GCC

NAME

SYNOPSIS

Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc.  

DESCRIPTION

Other options are passed on to one stage 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 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 would be -fno-foo. This manual documents only one of these two forms, whichever one is not the default.  

OPTIONS

Option Summary

Overall Options

-c -S -E -o file -combine -pipe -pass-exit-codes -x language -v -### --help[=class[,...]] --target-help --version -wrapper@file

C Language Options

-ansi -std=standard -fgnu89-inline -aux-info filename -fno-asm -fno-builtin -fno-builtin-function -fhosted -ffreestanding -fopenmp -fms-extensions -trigraphs -no-integrated-cpp -traditional -traditional-cpp -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 -fcheck-new -fconserve-space -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 -fno-nonansi-builtins -fno-operator-names -fno-optional-diags -fpermissive -frepo -fno-rtti -fstats -ftemplate-depth-n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline -fvisibility-inlines-hidden -fvisibility-ms-compat -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder -Weffc++ -Wstrict-null-sentinel -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo

Objective-C and Objective-C++ Language Options

-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers -fobjc-call-cxx-cdtors -fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept -Wno-protocol -Wselector -Wstrict-selector-match -Wundeclared-selector

Language Independent Options

-fmessage-length=n -fdiagnostics-show-location=[once|every-line] -fdiagnostics-show-option

Warning Options

-fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall -Waddress -Waggregate-return -Warray-bounds -Wno-attributes -Wno-builtin-macro-redefined -Wc++-compat -Wc++0x-compat -Wcast-align -Wcast-qual -Wchar-subscripts -Wclobbered -Wcomment -Wconversion -Wcoverage-mismatch -Wno-deprecated -Wno-deprecated-declarations -Wdisabled-optimization -Wno-div-by-zero -Wempty-body -Wenum-compare -Wno-endif-labels -Werror -Werror=* -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral -Wformat-security -Wformat-y2k -Wframe-larger-than=len -Wignored-qualifiers -Wimplicit -Wimplicit-function-declaration -Wimplicit-int -Winit-self -Winline -Wno-int-to-pointer-cast -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than=len -Wunsafe-loop-optimizations -Wlogical-op -Wlong-long -Wmain -Wmissing-braces -Wmissing-field-initializers -Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-noreturn -Wno-mudflap -Wno-multichar -Wnonnull -Wno-overflow -Woverlength-strings -Wpacked -Wpacked-bitfield-compat -Wpadded -Wparentheses -Wpedantic-ms-format -Wno-pedantic-ms-format -Wpointer-arith -Wno-pointer-to-int-cast -Wredundant-decls -Wreturn-type -Wsequence-point -Wshadow -Wsign-compare -Wsign-conversion -Wstack-protector -Wstrict-aliasing -Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=n -Wswitch -Wswitch-default -Wswitch-enum -Wsync-nand -Wsystem-headers -Wtrigraphs -Wtype-limits -Wundef -Wuninitialized -Wunknown-pragmas -Wno-pragmas -Wunreachable-code -Wunused -Wunused-function -Wunused-label -Wunused-parameter -Wunused-value -Wunused-variable -Wunused-but-set-parameter -Wunused-but-set-variable -Wvariadic-macros -Wvla -Wvolatile-register-var -Wwrite-strings

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

-dletters -dumpspecs -dumpmachine -dumpversion -fdbg-cnt-list -fdbg-cnt=counter-value-list -fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links -fdump-translation-unit[-n] -fdump-class-hierarchy[-n] -fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline -fdump-statistics -fdump-tree-all -fdump-tree-original[-n] -fdump-tree-optimized[-n] -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias -fdump-tree-ch -fdump-tree-ssa[-n] -fdump-tree-pre[-n] -fdump-tree-ccp[-n] -fdump-tree-dce[-n] -fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n] -fdump-tree-dom[-n] -fdump-tree-dse[-n] -fdump-tree-phiopt[-n] -fdump-tree-forwprop[-n] -fdump-tree-copyrename[-n] -fdump-tree-nrv -fdump-tree-vect -fdump-tree-sink -fdump-tree-sra[-n] -fdump-tree-fre[-n] -fdump-tree-vrp[-n] -ftree-vectorizer-verbose=n -fdump-tree-storeccp[-n] -fdump-final-insns=file -fcompare-debug[=opts] -fcompare-debug-second -feliminate-dwarf2-dups -feliminate-unused-debug-types -feliminate-unused-debug-symbols -femit-class-debug-always -fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report -fprofile-arcs -frandom-seed=string -fsched-verbose=n -fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose -ftest-coverage -ftime-report -fvar-tracking -fvar-tracking-assignments -fvar-tracking-assignments-toggle -g -glevel -gtoggle -gcoff -gdwarf-version -ggdb -gstabs -gstabs+ -gstrict-dwarf -gno-strict-dwarf -gvms -gxcoff -gxcoff+ -fno-merge-debug-strings -fno-dwarf2-cfi-asm -fdebug-prefix-map=old=new -femit-struct-debug-baseonly -femit-struct-debug-reduced -femit-struct-debug-detailed[=spec-list] -p -pg -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 -time[=file]

Optimization Options

-falign-functions[=n] -falign-jumps[=n] -falign-labels[=n] -falign-loops[=n] -fassociative-math -fauto-inc-dec -fbranch-probabilities -fbranch-target-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves -fcheck-data-deps -fconserve-stack -fcprop-registers -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range -fdata-sections -fdce -fdce -fdelayed-branch -fdelete-null-pointer-checks -fdse -fdse -fearly-inlining -fexpensive-optimizations -ffast-math -ffinite-math-only -ffloat-store -fforward-propagate -ffunction-sections -fgcse -fgcse-after-reload -fgcse-las -fgcse-lm -fgcse-sm -fif-conversion -fif-conversion2 -findirect-inlining -finline-functions -finline-functions-called-once -finline-limit=n -finline-small-functions -fipa-cp -fipa-cp-clone -fipa-matrix-reorg -fipa-pta -fipa-pure-const -fipa-reference -fipa-struct-reorg -fipa-type-escape -fira-algorithm=algorithm -fira-region=region -fira-coalesce -fno-ira-share-save-slots -fno-ira-share-spill-slots -fira-verbose=n -fivopts -fkeep-inline-functions -fkeep-static-consts -floop-block -floop-interchange -floop-strip-mine -fmerge-all-constants -fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves -fmove-loop-invariants -fmudflap -fmudflapir -fmudflapth -fno-branch-count-reg -fno-default-inline -fno-defer-pop -fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 -fno-sched-interblock -fno-sched-spec -fno-signed-zeros -fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls -fpeel-loops -fpredictive-commoning -fprefetch-loop-arrays -fprofile-correction -fprofile-dir=path -fprofile-generate -fprofile-generate=path -fprofile-use -fprofile-use=path -fprofile-values -freciprocal-math -fregmove -frename-registers -freorder-blocks -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -freschedule-modulo-scheduled-loops -frounding-math -frtl-abstract-sequences -fsched2-use-superblocks -fsched2-use-traces -fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n] -fschedule-insns -fschedule-insns2 -fsection-anchors -fsee -fselective-scheduling -fselective-scheduling2 -fsel-sched-pipelining -fsel-sched-pipelining-outer-loops -fsignaling-nans -fsingle-precision-constant -fsplit-ivs-in-unroller -fsplit-wide-types -fstack-protector -fstack-protector-all -fstrict-aliasing -fstrict-overflow -fthread-jumps -ftracer -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-inline-vars -ftree-coalesce-vars -ftree-copy-prop -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-fre -ftree-loop-im -ftree-loop-distribution -ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize -ftree-parallelize-loops=n -ftree-pre -ftree-reassoc -ftree-sink -ftree-sra -ftree-switch-conversion -ftree-ter -ftree-vect-loop-version -ftree-vectorize -ftree-vrp -funit-at-a-time -funroll-all-loops -funroll-loops -funsafe-loop-optimizations -funsafe-math-optimizations -funswitch-loops -fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb -fwhole-program --param name=value -O -O0 -O1 -O2 -O3 -Os

