxmega7", respectively. If mcu specifies a device, this built-in macro is set accordingly. For example, with -mmcu=atmega8 the macro is defined to 4.
- "__AVR_Device__"
- Setting -mmcu=device defines this built-in macro which
reflects the device's name. For example, -mmcu=atmega8 defines the
built-in macro "__AVR_ATmega8__",
-mmcu=attiny261a defines
"__AVR_ATtiny261A__", etc.
The built-in macros' names follow the scheme "__AVR_Device__" where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase.
If device is not a device but only a core architecture like avr51, this macro is not defined.
- "__AVR_DEVICE_NAME__"
- Setting -mmcu=device defines this built-in macro to the
device's name. For example, with -mmcu=atmega8 the macro is defined
to "atmega8".
If device is not a device but only a core architecture like avr51, this macro is not defined.
- "__AVR_XMEGA__"
- The device / architecture belongs to the XMEGA family of devices.
- "__AVR_HAVE_ELPM__"
- The device has the "ELPM" instruction.
- "__AVR_HAVE_ELPMX__"
- The device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.
- "__AVR_HAVE_MOVW__"
- The device has the "MOVW" instruction to perform 16-bit register-register moves.
- "__AVR_HAVE_LPMX__"
- The device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.
- "__AVR_HAVE_MUL__"
- The device has a hardware multiplier.
- "__AVR_HAVE_JMP_CALL__"
- The device has the "JMP" and "CALL" instructions. This is the case for devices with more than 8 KiB of program memory.
- "__AVR_HAVE_EIJMP_EICALL__"
- "__AVR_3_BYTE_PC__"
- The device has the "EIJMP" and "EICALL" instructions. This is the case for devices with more than 128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide.
- "__AVR_2_BYTE_PC__"
- The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.
- "__AVR_HAVE_8BIT_SP__"
- "__AVR_HAVE_16BIT_SP__"
- The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by -mtiny-stack.
- "__AVR_HAVE_SPH__"
- "__AVR_SP8__"
- The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by -mmcu= and in the cases of -mmcu=avr2 and -mmcu=avr25 also by -msp8.
- "__AVR_HAVE_RAMPD__"
- "__AVR_HAVE_RAMPX__"
- "__AVR_HAVE_RAMPY__"
- "__AVR_HAVE_RAMPZ__"
- The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special function register, respectively.
- "__NO_INTERRUPTS__"
- This macro reflects the -mno-interrupts command-line option.
- "__AVR_ERRATA_SKIP__"
- "__AVR_ERRATA_SKIP_JMP_CALL__"
- Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions are "SBRS", "SBRC", "SBIS", "SBIC" and "CPSE". The second macro is only defined if "__AVR_HAVE_JMP_CALL__" is also set.
- "__AVR_ISA_RMW__"
- The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).
- "__AVR_SFR_OFFSET__=offset"
- Instructions that can address I/O special function registers directly like "IN", "OUT", "SBI", etc. may use a different address as if addressed by an instruction to access RAM like "LD" or "STS". This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.
- "__AVR_SHORT_CALLS__"
- The -mshort-calls command line option is set.
- "__AVR_PM_BASE_ADDRESS__=addr"
- Some devices support reading from flash memory by means of "LD*" instructions. The flash memory is seen in the data address space at an offset of "__AVR_PM_BASE_ADDRESS__". If this macro is not defined, this feature is not available. If defined, the address space is linear and there is no need to put ".rodata" into RAM. This is handled by the default linker description file, and is currently available for "avrtiny" and "avrxmega3". Even more convenient, there is no need to use address spaces like "__flash" or features like attribute "progmem" and "pgm_read_*".
- "__WITH_AVRLIBC__"
- The compiler is configured to be used together with AVR-Libc. See the --with-avrlibc configure option.
Blackfin Options¶
- -mcpu=cpu[-sirevision]
- Specifies the name of the target Blackfin processor. Currently, cpu
can be one of bf512, bf514, bf516, bf518,
bf522, bf523, bf524, bf525, bf526,
bf527, bf531, bf532, bf533, bf534,
bf536, bf537, bf538, bf539, bf542,
bf544, bf547, bf548, bf549, bf542m,
bf544m, bf547m, bf548m, bf549m, bf561,
bf592.
The optional sirevision specifies the silicon revision of the target Blackfin processor. Any workarounds available for the targeted silicon revision are enabled. If sirevision is none, no workarounds are enabled. If sirevision is any, all workarounds for the targeted processor are enabled. The "__SILICON_REVISION__" macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If sirevision is none, the "__SILICON_REVISION__" is not defined. If sirevision is any, the "__SILICON_REVISION__" is defined to be 0xffff. If this optional sirevision is not used, GCC assumes the latest known silicon revision of the targeted Blackfin processor.
GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if -msim is not given.
Without this option, bf532 is used as the processor by default.
Note that support for bf561 is incomplete. For bf561, only the preprocessor macro is defined.
- -msim
- Specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. This option has effect only for bfin-elf toolchain. Certain other options, such as -mid-shared-library and -mfdpic, imply -msim.
- -momit-leaf-frame-pointer
- Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions.
- -mspecld-anomaly
- When enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. If this option is used, "__WORKAROUND_SPECULATIVE_LOADS" is defined.
- -mno-specld-anomaly
- Don't generate extra code to prevent speculative loads from occurring.
- -mcsync-anomaly
- When enabled, the compiler ensures that the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. If this option is used, "__WORKAROUND_SPECULATIVE_SYNCS" is defined.
- -mno-csync-anomaly
- Don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch.
- -mlow-64k
- When enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory.
- -mno-low-64k
- Assume that the program is arbitrarily large. This is the default.
- -mstack-check-l1
- Do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.
- -mid-shared-library
- Generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies -fPIC. With a bfin-elf target, this option implies -msim.
- -mno-id-shared-library
- Generate code that doesn't assume ID-based shared libraries are being used. This is the default.
- -mleaf-id-shared-library
- Generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries. That allows the compiler to use faster code for jumps and calls.
- -mno-leaf-id-shared-library
- Do not assume that the code being compiled won't link against any ID shared libraries. Slower code is generated for jump and call insns.
- Specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library but is no more space- or time-efficient than omitting this option.
- -msep-data
- Generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management by eliminating relocations against the text section.
- -mno-sep-data
- Generate code that assumes that the data segment follows the text segment. This is the default.
- -mlong-calls
- -mno-long-calls
- Tells the compiler to perform function calls by first loading the address
of the function into a register and then performing a subroutine call on
this register. This switch is needed if the target function lies outside
of the 24-bit addressing range of the offset-based version of subroutine
call instruction.
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.
- -mfast-fp
- Link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance.
- -minline-plt
- Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without -mfdpic.
- -mmulticore
- Build a standalone application for multicore Blackfin processors. This
option causes proper start files and link scripts supporting multicore to
be used, and defines the macro
"__BFIN_MULTICORE". It can only be used
with -mcpu=bf561[-sirevision].
This option can be used with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is used. In this model, the main function of Core B should be named as "coreb_main".
If this option is not used, the single-core application programming model is used.
- -mcorea
- Build a standalone application for Core A of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core A, and the macro "__BFIN_COREA" is defined. This option can only be used in conjunction with -mmulticore.
- -mcoreb
- Build a standalone application for Core B of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core B, and the macro "__BFIN_COREB" is defined. When this option is used, "coreb_main" should be used instead of "main". This option can only be used in conjunction with -mmulticore.
- -msdram
- Build a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro "__BFIN_SDRAM" is defined. The loader should initialize SDRAM before loading the application.
- -micplb
- Assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.
C6X Options¶
- -march=name
- This specifies the name of the target architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x.
- -mbig-endian
- Generate code for a big-endian target.
- -mlittle-endian
- Generate code for a little-endian target. This is the default.
- -msim
- Choose startup files and linker script suitable for the simulator.
- -msdata=default
- Put small global and static data in the ".neardata" section, which is pointed to by register "B14". Put small uninitialized global and static data in the ".bss" section, which is adjacent to the ".neardata" section. Put small read-only data into the ".rodata" section. The corresponding sections used for large pieces of data are ".fardata", ".far" and ".const".
- -msdata=all
- Put all data, not just small objects, into the sections reserved for small data, and use addressing relative to the "B14" register to access them.
- -msdata=none
- Make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the ".fardata" section, and all uninitialized data in the ".far" section. Put all constant data into the ".const" section.
CRIS Options¶
These options are defined specifically for the CRIS ports.
- -march=architecture-type
- -mcpu=architecture-type
- Generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where the default is v10.
- -mtune=architecture-type
- Tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for architecture-type are the same as for -march=architecture-type.
- -mmax-stack-frame=n
- Warn when the stack frame of a function exceeds n bytes.
- -metrax4
- -metrax100
- The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.