Preprocessor Options

-Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN -Dmacro[=defn] -E -H -idirafter dir -include file -imacros file -iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem dir -imultilib dir -isysroot dir -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -fworking-directory -remap -trigraphs -undef -Umacro -Wp,option -Xpreprocessor option

Assembler Option

-Wa,option -Xassembler option

Linker Options

object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -pie -rdynamic -s -static -static-libgcc -shared -shared-libgcc -symbolic -T script -Wl,option -Xlinker option -u symbol

Directory Options

-Bprefix -Idir -iquotedir -Ldir -specs=file -I- --sysroot=dir

Target Options

-V version -b machine

Machine Dependent Options

ARC Options -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section -mdata=data-section -mrodata=readonly-data-section

ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check -mno-apcs-stack-check -mapcs-float -mno-apcs-float -mapcs-reentrant -mno-apcs-reentrant -msched-prolog -mno-sched-prolog -mlittle-endian -mbig-endian -mwords-little-endian -mfloat-abi=name -msoft-float -mhard-float -mfpe -mthumb-interwork -mno-thumb-interwork -mcpu=name -march=name -mfpu=name -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 -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns -mpoke-function-name -mthumb -marm -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking -mtp=name -mword-relocations -mfix-cortex-m3-ldrd

AVR Options -mmcu=mcu -msize -mno-interrupts -mcall-prologues -mno-tablejump -mtiny-stack -mint8

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

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

CRX Options -mmac -mpush-args

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 -malpha-as -mgas -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

DEC Alpha/VMS Options -mvms-return-codes

FR30 Options -msmall-model -mno-lsim

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 -muclibc

H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300

HPPA Options -march=architecture-type -mbig-switch -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-big-switch -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

i386 and x86-64 Options -mtune=cpu-type -march=cpu-type -mfpmath=unit -masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387 -msoft-float -mno-wide-multiply -mrtd -malign-double -mpreferred-stack-boundary=num -mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe -mcrc32 -mrecip -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx -maes -mpclmul -mfused-madd -msse4a -m3dnow -mpopcnt -mabm -mfma4 -mxop -mlwp -mthreads -mno-align-stringops -minline-all-stringops -minline-stringops-dynamically -mstringop-strategy=alg -mpush-args -maccumulate-outgoing-args -m128bit-long-double -m96bit-long-double -mregparm=num -msseregparm -mveclibabi=type -mpc32 -mpc64 -mpc80 -mstackrealign -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model -m32 -m64 -mlarge-data-threshold=num -msse2avx

IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop -mregister-names -mno-sdata -mconstant-gp -mauto-pic -minline-float-divide-min-latency -minline-float-divide-max-throughput -minline-int-divide-min-latency -minline-int-divide-max-throughput -minline-sqrt-min-latency -minline-sqrt-max-throughput -mno-dwarf2-asm -mearly-stop-bits -mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-type -mt -pthread -milp32 -mlp64 -mno-sched-br-data-spec -msched-ar-data-spec -mno-sched-control-spec -msched-br-in-data-spec -msched-ar-in-data-spec -msched-in-control-spec -msched-ldc -mno-sched-control-ldc -mno-sched-spec-verbose -mno-sched-prefer-non-data-spec-insns -mno-sched-prefer-non-control-spec-insns -mno-sched-count-spec-in-critical-path

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

M68hc1x Options -m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 -mauto-incdec -minmax -mlong-calls -mshort -msoft-reg-count=count

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

MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 -mips64r2 -mips16 -mno-mips16 -mflip-mips16 -minterlink-mips16 -mno-interlink-mips16 -mabi=abi -mabicalls -mno-abicalls -mshared -mno-shared -mplt -mno-plt -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfp64 -mhard-float -msoft-float -msingle-float -mdouble-float -mdsp -mno-dsp -mdspr2 -mno-dspr2 -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 -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 -mfix-r10000 -mno-fix-r10000 -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 -mfp-exceptions -mno-fp-exceptions -mvr4130-align -mno-vr4130-align

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 -mam33 -mno-am33 -mam33-2 -mno-am33-2 -mreturn-pointer-on-d0 -mno-crt0 -mrelax

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 -msplit -mno-split -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.

RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mpower -mno-power -mpower2 -mno-power2 -mpowerpc -mpowerpc64 -mno-powerpc -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 -mnew-mnemonics -mold-mnemonics -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 -mstring -mno-string -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 -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 -misel -mno-isel -misel=yes -misel=no -mspe -mno-spe -mspe=yes -mspe=no -mpaired -mgen-cell-microcode -mwarn-cell-microcode -mvrsave -mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -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 -pthread

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 -mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard

Score Options -meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u -mscore7 -mscore7d

SH Options -m1 -m2 -m2e -m3 -m3e -m4-nofpu -m4-single-only -m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu -m5-compact -m5-compact-nofpu -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave -mieee -mbitops -misize -minline-ic_invalidate -mpadstruct -mspace -mprefergot -musermode -multcost=number -mdiv=strategy -mdivsi3_libfunc=name -mfixed-range=register-range -madjust-unroll -mindexed-addressing -mgettrcost=number -mpt-fixed -minvalid-symbols

SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -m32 -m64 -mapp-regs -mno-app-regs -mfaster-structs -mno-faster-structs -mfpu -mno-fpu -mhard-float -msoft-float -mhard-quad-float -msoft-quad-float -mimpure-text -mno-impure-text -mlittle-endian -mstack-bias -mno-stack-bias -munaligned-doubles -mno-unaligned-doubles -mv8plus -mno-v8plus -mvis -mno-vis -threads -pthreads -pthread

SPU Options -mwarn-reloc -merror-reloc -msafe-dma -munsafe-dma -mbranch-hints -msmall-mem -mlarge-mem -mstdmain -mfixed-range=register-range

System V Options -Qy -Qn -YP,paths -Ym,dir

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 -mv850e1 -mv850e -mv850 -mbig-switch

VAX Options -mg -mgnu -munix

VxWorks Options -mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy -Xbind-now

x86-64 Options See i386 and x86-64 Options.

i386 and x86-64 Windows Options -mconsole -mcygwin -mno-cygwin -mdll -mnop-fun-dllimport -mthread -mwin32 -mwindows

Xstormy16 Options -msim

Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mserialize-volatile -mno-serialize-volatile -mtext-section-literals -mno-text-section-literals -mtarget-align -mno-target-align -mlongcalls -mno-longcalls

zSeries Options See S/390 and zSeries Options.