- -mmul-bug-workaround
- -mno-mul-bug-workaround
- Work around a bug in the "muls" and "mulu" instructions for CPU models where it applies. This option is active by default.
- -mpdebug
- Enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect of turning off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file.
- -mcc-init
- Do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes.
- -mno-side-effects
- Do not emit instructions with side effects in addressing modes other than post-increment.
- -mstack-align
- -mno-stack-align
- -mdata-align
- -mno-data-align
- -mconst-align
- -mno-const-align
- These options (no- options) arrange (eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options.
- -m32-bit
- -m16-bit
- -m8-bit
- Similar to the stack- data- and const-align options above, these options arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
- -mno-prologue-epilogue
- -mprologue-epilogue
- With -mno-prologue-epilogue, the normal function prologue and epilogue which set up the stack frame are omitted and no return instructions or return sequences are generated in the code. Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated.
- -mno-gotplt
- -mgotplt
- With -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for functions from the PLT part of the GOT rather than (traditional on other architectures) calls to the PLT. The default is -mgotplt.
- -melf
- Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.
- -mlinux
- Legacy no-op option only recognized with the cris-axis-linux-gnu target.
- -sim
- This option, recognized for the cris-axis-elf, arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are allocated consecutively.
- -sim2
- Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000.
CR16 Options¶
These options are defined specifically for the CR16 ports.
- -mmac
- Enable the use of multiply-accumulate instructions. Disabled by default.
- -mcr16cplus
- -mcr16c
- Generate code for CR16C or CR16C+ architecture. CR16C+ architecture is default.
- -msim
- Links the library libsim.a which is in compatible with simulator. Applicable to ELF compiler only.
- -mint32
- Choose integer type as 32-bit wide.
- -mbit-ops
- Generates "sbit"/"cbit" instructions for bit manipulations.
- -mdata-model=model
- Choose a data model. The choices for model are near, far or medium. medium is default. However, far is not valid with -mcr16c, as the CR16C architecture does not support the far data model.
Darwin Options¶
These options are defined for all architectures running the Darwin operating system.
FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this.
The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library). The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files.
- -Fdir
- Add the framework directory dir to the head of the list of
directories to be searched for header files. These directories are
interleaved with those specified by -I options and are scanned in a
left-to-right order.
A framework directory is a directory with frameworks in it. A framework is a directory with a Headers and/or PrivateHeaders directory contained directly in it that ends in .framework. The name of a framework is the name of this directory excluding the .framework. Headers associated with the framework are found in one of those two directories, with Headers being searched first. A subframework is a framework directory that is in a framework's Frameworks directory. Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they occur in the same framework. A subframework should not have the same name as a framework; a warning is issued if this is violated. Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this. The standard frameworks can be found in /System/Library/Frameworks and /Library/Frameworks. An example include looks like "#include <Framework/header.h>", where Framework denotes the name of the framework and header.h is found in the PrivateHeaders or Headers directory.
- -iframeworkdir
- Like -F except the directory is a treated as a system directory. The main difference between this -iframework and -F is that with -iframework the compiler does not warn about constructs contained within header files found via dir. This option is valid only for the C family of languages.
- -gused
- Emit debugging information for symbols that are used. For stabs debugging format, this enables -feliminate-unused-debug-symbols. This is by default ON.
- -gfull
- Emit debugging information for all symbols and types.
- -mmacosx-version-min=version
- The earliest version of MacOS X that this executable will run on is
version. Typical values of version include
10.1, 10.2, and
10.3.9.
If the compiler was built to use the system's headers by default, then the default for this option is the system version on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and code bases as possible.
- -mkernel
- Enable kernel development mode. The -mkernel option sets -static, -fno-common, -fno-use-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable. This mode also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets.
- -mone-byte-bool
- Override the defaults for "bool" so that
"sizeof(bool)==1". By default
"sizeof(bool)" is
4 when compiling for Darwin/PowerPC and
1 when compiling for Darwin/x86, so this option
has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated without that switch. Using this switch may require recompiling all other modules in a program, including system libraries. Use this switch to conform to a non-default data model.
- -mfix-and-continue
- -ffix-and-continue
- -findirect-data
- Generate code suitable for fast turnaround development, such as to allow GDB to dynamically load .o files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility.
- -all_load
- Loads all members of static archive libraries. See man ld(1) for more information.
- -arch_errors_fatal
- Cause the errors having to do with files that have the wrong architecture to be fatal.
- -bind_at_load
- Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.
- -bundle
- Produce a Mach-o bundle format file. See man ld(1) for more information.
- -bundle_loader executable
- This option specifies the executable that will load the build output file being linked. See man ld(1) for more information.
- -dynamiclib
- When passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin libtool command.
- -force_cpusubtype_ALL
- This causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option.
- -allowable_client client_name
- -client_name
- -compatibility_version
- -current_version
- -dead_strip
- -dependency-file
- -dylib_file
- -dylinker_install_name
- -dynamic
- -exported_symbols_list
- -filelist
- -flat_namespace
- -force_flat_namespace
- -headerpad_max_install_names
- -image_base
- -init
- -install_name
- -keep_private_externs
- -multi_module
- -multiply_defined
- -multiply_defined_unused
- -noall_load
- -no_dead_strip_inits_and_terms
- -nofixprebinding
- -nomultidefs
- -noprebind
- -noseglinkedit
- -pagezero_size
- -prebind
- -prebind_all_twolevel_modules
- -private_bundle
- -read_only_relocs
- -sectalign
- -sectobjectsymbols
- -whyload
- -seg1addr
- -sectcreate
- -sectobjectsymbols
- -sectorder
- -segaddr
- -segs_read_only_addr
- -segs_read_write_addr
- -seg_addr_table
- -seg_addr_table_filename
- -seglinkedit
- -segprot
- -segs_read_only_addr
- -segs_read_write_addr
- -single_module
- -static
- -sub_library
- -sub_umbrella
- -twolevel_namespace
- -umbrella
- -undefined
- -unexported_symbols_list
- -weak_reference_mismatches
- -whatsloaded
- These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.
DEC Alpha Options¶
These -m options are defined for the DEC Alpha implementations:
- -mno-soft-float
- -msoft-float
- Use (do not use) the hardware floating-point instructions for
floating-point operations. When -msoft-float is specified,
functions in libgcc.a are used to perform floating-point
operations. Unless they are replaced by routines that emulate the
floating-point operations, or compiled in such a way as to call such
emulations routines, these routines issue floating-point operations. If
you are compiling for an Alpha without floating-point operations, you must
ensure that the library is built so as not to call them.
Note that Alpha implementations without floating-point operations are required to have floating-point registers.
- -mfp-reg
- -mno-fp-regs
- Generate code that uses (does not use) the floating-point register set.
-mno-fp-regs implies -msoft-float. If the floating-point
register set is not used, floating-point operands are passed in integer
registers as if they were integers and floating-point results are passed
in $0 instead of $f0. This
is a non-standard calling sequence, so any function with a floating-point
argument or return value called by code compiled with -mno-fp-regs
must also be compiled with that option.
A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.
- -mieee
- The Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating-point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE-compliant code except that the inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro "_IEEE_FP" is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact.
- -mieee-with-inexact
- This is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this option causes the generated code to implement fully-compliant IEEE math. In addition to "_IEEE_FP", "_IEEE_FP_EXACT" is defined as a preprocessor macro. On some Alpha implementations the resulting code may execute significantly slower than the code generated by default. Since there is very little code that depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this option -ieee_with_inexact.
- -mfp-trap-mode=trap-mode
- This option controls what floating-point related traps are enabled. Other Alpha compilers call this option -fptm trap-mode. The trap mode can be set to one of four values:
- n
- This is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).
- u
- In addition to the traps enabled by n, underflow traps are enabled as well.
- su
- Like u, but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).
- sui
- Like su, but inexact traps are enabled as well.
- -mfp-rounding-mode=rounding-mode
- Selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The rounding-mode can be one of:
- n
- Normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.
- m
- Round towards minus infinity.
- c
- Chopped rounding mode. Floating-point numbers are rounded towards zero.
- d
- Dynamic rounding mode. A field in the floating-point control register (fpcr, see Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. Thus, unless your program modifies the fpcr, d corresponds to round towards plus infinity.
- -mtrap-precision=trap-precision
- In the Alpha architecture, floating-point traps are imprecise. This means without software assistance it is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating-point trap. Depending on the requirements of an application, different levels of precisions can be selected:
- p
- Program precision. This option is the default and means a trap handler can only identify which program caused a floating-point exception.
- f
- Function precision. The trap handler can determine the function that caused a floating-point exception.
- i
- Instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.
Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.
- -mieee-conformant
- This option marks the generated code as IEEE conformant. You must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file.