Code Generation Options

-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions -funwind-tables -fasynchronous-unwind-tables -finhibit-size-directive -finstrument-functions -finstrument-functions-exclude-function-list=sym,sym,... -finstrument-functions-exclude-file-list=file,file,... -fno-common -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-jump-tables -frecord-gcc-switches -freg-struct-return -fshort-enums -fshort-double -fshort-wchar -fverbose-asm -fpack-struct[=n] -fstack-check -fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit -fargument-alias -fargument-noalias -fargument-noalias-global -fargument-noalias-anything -fleading-underscore -ftls-model=model -ftrapv -fwrapv -fbounds-check -fvisibility

Options Controlling the Kind of Output

For any given input file, the file name suffix determines what kind of compilation is done:

file.c

C source code which must be preprocessed.

file.i

C source code which should not be preprocessed.

file.ii

C++ source code which 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 which 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 which should not be preprocessed.

file.h

C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header.

file.cc

file.cp

file.cxx

file.cpp

file.CPP

file.c++

file.C

C++ source code which 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 which must be preprocessed.

file.mii

Objective-C++ source code which 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.

file.f

file.for

file.ftn

Fixed form Fortran source code which should not be preprocessed.

file.F

file.FOR

file.fpp

file.FPP

file.FTN

Fixed form Fortran source code which must be preprocessed (with the traditional preprocessor).

file.f90

file.f95

file.f03

file.f08

Free form Fortran source code which should not be preprocessed.

file.F90

file.F95

file.F03

file.F08

Free form Fortran source code which must be preprocessed (with the traditional preprocessor).

file.ads

Ada source code file which 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 which 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  c-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
        java

-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).

-pass-exit-codes

Normally the gcc program will exit 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 will instead return with numerically highest error produced by any phase that returned an error indication. The C, C++, and Fortran frontends return 4, if an internal compiler error is encountered.

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 which don't require preprocessing are ignored.

-o file

Place output in file file. This applies regardless 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 all command arguments are quoted. This is useful for shell scripts to capture the driver-generated command lines.

-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.

-combine

If you are compiling multiple source files, this option tells the driver to pass all the source files to the compiler at once (for those languages for which the compiler can handle this). This will allow intermodule analysis (IMA) to be performed by the compiler. Currently the only language for which this is supported is C. If you pass source files for multiple languages to the driver, using this option, the driver will invoke the compiler(s) that support IMA once each, passing each compiler all the source files appropriate for it. For those languages that do not support IMA this option will be ignored, and the compiler will be invoked once for each source file in that language. If you use this option in conjunction with -save-temps, the compiler will generate multiple pre-processed files (one for each source file), but only one (combined) .o or .s file.

--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 will also be 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 which have no documentation associated with them will also be 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

This will display all of the optimization options supported by the compiler.

warnings

This will display all of the options controlling warning messages produced by the compiler.

target

This will display target-specific options. Unlike the --target-help option however, target-specific options of the linker and assembler will not be displayed. This is because those tools do not currently support the extended --help= syntax.

params

This will display the values recognized by the --param option.

language

This will display the options supported for language, where language is the name of one of the languages supported in this version of GCC.

common

This will display the options that are common to all languages.

These are the supported qualifiers:

undocumented

Display only those options which are undocumented.

joined

Display options which take an argument that appears after an equal sign in the same continuous piece of text, such as: --help=target.

separate

Display options which take 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 the following can be used:

        --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), which have a description the following can be used:

        --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 by so much that there is nothing to display. One case where it does work however is when one of the classes is target. So for example to display all the target-specific optimization options the following can be used:

        --help=target,optimizers

The --help= option can be repeated on the command line. Each successive use will display 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.

-wrapper

Invoke all subcommands under a wrapper program. It takes a single comma separated list as an argument, which will be used to invoke the wrapper:

        gcc -c t.c -wrapper gdb,--args

This will invoke all subprograms of gcc under ``gdb --args'', thus cc1 invocation will be ``gdb --args cc1 ...''.

@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

However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and treats .c, .h and .i files as C++ source files instead of C source files unless -x is used, and automatically specifies linking against the C++ library. 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

-ansi

In C mode, this is equivalent to -std=c89. 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, -pedantic 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 would normally be 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 c89 or c++98, and GNU dialects of those standards, such as gnu89 or gnu++98. By specifying a base standard, the compiler will accept all programs following that standard and those using GNU extensions that do not contradict it. For example, -std=c89 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, by specifying a GNU dialect of a standard, all features the compiler support are enabled, even when those features change the meaning of the base standard and some strict-conforming programs may be rejected. The particular standard is used by -pedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu89 -pedantic would warn about C++ style // comments, while -std=gnu99 -pedantic would not.

A value for this option must be provided; possible values are

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. Note that this standard is not yet fully supported; see <http://gcc.gnu.org/gcc-4.4/c99status.html> for more information. The names c9x and iso9899:199x are deprecated.

gnu89

GNU dialect of ISO C90 (including some C99 features). This is the default for C code.

gnu99

gnu9x

GNU dialect of ISO C99. When ISO C99 is fully implemented in GCC, this will become the default. The name gnu9x is deprecated.

c++98

The 1998 ISO C++ standard plus amendments. Same as -ansi for C++ code.

gnu++98

GNU dialect of -std=c++98. This is the default for C++ code.

c++0x

The working draft of the upcoming ISO C++0x standard. This option enables experimental features that are likely to be included in C++0x. The working draft is constantly changing, and any feature that is enabled by this flag may be removed from future versions of GCC if it is not part of the C++0x standard.

gnu++0x

GNU dialect of -std=c++0x. This option enables experimental features that may be removed in future versions of GCC.

-fgnu89-inline

The option -fgnu89-inline tells GCC to use the traditional GNU semantics for "inline" functions when in C99 mode.
  This option is accepted and ignored by GCC versions 4.1.3 up to but not including 4.3. In GCC versions 4.3 and later it changes the behavior of GCC 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 was first supported in GCC 4.3. This option is not supported in C89 or gnu89 mode.

The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which semantics are in effect for "inline" functions.

-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.

-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 that 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))

-fhosted

Assert that compilation takes place in 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 takes place in 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.

-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 v2.5 <http://www.openmp.org/>. This option implies -pthread, and thus is only supported on targets that have support for -pthread.

-fms-extensions

Accept some non-standard constructs used in Microsoft header files.

Some cases of unnamed fields in structures and unions are only accepted with this option.

-trigraphs

Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance) implies -trigraphs.

-no-integrated-cpp

Performs a compilation in two passes: preprocessing and compiling. This option allows a user supplied ``cc1'', ``cc1plus'', or ``cc1obj'' via the -B option. The user supplied compilation step can then add in an additional preprocessing step after normal preprocessing but before compiling. The default is to use the integrated cpp (internal cpp)

The semantics of this option will change if ``cc1'', ``cc1plus'', and ``cc1obj'' are merged.

-traditional

-traditional-cpp

Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler. They are now only supported with the -E switch. The preprocessor continues to support a pre-standard mode. See the GNU CPP manual for details.

-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.

Options Controlling C++ Dialect

        g++ -g -frepo -O -c firstClass.C

In this example, only -frepo is an option meant only for C++ programs; you can use the other options with any language supported by GCC.

Here is a list of options that are only for compiling C++ programs:

-fabi-version=n

Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++ 3.4. Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 0 will always be the version that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change as ABI bugs are fixed.

The default is version 2.

-fno-access-control

Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.

-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" will only return 0 if it is declared throw(), in which case the compiler will always check 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).

-fconserve-space

Put uninitialized or runtime-initialized global variables into the common segment, as C does. This saves space in the executable at the cost of not diagnosing duplicate definitions. If you compile with this flag and your program mysteriously crashes after "main()" has completed, you may have an object that is being destroyed twice because two definitions were merged.

This option is no longer useful on most targets, now that support has been added for putting variables into BSS without making them common.

-fno-deduce-init-list

Disable 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 option is present because this deduction is an extension to the current specification in the C++0x working draft, and there was some concern about potential overload resolution problems.