- -mbuild-constants
- Normally GCC examines a 32- or 64-bit integer constant to see if it can
construct it from smaller constants in two or three instructions. If it
cannot, it outputs the constant as a literal and generates code to load it
from the data segment at run time.
Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six).
You typically use this option to build a shared library dynamic loader. Itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment.
- -mbwx
- -mno-bwx
- -mcix
- -mno-cix
- -mfix
- -mno-fix
- -mmax
- -mno-max
- Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via -mcpu= option or that of the CPU on which GCC was built if none is specified.
- -mfloat-vax
- -mfloat-ieee
- Generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision.
- -mexplicit-relocs
- -mno-explicit-relocs
- Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros. Use of these macros does not allow optimal instruction scheduling. GNU binutils as of version 2.12 supports a new syntax that allows the compiler to explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, as GCC detects the capabilities of the assembler when it is built and sets the default accordingly.
- -msmall-data
- -mlarge-data
- When -mexplicit-relocs is in effect, static data is accessed via
gp-relative relocations. When -msmall-data is used, objects
8 bytes long or smaller are placed in a small data area (the
".sdata" and
".sbss" sections) and are accessed via
16-bit relocations off of the $gp register. This
limits the size of the small data area to 64KB, but allows the variables
to be directly accessed via a single instruction.
The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that require more than 2GB of data must use "malloc" or "mmap" to allocate the data in the heap instead of in the program's data segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
- -msmall-text
- -mlarge-text
- When -msmall-text is used, the compiler assumes that the code of
the entire program (or shared library) fits in 4MB, and is thus reachable
with a branch instruction. When -msmall-data is used, the compiler
can assume that all local symbols share the same
$gp value, and thus reduce the number of
instructions required for a function call from 4 to 1.
The default is -mlarge-text.
- -mcpu=cpu_type
- Set the instruction set and instruction scheduling parameters for machine
type cpu_type. You can specify either the EV style name or
the corresponding chip number. GCC supports scheduling parameters for the
EV4, EV5 and EV6 family of processors and chooses the default values for
the instruction set from the processor you specify. If you do not specify
a processor type, GCC defaults to the processor on which the compiler was
built.
Supported values for cpu_type are
- ev4
- ev45
- 21064
- Schedules as an EV4 and has no instruction set extensions.
- ev5
- 21164
- Schedules as an EV5 and has no instruction set extensions.
- ev56
- 21164a
- Schedules as an EV5 and supports the BWX extension.
- pca56
- 21164pc
- 21164PC
- Schedules as an EV5 and supports the BWX and MAX extensions.
- ev6
- 21264
- Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
- ev67
- 21264a
- Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
Native toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor.
- -mtune=cpu_type
- Set only the instruction scheduling parameters for machine type
cpu_type. The instruction set is not changed.
Native toolchains also support the value native, which selects the best architecture option for the host processor. -mtune=native has no effect if GCC does not recognize the processor.
- -mmemory-latency=time
- Sets the latency the scheduler should assume for typical memory references
as seen by the application. This number is highly dependent on the memory
access patterns used by the application and the size of the external cache
on the machine.
Valid options for time are
FR30 Options¶
These options are defined specifically for the FR30 port.
- -msmall-model
- Use the small address space model. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range.
- -mno-lsim
- Assume that runtime support has been provided and so there is no need to include the simulator library (libsim.a) on the linker command line.
FT32 Options¶
These options are defined specifically for the FT32 port.
- -msim
- Specifies that the program will be run on the simulator. This causes an alternate runtime startup and library to be linked. You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed.
- -mlra
- Enable Local Register Allocation. This is still experimental for FT32, so by default the compiler uses standard reload.
- -mnodiv
- Do not use div and mod instructions.
- -mft32b
- Enable use of the extended instructions of the FT32B processor.
- -mcompress
- Compress all code using the Ft32B code compression scheme.
- -mnopm
- Do not generate code that reads program memory.
FRV Options¶
- -mgpr-32
- Only use the first 32 general-purpose registers.
- -mgpr-64
- Use all 64 general-purpose registers.
- -mfpr-32
- Use only the first 32 floating-point registers.
- -mfpr-64
- Use all 64 floating-point registers.
- -mhard-float
- Use hardware instructions for floating-point operations.
- -msoft-float
- Use library routines for floating-point operations.
- -malloc-cc
- Dynamically allocate condition code registers.
- -mfixed-cc
- Do not try to dynamically allocate condition code registers, only use "icc0" and "fcc0".
- -mdword
- Change ABI to use double word insns.
- -mno-dword
- Do not use double word instructions.
- -mdouble
- Use floating-point double instructions.
- -mno-double
- Do not use floating-point double instructions.
- -mmedia
- Use media instructions.
- -mno-media
- Do not use media instructions.
- -mmuladd
- Use multiply and add/subtract instructions.
- -mno-muladd
- Do not use multiply and add/subtract instructions.
- -mfdpic
- Select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits. With a bfin-elf target, this option implies -msim.
- -minline-plt
- Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without -mfdpic. It's enabled by default if optimizing for speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the command line.
- -mTLS
- Assume a large TLS segment when generating thread-local code.
- -mtls
- Do not assume a large TLS segment when generating thread-local code.
- -mgprel-ro
- Enable the use of "GPREL" relocations in the FDPIC ABI for data that is known to be in read-only sections. It's enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win. If it is not, -mno-gprel-ro can be used to disable it.
- -multilib-library-pic
- Link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic. You should never have to use it explicitly.
- -mlinked-fp
- Follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option is enabled by default and can be disabled with -mno-linked-fp.
- -mlong-calls
- Use indirect addressing to call functions outside the current compilation unit. This allows the functions to be placed anywhere within the 32-bit address space.
- -malign-labels
- Try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option only has an effect when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to existing ones.
- -mlibrary-pic
- Generate position-independent EABI code.
- -macc-4
- Use only the first four media accumulator registers.
- -macc-8
- Use all eight media accumulator registers.
- -mpack
- Pack VLIW instructions.
- -mno-pack
- Do not pack VLIW instructions.
- -mno-eflags
- Do not mark ABI switches in e_flags.
- -mcond-move
- Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
- -mno-cond-move
- Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
- -mscc
- Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
- -mno-scc
- Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
- -mcond-exec
- Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
- -mno-cond-exec
- Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
- -mvliw-branch
- Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
- -mno-vliw-branch
- Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
- -mmulti-cond-exec
- Enable optimization of "&&" and
"||" in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
- -mno-multi-cond-exec
- Disable optimization of "&&" and
"||" in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
- -mnested-cond-exec
- Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
- -mno-nested-cond-exec
- Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
- -moptimize-membar
- This switch removes redundant "membar" instructions from the compiler-generated code. It is enabled by default.
- -mno-optimize-membar
- This switch disables the automatic removal of redundant "membar" instructions from the generated code.
- -mtomcat-stats
- Cause gas to print out tomcat statistics.
- -mcpu=cpu
- Select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple.
GNU/Linux Options¶
These -m options are defined for GNU/Linux targets:
- -mglibc
- Use the GNU C library. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets.
- -muclibc
- Use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
- -mmusl
- Use the musl C library. This is the default on *-*-linux-*musl* targets.
- -mbionic
- Use Bionic C library. This is the default on *-*-linux-*android* targets.
- -mandroid
- Compile code compatible with Android platform. This is the default on
*-*-linux-*android* targets.
When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the preprocessor macro "__ANDROID__" to be defined.
- -tno-android-cc
- Disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
- -tno-android-ld
- Disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.
H8/300 Options¶
These -m options are defined for the H8/300 implementations:
- -mrelax
- Shorten some address references at link time, when possible; uses the linker option -relax.
- -mh
- Generate code for the H8/300H.
- -ms
- Generate code for the H8S.
- -mn
- Generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms.
- -ms2600
- Generate code for the H8S/2600. This switch must be used with -ms.
- -mexr
- Extended registers are stored on stack before execution of function with monitor attribute. Default option is -mexr. This option is valid only for H8S targets.
- -mno-exr
- Extended registers are not stored on stack before execution of function with monitor attribute. Default option is -mno-exr. This option is valid only for H8S targets.
- -mint32
- Make "int" data 32 bits by default.
- -malign-300
- On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries. This option has no effect on the H8/300.
HPPA Options¶
These -m options are defined for the HPPA family of computers:
- -march=architecture-type
- Generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but not the other way around.
- -mpa-risc-1-0
- -mpa-risc-1-1
- -mpa-risc-2-0
- Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
- -mcaller-copies
- The caller copies function arguments passed by hidden reference. This option should be used with care as it is not compatible with the default 32-bit runtime. However, only aggregates larger than eight bytes are passed by hidden reference and the option provides better compatibility with OpenMP.
- -mjump-in-delay
- This option is ignored and provided for compatibility purposes only.
- -mdisable-fpregs
- Prevent floating-point registers from being used in any manner. This is necessary for compiling kernels that perform lazy context switching of floating-point registers. If you use this option and attempt to perform floating-point operations, the compiler aborts.