-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, and versions of G++ before 4.1 always worked that way. However, in ISO C++ a friend function which is not declared in an enclosing scope can only be found using argument dependent lookup. This option causes friends to be injected as they were in earlier releases.

This option is for compatibility, and may be removed in a future release of G++.

-fno-elide-constructors

The C++ standard allows an implementation to omit creating a temporary which 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.

-fno-enforce-eh-specs

Don't generate code to check for violation of exception specifications at runtime. 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 will still optimize based on the specifications, so throwing an unexpected exception will result in undefined behavior.

-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++.

The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.

-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. -ansi implies -fno-gnu-keywords.

-fno-implicit-templates

Never emit code for non-inline templates which 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 will 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 will cause linker errors if these functions are not inlined everywhere they are called.

-fms-extensions

Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.

-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.

-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 will allow some nonconforming code to compile.

-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++ runtime 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 it will generate it as needed. The dynamic_cast operator can still be used for casts that do not require runtime type information, i.e. casts to "void *" or to unambiguous base classes.

-fstats

Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to the G++ development team.

-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.

-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 will only work if your C library supports "__cxa_atexit".

-fno-use-cxa-get-exception-ptr

Don't use the "__cxa_get_exception_ptr" runtime routine. This will cause "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 methods where the addresses of the two functions were 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 will have 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 which are defined in more than one different shared object: those declarations are permitted if they would have been permitted when this option was not used.

In new code it is better to use -fvisibility=hidden and export those classes which 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 will be different, so changing one will 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++ will use weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it will result 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:

-fno-default-inline

Do not assume inline for functions defined inside a class scope.
  Note that these functions will have linkage like inline functions; they just won't be inlined by default.

-Wabi (C, Objective-C, C++ and Objective-C++ only)

Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. 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 will be 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.

The known incompatibilities at this point include:

*

Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the same byte as a base class. For example:

        struct A { virtual void f(); int f1 : 1; };
        struct B : public A { int f2 : 1; };

In this case, G++ will place "B::f2" into the same byte as"A::f1"; other compilers will not. You can avoid this problem by explicitly padding "A" so that its size is a multiple of the byte size on your platform; that will cause G++ and other compilers to layout "B" identically.

*

Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out virtual bases. For example:

        struct A { virtual void f(); char c1; };
        struct B { B(); char c2; };
        struct C : public A, public virtual B {};

In this case, G++ will not place "B" into the tail-padding for "A"; other compilers will. You can avoid this problem by explicitly padding "A" so that its size is a multiple of its alignment (ignoring virtual base classes); that will cause G++ and other compilers to layout "C" identically.

*

Incorrect handling of bit-fields with declared widths greater than that of their underlying types, when the bit-fields appear in a union. For example:

        union U { int i : 4096; };

Assuming that an "int" does not have 4096 bits, G++ will make the union too small by the number of bits in an "int".

*

Empty classes can be placed at incorrect offsets. For example:

        struct A {};
        
        struct B {
          A a;
          virtual void f ();
        };
        
        struct C : public B, public A {};

G++ will place the "A" base class of "C" at a nonzero offset; it should be placed at offset zero. G++ mistakenly believes that the "A" data member of "B" is already at offset zero.

*

Names of template functions whose types involve "typename" or template template parameters can be mangled incorrectly.

        template <typename Q>
        void f(typename Q::X) {}
        
        template <template <typename> class Q>
        void f(typename Q<int>::X) {}

Instantiations of these templates may be mangled incorrectly.

It also warns psABI related changes. The known psABI changes at this point include:

*

For SYSV/x86-64, when passing union with long double, it is changed to pass in memory as specified in psABI. For example:

        union U {
          long double ld;
          int i;
        };

"union U" will always be passed in memory.

-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.

-Wnon-virtual-dtor (C++ and Objective-C++ only)

Warn when a class has virtual functions and accessible non-virtual destructor, in which case it would be possible but unsafe to delete an instance of a derived class through a pointer to the base class. This warning is also enabled if -Weffc++ is specified.

-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 will rearrange 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.

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++ book:

*

Item 11: Define a copy constructor and an assignment operator for classes with dynamically allocated memory.

*

Item 12: Prefer initialization to assignment in constructors.

*

Item 14: Make destructors virtual in base classes.

*

Item 15: Have "operator=" return a reference to *this.

*

Item 23: Don't try to return a reference when you must return an object.

Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book:

*

Item 6: Distinguish between prefix and postfix forms of increment and decrement operators.

*

Item 7: Never overload "&&", "||", or ",".

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 also 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 not 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-templatized friend functions are declared within a template. Since the advent of explicit template specification support in G++, if the name of the friend is an unqualified-id (i.e., friend foo(int)), the C++ language specification demands that the friend declare or define an ordinary, nontemplate function. (Section 14.5.3). Before G++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization of a templatized function. Because this non-conforming behavior is no longer the default behavior for G++, -Wnon-template-friend allows the compiler to check existing code for potential trouble spots and is on by default. This new compiler behavior can be turned off with -Wno-non-template-friend which keeps the conformant compiler code but disables the helpful warning.

-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();

will fail 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++ would try to preserve unsignedness, but the standard mandates the current behavior.

        struct A {
          operator int ();
          A& operator = (int);
        };
        
        main ()
        {
          A a,b;
          a = b;
        }

In this example, G++ will synthesize a default A& operator = (const A&);, while cfront will use the user-defined operator =.

Options Controlling Objective-C and Objective-C++ Dialects

This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs, but 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, will override 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 (e.g., "[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. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

-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 that will run 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 that will run all such default destructors, in reverse order.

The "- (id) .cxx_construct" and/or "- (void) .cxx_destruct" methods thusly generated will 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 will be invoked by the runtime immediately after a new object instance is allocated; the "- (void) .cxx_destruct" methods will be 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++ and Java. This option is unavailable in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier.

          @try {
            ...
               @throw expr;
            ...
          }
          @catch (AnObjCClass *exc) {
            ...
              @throw expr;
            ...
              @throw;
            ...
          }
          @catch (AnotherClass *exc) {
            ...
          }
          @catch (id allOthers) {
            ...
          }
          @finally {
            ...
              @throw expr;
            ...
          }

The @throw statement may appear anywhere in an Objective-C or Objective-C++ program; when used inside of a @catch block, the @throw may appear without an argument (as shown above), in which case the object caught by the @catch will be rethrown.

Note that only (pointers to) Objective-C objects may be thrown and caught using this scheme. When an object is thrown, it will be caught by the nearest @catch clause capable of handling objects of that type, analogously to how "catch" blocks work in C++ and Java. A "@catch(id ...)" clause (as shown above) may also be provided to catch any and all Objective-C exceptions not caught by previous @catch clauses (if any).

The @finally clause, if present, will be executed upon exit from the immediately preceding "@try ... @catch" section. This will happen regardless of whether any exceptions are thrown, caught or rethrown inside the "@try ... @catch" section, analogously to the behavior of the "finally" clause in Java.

There are several caveats to using the new exception mechanism:

*

Although currently designed to be binary compatible with "NS_HANDLER"-style idioms provided by the "NSException" class, the new exceptions can only be used on Mac OS X 10.3 (Panther) and later systems, due to additional functionality needed in the (NeXT) Objective-C runtime.

*

As mentioned above, the new exceptions do not support handling types other than Objective-C objects. Furthermore, when used from Objective-C++, the Objective-C exception model does not interoperate with C++ exceptions at this time. This means you cannot @throw an exception from Objective-C and "catch" it in C++, or vice versa (i.e., "throw ... @catch").