- -mdisable-indexing
- Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.
- -mno-space-regs
- Generate code that assumes the target has no space registers. This allows
GCC to generate faster indirect calls and use unscaled index address
modes.
Such code is suitable for level 0 PA systems and kernels.
- -mfast-indirect-calls
- Generate code that assumes calls never cross space boundaries. This allows
GCC to emit code that performs faster indirect calls.
This option does not work in the presence of shared libraries or nested functions.
- -mfixed-range=register-range
- Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
- -mlong-load-store
- Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the +k option to the HP compilers.
- -mportable-runtime
- Use the portable calling conventions proposed by HP for ELF systems.
- -mgas
- Enable the use of assembler directives only GAS understands.
- -mschedule=cpu-type
- Schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700 7100, 7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is 8000.
- -mlinker-opt
- Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs.
- -msoft-float
- Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all HPPA
targets. Normally the facilities of the machine's usual C compiler are
used, but this cannot be done directly in cross-compilation. You must make
your own arrangements to provide suitable library functions for
cross-compilation.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.
- -msio
- Generate the predefine, "_SIO", for server IO. The default is -mwsio. This generates the predefines, "__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. These options are available under HP-UX and HI-UX.
- -mgnu-ld
- Use options specific to GNU ld. This passes -shared to ld when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which `gcc -print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
- -mhp-ld
- Use options specific to HP ld. This passes -b to ld when building a shared library and passes +Accept TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which `gcc -print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
- -mlong-calls
- Generate code that uses long call sequences. This ensures that a call is
always able to reach linker generated stubs. The default is to generate
long calls only when the distance from the call site to the beginning of
the function or translation unit, as the case may be, exceeds a predefined
limit set by the branch type being used. The limits for normal calls are
7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X
architectures. Sibcalls are always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.
It is normally not desirable to use this option as it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long.
- -munix=unix-std
- Generate compiler predefines and select a startfile for the specified UNIX
standard. The choices for unix-std are 93, 95 and
98. 93 is supported on all HP-UX versions. 95 is
available on HP-UX 10.10 and later. 98 is available on HP-UX 11.11
and later. The default values are 93 for HP-UX 10.00, 95 for
HP-UX 10.10 though to 11.00, and 98 for HP-UX 11.11 and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98 provides additional predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. Thus, extreme care is needed in using this option.
Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable "__xpg4_extended_mask" as appropriate. Most GNU software doesn't provide this capability.
- -nolibdld
- Suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later.
- -static
- The HP-UX implementation of setlocale in libc has a dependency on
libdld.sl. There isn't an archive version of libdld.sl. Thus, when the
-static option is specified, special link options are needed to
resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The -nolibdld option can be used to prevent the GCC driver from adding these link options.
- -threads
- Add support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker.
IA-64 Options¶
These are the -m options defined for the Intel IA-64 architecture.
- -mbig-endian
- Generate code for a big-endian target. This is the default for HP-UX.
- -mlittle-endian
- Generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.
- -mgnu-as
- -mno-gnu-as
- Generate (or don't) code for the GNU assembler. This is the default.
- -mgnu-ld
- -mno-gnu-ld
- Generate (or don't) code for the GNU linker. This is the default.
- -mno-pic
- Generate code that does not use a global pointer register. The result is not position independent code, and violates the IA-64 ABI.
- -mvolatile-asm-stop
- -mno-volatile-asm-stop
- Generate (or don't) a stop bit immediately before and after volatile asm statements.
- -mregister-names
- -mno-register-names
- Generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler output more readable.
- -mno-sdata
- -msdata
- Disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs.
- -mconstant-gp
- Generate code that uses a single constant global pointer value. This is useful when compiling kernel code.
- -mauto-pic
- Generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling firmware code.
- -minline-float-divide-min-latency
- Generate code for inline divides of floating-point values using the minimum latency algorithm.
- -minline-float-divide-max-throughput
- Generate code for inline divides of floating-point values using the maximum throughput algorithm.
- -mno-inline-float-divide
- Do not generate inline code for divides of floating-point values.
- -minline-int-divide-min-latency
- Generate code for inline divides of integer values using the minimum latency algorithm.
- -minline-int-divide-max-throughput
- Generate code for inline divides of integer values using the maximum throughput algorithm.
- -mno-inline-int-divide
- Do not generate inline code for divides of integer values.