The -fobjc-exceptions switch also enables the use of synchronization blocks for thread-safe execution:

          @synchronized (ObjCClass *guard) {
            ...
          }

Upon entering the @synchronized block, a thread of execution shall first check whether a lock has been placed on the corresponding "guard" object by another thread. If it has, the current thread shall wait until the other thread relinquishes its lock. Once "guard" becomes available, the current thread will place its own lock on it, execute the code contained in the @synchronized block, and finally relinquish the lock (thereby making "guard" available to other threads).

Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized. Note that throwing exceptions out of @synchronized blocks is allowed, and will cause the guarding object to be unlocked properly.

-fobjc-gc

Enable garbage collection (GC) in Objective-C and Objective-C++ programs.

-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.

-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 will omit such warnings if any differences found are confined to types which 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

-fmessage-length=n

Try to format error messages so that they fit on lines of about n characters. The default is 72 characters for g++ and 0 for the rest of the front ends supported by GCC. If n is zero, then no line-wrapping will be done; each error message will appear on a single line.

-fdiagnostics-show-location=once

Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit once source location information; 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-show-option

This option instructs the diagnostic machinery to add text to each diagnostic emitted, which indicates which command line option directly controls that diagnostic, when such an option is known to the diagnostic machinery.

-Wcoverage-mismatch

Warn if feedback profiles do not match when using the -fprofile-use option. If a source file was changed between -fprofile-gen and -fprofile-use, the files with the profile feedback can fail to match the source file and GCC can not use the profile feedback information. By default, GCC emits an error message in this case. The option -Wcoverage-mismatch emits a warning instead of an error. GCC does not use appropriate feedback profiles, so using this option can result in poorly optimized code. This option is useful only in the case of very minor changes such as bug fixes to an existing code-base.

Options to Request or Suppress Warnings

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.

-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. You can use the -fdiagnostics-show-option option to have each controllable warning amended with the option which controls it, to determine what to use with this option.

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 -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.

-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 will 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.

-pedantic 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 -pedantic 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 -pedantic. 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 gnu89 or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -pedantic are given where they are required by the base standard. (It would 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

Like -pedantic, except that errors are produced rather than warnings.

-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 (only with -O2) -Wc++0x-compat -Wchar-subscripts -Wimplicit-int -Wimplicit-function-declaration -Wcomment -Wformat -Wmain (only for C/ObjC and unless -ffreestanding) -Wmissing-braces -Wnonnull -Wparentheses -Wpointer-sign -Wreorder -Wreturn-type -Wsequence-point -Wsign-compare (only in C++) -Wstrict-aliasing -Wstrict-overflow=1 -Wswitch -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 -Wempty-body -Wignored-qualifiers -Wmissing-field-initializers -Wmissing-parameter-type (C only) -Wold-style-declaration (C only) -Woverride-init -Wsign-compare -Wtype-limits -Wuninitialized -Wunused-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 which has been declared register.

*

(C++ only) Taking the address of a variable which has been declared register.

*

(C++ only) A base class is not initialized in a derived class' copy constructor.

-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.

-Wcomment

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.

-Wformat

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 -pedantic is used with -Wformat, warnings will be 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).

Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.

-Wformat is included in -Wall. For more control over some aspects of format checking, the options -Wformat-y2k, -Wno-format-extra-args, -Wno-format-zero-length, -Wformat-nonliteral, -Wformat-security, and -Wformat=2 are available, but are not included in -Wall.

-Wformat-y2k

If -Wformat is specified, also warn about "strftime" formats which may yield only a two-digit year.

-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 will suppress the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed.

-Wno-format-zero-length (C and Objective-C only)

If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed.

-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=2

Enable -Wformat plus format checks not included in -Wformat. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.

-Wnonnull (C and Objective-C only)

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.

-Winit-self (C, C++, Objective-C and Objective-C++ only)

Warn about uninitialized variables which are initialized with themselves. Note this option can only be used with the -Wuninitialized option.

For example, GCC will warn about "i" being uninitialized in the following snippet only when -Winit-self has been specified:

        int f()
        {
          int i = i;
          return i;
        }

-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

Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.

-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.

-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 -pedantic.

-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.

        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.

-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 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 will issue 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" could belong to the enclosing "if". The resulting code would look like this:

        {
          if (a)
            {
              if (b)
                foo ();
              else
                bar ();
            }
        }

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 defines 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 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++.

-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), and about a "return" statement with a expression in a function whose return-type is "void".

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 -Wall.

-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. 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.

-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.

-Wtrigraphs

Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within comments are not warned about). This warning is enabled by -Wall.

-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.

-Wunused-but-set-variable

Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration).

To suppress this warning use the unused attribute.

-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-parameter

Warn whenever a function parameter is unused aside from its declaration.

To suppress this warning use the unused attribute.

-Wunused-variable

Warn whenever a local variable or non-constant static variable is unused aside from its declaration. This warning is enabled by -Wall.

To suppress this warning use the unused attribute.

-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] will cause a warning, while x[(void)i,j] will 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 which 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 which 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 will depend 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.

These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct despite 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. Here is another common case:

        {
          int save_y;
          if (change_y) save_y = y, y = new_y;
          ...
          if (change_y) y = save_y;
        }

This has no bug because "save_y" is used only if it is set.

This option also warns when a non-volatile automatic variable might be changed by a call to "longjmp". These warnings as well are possible only in optimizing compilation.

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 which 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 which is not understood by GCC. If this command line option is used, warnings will even be issued for unknown pragmas in system header files. This is not the case if the warnings were 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 which 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 which 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=n, with n=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 frontend 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 frontend 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 punn+dereference pattern in the frontend: "*(int*)&some_float". If optimization is enabled, it also runs in the backend, 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 -fstrict-overflow is active. 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 which 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 which 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 will require, in particular when determining whether a loop will be executed at all.

-Wstrict-overflow=1

Warn about cases which are both questionable and easy to avoid. For example: "x + 1 > x"; with -fstrict-overflow, the compiler will simplify this 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 -fstrict-overflow is in effect, 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" will be simplified to "x > 0".

-Wstrict-overflow=4

Also warn about other simplifications not covered by the above cases. For example: "(x * 10) / 5" will be 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" will be 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 will give a very large number of false positives.

-Warray-bounds

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.

-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 will not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.

-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 would 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 which 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 does not in ISO C.

*

In traditional C, some preprocessor directives did not exist. Traditional preprocessors would only consider 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 would ignore 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 would not recognize #elif, so it 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 would cause 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 will 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 and was not supported by GCC versions before GCC 3.0.

-Wundef

Warn if an undefined identifier is evaluated in an #if directive.

-Wno-endif-labels

Do not warn whenever an #else or an #endif are followed by text.

-Wshadow

Warn whenever a local variable shadows another local variable, parameter or global variable or whenever a built-in function is shadowed.

-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.

-Wunsafe-loop-optimizations

Warn if the loop cannot be optimized because the compiler could not assume anything on the bounds of the loop indices. With -funsafe-loop-optimizations warn if the compiler made such assumptions.

-Wno-pedantic-ms-format (MinGW targets only)

Disables the warnings about non-ISO "printf" / "scanf" format width specifiers "I32", "I64", and "I" used on Windows targets depending on the MS runtime, when you are using the options -Wformat and -pedantic without gnu-extensions.

-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 -pedantic.

-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.

-Wbad-function-cast (C and Objective-C only)

Warn whenever a function call is cast to a non-matching type. For example, warn if "int malloc()" is cast to "anything *".

-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++0x-compat (C++ and Objective-C++ only)

Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 200x, e.g., identifiers in ISO C++ 1998 that will become keywords in ISO C++ 200x. 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 *".

-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.

-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 will get a warning. These warnings will 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 will just be 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.

-Wclobbered

Warn for variables that might be changed by longjmp or vfork. This warning is also enabled by -Wextra.

-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 conversions between "NULL" and non-pointer types; confusing overload resolution for user-defined conversions; and conversions that will 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.

-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 (C++ and Objective-C++ only)

Warn about a comparison between values of different enum types. This warning is enabled by default.

-Wsign-compare

Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. This warning is also enabled by -Wextra; to get the other warnings of -Wextra without this warning, use -Wextra -Wno-sign-compare.

-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.

-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.

-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-attributes

Do not warn if an unexpected "__attribute__" is used, such as unrecognized attributes, function attributes applied to variables, etc. This will not stop errors for incorrect use of supported attributes.

-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 which 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. The aim is to detect global functions that fail to be declared in header files.

-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 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 would cause 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 would not trigger a warning:

        struct s { int f, g, h; };
        struct s x = { .f = 3, .g = 4 };

This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers.

-Wmissing-noreturn

Warn about functions which might be candidates for attribute "noreturn". Note these are only possible candidates, not absolute ones. Care should be taken to manually verify functions actually do not ever return before adding the "noreturn" attribute, otherwise subtle code generation bugs could be introduced. You will not get a warning for "main" in hosted C environments.

-Wmissing-format-attribute

Warn about function pointers which might be candidates for "format" attributes. Note these are only possible candidates, not absolute ones. GCC will guess 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 will also warn about function definitions which might be candidates for "format" attributes. Again, these are only possible candidates. GCC will guess 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.

-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 which have not been normalized; this option controls that warning.

There are four levels of warning that GCC supports. The default is -Wnormalized=nfc, which warns about any identifier which is not in the ISO 10646 ``C'' normalized form, NFC. NFC is the recommended form for most uses.

Unfortunately, there are some characters which ISO C and ISO C++ allow in identifiers that when turned into NFC aren't allowable as 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. You would only want to do this if you were 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'', will display just like a regular "n" which 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 will warn 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 is unable to 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.

-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.

-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" will be 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.

-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.

-Wnested-externs (C and Objective-C only)

Warn if an "extern" declaration is encountered within a function.

-Wunreachable-code

Warn if the compiler detects that code will never be executed.

This option is intended to warn when the compiler detects that at least a whole line of source code will never be executed, because some condition is never satisfied or because it is after a procedure that never returns.

It is possible for this option to produce a warning even though there are circumstances under which part of the affected line can be executed, so care should be taken when removing apparently-unreachable code.

For instance, when a function is inlined, a warning may mean that the line is unreachable in only one inlined copy of the function.

This option is not made part of -Wall because in a debugging version of a program there is often substantial code which checks correct functioning of the program and is, hopefully, unreachable because the program does work. Another common use of unreachable code is to provide behavior which is selectable at compile-time.

-Winline

Warn if a function can not be inlined and it was declared as inline. Even with this option, the compiler will 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 1998 ISO C++ standard, applying offsetof to a non-POD type is undefined. In existing C++ implementations, however, offsetof typically gives meaningful results even when applied to certain kinds of non-POD types. (Such as a simple struct that fails to be a POD type only by virtue of having a constructor.) 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.

-Wno-int-to-pointer-cast (C and Objective-C only)

Suppress warnings from casts to pointer type of an integer of a different size.

-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 can't be used.

-Wlong-long

Warn if long long type is used. This is default. To inhibit the warning messages, use -Wno-long-long. Flags -Wlong-long and -Wno-long-long are taken into account only when -pedantic flag is used.

-Wvariadic-macros

Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU alternate syntax when in pedantic ISO C99 mode. This is default. To inhibit the warning messages, use -Wno-variadic-macros.

-Wvla

Warn if variable length array is used in the code. -Wno-vla will prevent the -pedantic warning of the variable length array.

-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 were unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC will refuse 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 -pedantic, 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 will not be protected against stack smashing.

-Wno-mudflap

Suppress warnings about constructs that cannot be instrumented by -fmudflap.

-Woverlength-strings

Warn about string constants which are longer than the ``minimum maximum'' length specified in the C standard. Modern compilers generally allow string constants which 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 C89, 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 -pedantic, and can be disabled with -Wno-overlength-strings.

Options for Debugging Your Program or GCC

-g

Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF 2). 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 will probably make 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).

GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally produce surprising results: 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 were already at hand; some statements may execute in different places because they were moved out of loops.

Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.

The following options are useful when GCC is generated with the capability for more than one debugging format.

-ggdb

Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.

-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 which is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler.

-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 will increase the size of debugging information by as much as a factor of two.

-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.

-gcoff

Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4.

-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.

-gdwarf-version

Produce debugging information in DWARF format (if that is supported). This is the format used by DBX on IRIX 6. The value of version may be either 2 or 3; the default version is 3.

Note that with DWARF version 2 some ports require, and will always use, some non-conflicting DWARF 3 extensions in the unwind tables.

-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.

-gvms

Produce debugging information in VMS debug format (if that is supported). This is the format used by DEBUG on VMS systems.

-glevel

-ggdblevel

-gstabslevel

-gcofflevel

-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, but no information about local variables and no line numbers.

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-2 does not accept a concatenated debug level, because GCC used to support an option -gdwarf that meant to generate debug information in version 1 of the DWARF format (which is very different from version 2), and it would have been too confusing. That debug format is long obsolete, but the option cannot be changed now. Instead use an additional -glevel option to change the debug level for DWARF.

-gtoggle

Turn off generation of debug info, if leaving out this option would have generated 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.

-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 will be 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 will reject 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 side-effect compiler outputs to files or to the standard output. 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.

-feliminate-dwarf2-dups

Compress DWARF2 debugging information by eliminating duplicated information about each symbol. This option only makes sense when generating DWARF2 debugging information with -gdwarf-2.

-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 was 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 2.

-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 was 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 2.

-femit-struct-debug-detailed[=spec-list]

Specify the struct-like types for which the compiler will generate 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 will serve 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 would be legal, 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 will 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 types declared in foo.c and foo.h will have debug information, but types declared in other header will not. 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 2.

-fno-merge-debug-strings

Direct the linker to not merge together strings in the debugging information which 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 in directory old, record debugging information describing them as in new instead.

-fno-dwarf2-cfi-asm

Emit DWARF 2 unwind info as compiler generated ".eh_frame" section instead of using GAS ".cfi_*" directives.

-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.

-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.

-fmem-report

Makes the compiler print some statistics about permanent memory allocation when it finishes.

-fpre-ipa-mem-report

-fpost-ipa-mem-report

Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.

-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. 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.

*

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. Also "fork" calls are detected and correctly handled (double counting will not happen).

*

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 will match the source files more closely, if you do not optimize.

-fdbg-cnt-list

Print the name and the counter upperbound for all debug counters.

-fdbg-cnt=counter-value-list

Set the internal debug counter upperbound. counter-value-list is a comma-separated list of name:value pairs which sets the upperbound of each debug counter name to value. All debug counters have the initial upperbound of UINT_MAX, thus dbg_cnt() returns true always unless the upperbound is set by this option. e.g. With -fdbg-cnt=dce:10,tail_call:0 dbg_cnt(dce) will return true only for first 10 invocations and dbg_cnt(tail_call) will return false always.

-dletters

-fdump-rtl-pass

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. dumpname 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. These switches may have different effects 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 sub-expression 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 pro and epilogues.

-fdump-rtl-regmove

Dump after the register move pass.