- -minline-sqrt-min-latency
- Generate code for inline square roots using the minimum latency algorithm.
- -minline-sqrt-max-throughput
- Generate code for inline square roots using the maximum throughput algorithm.
- -mno-inline-sqrt
- Do not generate inline code for "sqrt".
- -mfused-madd
- -mno-fused-madd
- Do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions.
- -mno-dwarf2-asm
- -mdwarf2-asm
- Don't (or do) generate assembler code for the DWARF line number debugging info. This may be useful when not using the GNU assembler.
- -mearly-stop-bits
- -mno-early-stop-bits
- Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but does not always do so.
- -mfixed-range=register-range
- Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
- -mtls-size=tls-size
- Specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
- -mtune=cpu-type
- Tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1, merced, itanium2, and mckinley.
- -milp32
- -mlp64
- Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags.
- -mno-sched-br-data-spec
- -msched-br-data-spec
- (Dis/En)able data speculative scheduling before reload. This results in generation of "ld.a" instructions and the corresponding check instructions ("ld.c" / "chk.a"). The default setting is disabled.
- -msched-ar-data-spec
- -mno-sched-ar-data-spec
- (En/Dis)able data speculative scheduling after reload. This results in generation of "ld.a" instructions and the corresponding check instructions ("ld.c" / "chk.a"). The default setting is enabled.
- -mno-sched-control-spec
- -msched-control-spec
- (Dis/En)able control speculative scheduling. This feature is available only during region scheduling (i.e. before reload). This results in generation of the "ld.s" instructions and the corresponding check instructions "chk.s". The default setting is disabled.
- -msched-br-in-data-spec
- -mno-sched-br-in-data-spec
- (En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload. This is effective only with -msched-br-data-spec enabled. The default setting is enabled.
- -msched-ar-in-data-spec
- -mno-sched-ar-in-data-spec
- (En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after reload. This is effective only with -msched-ar-data-spec enabled. The default setting is enabled.
- -msched-in-control-spec
- -mno-sched-in-control-spec
- (En/Dis)able speculative scheduling of the instructions that are dependent on the control speculative loads. This is effective only with -msched-control-spec enabled. The default setting is enabled.
- -mno-sched-prefer-non-data-spec-insns
- -msched-prefer-non-data-spec-insns
- If enabled, data-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the data speculation much more conservative. The default setting is disabled.
- -mno-sched-prefer-non-control-spec-insns
- -msched-prefer-non-control-spec-insns
- If enabled, control-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the control speculation much more conservative. The default setting is disabled.
- -mno-sched-count-spec-in-critical-path
- -msched-count-spec-in-critical-path
- If enabled, speculative dependencies are considered during computation of the instructions priorities. This makes the use of the speculation a bit more conservative. The default setting is disabled.
- -msched-spec-ldc
- Use a simple data speculation check. This option is on by default.
- -msched-control-spec-ldc
- Use a simple check for control speculation. This option is on by default.
- -msched-stop-bits-after-every-cycle
- Place a stop bit after every cycle when scheduling. This option is on by default.
- -msched-fp-mem-deps-zero-cost
- Assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is disabled by default.
- -msel-sched-dont-check-control-spec
- Generate checks for control speculation in selective scheduling. This flag is disabled by default.
- -msched-max-memory-insns=max-insns
- Limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is 1.
- -msched-max-memory-insns-hard-limit
- Makes the limit specified by msched-max-memory-insns a hard limit, disallowing more than that number in an instruction group. Otherwise, the limit is "soft", meaning that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled.
LM32 Options¶
These -m options are defined for the LatticeMico32 architecture:
- -mbarrel-shift-enabled
- Enable barrel-shift instructions.
- -mdivide-enabled
- Enable divide and modulus instructions.
- -mmultiply-enabled
- Enable multiply instructions.
- -msign-extend-enabled
- Enable sign extend instructions.
- -muser-enabled
- Enable user-defined instructions.
M32C Options¶
- -mcpu=name
- Select the CPU for which code is generated. name may be one of r8c for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
- -msim
- Specifies that the program will be run on the simulator. This causes an alternate runtime library to be linked in which supports, for example, file I/O. You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed.
- -memregs=number
- Specifies the number of memory-based pseudo-registers GCC uses during code generation. These pseudo-registers are used like real registers, so there is a tradeoff between GCC's ability to fit the code into available registers, and the performance penalty of using memory instead of registers. Note that all modules in a program must be compiled with the same value for this option. Because of that, you must not use this option with GCC's default runtime libraries.