-fdump-rtl-sched1

-fdump-rtl-sched2

-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.

-fdump-rtl-see

Dump after sign 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

-fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4 and -fdump-rtl-split5 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.

-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.

-dm

Print statistics on memory usage, at the end of the run, to standard error.

-dp

Annotate the assembler output with a comment indicating which pattern and alternative was used. The length of each instruction is also printed.

-dP

Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.

-dv

For each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with VCG to file.pass.vcg.

-dx

Just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand.

-dy

Dump debugging information during parsing, to standard error.

-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.

-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-translation-unit (C++ only)

-fdump-translation-unit-options (C++ only)

Dump a representation of the tree structure for the entire translation unit to a file. The file name is made by appending .tu to the source file name. If the -options form is used, options controls the details of the dump as described for the -fdump-tree options.

-fdump-class-hierarchy (C++ only)

-fdump-class-hierarchy-options (C++ only)

Dump a representation of each class's hierarchy and virtual function table layout to a file. The file name is made by appending .class to the source file name. If the -options form is used, options controls the details of the dump as described for the -fdump-tree options.

-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. The following dumps are possible:

all

Enables all inter-procedural analysis dumps.

cgraph

Dumps information about call-graph optimization, unused function removal, and inlining decisions.

inline

Dump after function inlining.

-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. If the -option form is used, -stats will cause counters to be summed over the whole compilation unit while -details will dump every event as the passes generate them. The default with no option is to sum counters for each function compiled.

-fdump-tree-switch

-fdump-tree-switch-options

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. If the -options form is used, options is a list of - separated options that control the details of the dump. Not all options are applicable to all dumps, those which are not meaningful will be 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.

slim

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.

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).

stats

Enable dumping various statistics about the pass (not honored by every dump option).

blocks

Enable showing basic block boundaries (disabled in raw dumps).

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.

all

Turn on all options, except raw, slim, verbose and lineno.

The following tree dumps are possible:

original

Dump before any tree based optimization, to file.original.

optimized

Dump after all tree based optimization, to file.optimized.

gimple

Dump each function before and after the gimplification pass to a file. The file name is made by appending .gimple to the source file name.

cfg

Dump the control flow graph of each function to a file. The file name is made by appending .cfg to the source file name.

vcg

Dump the control flow graph of each function to a file in VCG format. The file name is made by appending .vcg to the source file name. Note that if the file contains more than one function, the generated file cannot be used directly by VCG. You will need to cut and paste each function's graph into its own separate file first.

ch

Dump each function after copying loop headers. The file name is made by appending .ch to the source file name.

ssa

Dump SSA related information to a file. The file name is made by appending .ssa to the source file name.

alias

Dump aliasing information for each function. The file name is made by appending .alias to the source file name.

ccp

Dump each function after CCP. The file name is made by appending .ccp to the source file name.

storeccp

Dump each function after STORE-CCP. The file name is made by appending .storeccp to the source file name.

pre

Dump trees after partial redundancy elimination. The file name is made by appending .pre to the source file name.

fre

Dump trees after full redundancy elimination. The file name is made by appending .fre to the source file name.

copyprop

Dump trees after copy propagation. The file name is made by appending .copyprop to the source file name.

store_copyprop

Dump trees after store copy-propagation. The file name is made by appending .store_copyprop to the source file name.

dce

Dump each function after dead code elimination. The file name is made by appending .dce to the source file name.

mudflap

Dump each function after adding mudflap instrumentation. The file name is made by appending .mudflap to the source file name.

sra

Dump each function after performing scalar replacement of aggregates. The file name is made by appending .sra to the source file name.

sink

Dump each function after performing code sinking. The file name is made by appending .sink to the source file name.

dom

Dump each function after applying dominator tree optimizations. The file name is made by appending .dom to the source file name.

dse

Dump each function after applying dead store elimination. The file name is made by appending .dse to the source file name.

phiopt

Dump each function after optimizing PHI nodes into straightline code. The file name is made by appending .phiopt to the source file name.

forwprop

Dump each function after forward propagating single use variables. The file name is made by appending .forwprop to the source file name.

copyrename

Dump each function after applying the copy rename optimization. The file name is made by appending .copyrename to the source file name.

nrv

Dump each function after applying the named return value optimization on generic trees. The file name is made by appending .nrv to the source file name.

vect

Dump each function after applying vectorization of loops. The file name is made by appending .vect to the source file name.

vrp

Dump each function after Value Range Propagation (VRP). The file name is made by appending .vrp to the source file name.

all

Enable all the available tree dumps with the flags provided in this option.

-ftree-vectorizer-verbose=n

This option controls the amount of debugging output the vectorizer prints. This information is written to standard error, unless -fdump-tree-all or -fdump-tree-vect is specified, in which case it is output to the usual dump listing file, .vect. For n=0 no diagnostic information is reported. If n=1 the vectorizer reports each loop that got vectorized, and the total number of loops that got vectorized. If n=2 the vectorizer also reports non-vectorized loops that passed the first analysis phase (vect_analyze_loop_form) - i.e. countable, inner-most, single-bb, single-entry/exit loops. This is the same verbosity level that -fdump-tree-vect-stats uses. Higher verbosity levels mean either more information dumped for each reported loop, or same amount of information reported for more loops: If n=3, alignment related information is added to the reports. If n=4, data-references related information (e.g. memory dependences, memory access-patterns) is added to the reports. If n=5, the vectorizer reports also non-vectorized inner-most loops that did not pass the first analysis phase (i.e., may not be countable, or may have complicated control-flow). If n=6, the vectorizer reports also non-vectorized nested loops. For n=7, all the information the vectorizer generates during its analysis and transformation is reported. This is the same verbosity level that -fdump-tree-vect-details uses.

-frandom-seed=string

This option provides a seed that GCC uses when it would otherwise use random numbers. It is used to generate 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 should be different for every file you compile.

-fsched-verbose=n

On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints. This information is written to standard error, unless -fdump-rtl-sched1 or -fdump-rtl-sched2 is specified, in which case it is output to the usual dump listing file, .sched or .sched2 respectively. However for n greater than nine, the output is always printed to standard error.

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.

-save-temps

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 would produce 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.

-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.

-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.

It can be enabled even if var-tracking is disabled, in which case annotations will be created and maintained, but discarded at the end.

-fvar-tracking-assignments-toggle

Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.

-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 @samp{-, 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-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 will search---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 will be 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)---and don't do anything else.

-dumpspecs

Print the compiler's built-in specs---and don't do anything else. (This is used when GCC itself is being built.)

-feliminate-unused-debug-types

Normally, when producing DWARF2 output, GCC will emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit. Sometimes this is useful, such as 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. With this option, GCC will avoid producing debug symbol output for types that are nowhere used in the source file being compiled.

Options That Control Optimization

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 would 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.

-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 -fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdse -fguess-branch-probability -fif-conversion2 -fif-conversion -finline-small-functions -fipa-pure-const -fipa-reference -fmerge-constants -fsplit-wide-types -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-fre -ftree-sra -ftree-ter -funit-at-a-time

-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.

-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 -fexpensive-optimizations -fgcse -fgcse-lm -findirect-inlining -foptimize-sibling-calls -fpeephole2 -fregmove -freorder-blocks -freorder-functions -frerun-cse-after-loop -fsched-interblock -fsched-spec -fschedule-insns -fschedule-insns2 -fstrict-aliasing -fstrict-overflow -ftree-switch-conversion -ftree-pre -ftree-vrp

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 -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload and -ftree-vectorize options.

-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. It also performs further optimizations designed to reduce code size.

-Os disables the following optimization flags: -falign-functions -falign-jumps -falign-loops -falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays -ftree-vect-loop-version

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 would be -fno-foo. In the table below, only one of the forms is listed---the one you typically will 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-default-inline

Do not make member functions inline by default merely because they are defined inside the class scope (C++ only). Otherwise, when you specify -O, member functions defined inside class scope are compiled inline by default; i.e., you don't need to add inline in front of the member function name.

-fno-defer-pop

Always pop the arguments to each function call as soon as that function returns. For machines which 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 -O2, -O3, -Os.

-fomit-frame-pointer

Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on some machines.

On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro "FRAME_POINTER_REQUIRED" controls whether a target machine supports this flag.

Enabled at levels -O, -O2, -O3, -Os.

-foptimize-sibling-calls

Optimize sibling and tail recursive calls.

Enabled at levels -O2, -O3, -Os.

-fno-inline

Don't pay attention to the "inline" keyword. Normally this option is used to keep the compiler from expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline.

-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.

Enabled at level -O2.

-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 level -O2.

-finline-functions

Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be 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 level -O3.

-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.

-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.

-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 C89. In C++, emit any and all inline functions into the object file.

-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 the variable was 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 will result 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 will be deleted which will trigger the generation of reg-moves based on the life-range analysis. This option is effective only with -fmodulo-sched enabled.

-fno-branch-count-reg

Do not use ``decrement and branch'' instructions on a count register, but instead generate a sequence of instructions that decrement a register, compare it against zero, 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.

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.

-fmudflap -fmudflapth -fmudflapir

For front-ends that support it (C and C++), instrument all risky pointer/array dereferencing operations, some standard library string/heap functions, and some other associated constructs with range/validity tests. Modules so instrumented should be immune to buffer overflows, invalid heap use, and some other classes of C/C++ programming errors. The instrumentation relies on a separate runtime library (libmudflap), which will be linked into a program if -fmudflap is given at link time. Run-time behavior of the instrumented program is controlled by the MUDFLAP_OPTIONS environment variable. See "env MUDFLAP_OPTIONS=-help a.out" for its options.

Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-threaded. Use -fmudflapir, in addition to -fmudflap or -fmudflapth, if instrumentation should ignore pointer reads. This produces less instrumentation (and therefore faster execution) and still provides some protection against outright memory corrupting writes, but allows erroneously read data to propagate within a program.

-fthread-jumps

Perform optimizations where we 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 will follow 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 which 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 has been 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 runtime 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 will attempt to move loads which 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 gcse is enabled.

-fgcse-sm

When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This pass will attempt 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 cleanup redundant spilling.

-funsafe-loop-optimizations

If given, the loop optimizer will assume that loop indices do not overflow, and that the loops with nontrivial exit condition are not infinite. This enables a wider range of loop optimizations even if the loop optimizer itself cannot prove that these assumptions are valid. Using -Wunsafe-loop-optimizations, the compiler will warn you if it finds this kind of loop.

-fcrossjumping

Perform cross-jumping transformation. This transformation unifies equivalent code and save 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 include 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 "if-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.

-fdelete-null-pointer-checks

Use global dataflow analysis to identify and eliminate useless checks for null pointers. The compiler assumes that dereferencing a null pointer would have halted the program. If a pointer is checked after it has already been dereferenced, it cannot be null.

In some environments, this assumption is not true, and programs can safely dereference null pointers. Use -fno-delete-null-pointer-checks to disable this optimization for programs which depend on that behavior.

Enabled at levels -O2, -O3, -Os.

-fexpensive-optimizations

Perform a number of minor optimizations that are relatively expensive.

Enabled at levels -O2, -O3, -Os.

-foptimize-register-move

-fregmove

Attempt to reassign register numbers in move instructions and as operands of other simple instructions in order to maximize the amount of register tying. This is especially helpful on machines with two-operand instructions.

Note -fregmove and -foptimize-register-move are the same optimization.

Enabled at levels -O2, -O3, -Os.

-fira-algorithm=algorithm

Use specified coloring algorithm for the integrated register allocator. The algorithm argument should be "priority" or "CB". The first algorithm specifies Chow's priority coloring, the second one specifies Chaitin-Briggs coloring. The second algorithm can be unimplemented for some architectures. If it is implemented, it is the default because Chaitin-Briggs coloring as a rule generates a better code.

-fira-region=region

Use specified regions for the integrated register allocator. The region argument should be one of "all", "mixed", or "one". The first value means using all loops as register allocation regions, the second value which is the default means using all loops except for loops with small register pressure as the regions, and third one means using all function as a single region. The first value can give best result for machines with small size and irregular register set, the third one results in faster and generates decent code and the smallest size code, and the default value usually give the best results in most cases and for most architectures.

-fira-coalesce

Do optimistic register coalescing. This option might be profitable for architectures with big regular register files.

-fno-ira-share-save-slots

Switch off sharing stack slots used for saving call used hard registers living through a call. Each hard register will get a separate stack slot and as a result function stack frame will be bigger.

-fno-ira-share-spill-slots

Switch off sharing stack slots allocated for pseudo-registers. Each pseudo-register which did not get a hard register will get a separate stack slot and as a result function stack frame will be bigger.

-fira-verbose=n

Set up how verbose dump file for the integrated register allocator will be. Default value is 5. If the value is greater or equal to 10, the dump file will be stderr as if the value were n minus 10.

-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, -Os.

-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-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 will be 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) will be examined for a dependency on a stalled insn that is 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, do use superblock scheduling algorithm. Superblock scheduling allows motion across basic block boundaries resulting on 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.

-fsched2-use-traces

Use -fsched2-use-superblocks algorithm when scheduling after register allocation and additionally perform code duplication in order to increase the size of superblocks using tracer pass. See -ftracer for details on trace formation.

This mode should produce faster but significantly longer programs. Also without -fbranch-probabilities the traces constructed may not match the reality and hurt the performance. This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

-fsee

Eliminate redundant sign extension instructions and move the non-redundant ones to optimal placement using lazy code motion (LCM).

-freschedule-modulo-scheduled-loops

The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled we may want to prevent the later scheduling passes from changing its schedule, we use this option to control that.

-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 until 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 until -fsel-sched-pipelining is turned on.

-fcaller-saves

Enable values to be allocated in registers that will be 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 than would otherwise be produced.

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.

-fconserve-stack

Attempt to minimize stack usage. The compiler will attempt 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.

-ftree-pre

Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3.

-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-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 cannot escape the compilation unit. Enabled by default at -O and higher.

-fipa-struct-reorg

Perform structure reorganization optimization, that change C-like structures layout in order to better utilize spatial locality. This transformation is affective for programs containing arrays of structures. Available in two compilation modes: profile-based (enabled with -fprofile-generate) or static (which uses built-in heuristics). Require -fipa-type-escape to provide the safety of this transformation. It works only in whole program mode, so it requires -fwhole-program and -combine to be enabled. Structures considered cold by this transformation are not affected (see --param struct-reorg-cold-struct-ratio=value).

With this flag, the program debug info reflects a new structure layout.

-fipa-pta

Perform interprocedural pointer analysis. This option is experimental and does not affect generated code.

-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 will perform 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-matrix-reorg

Perform matrix flattening and transposing. Matrix flattening tries to replace a m-dimensional matrix with its equivalent n-dimensional matrix, where n < m. This reduces the level of indirection needed for accessing the elements of the matrix. The second optimization is matrix transposing that attempts to change the order of the matrix's dimensions in order to improve cache locality. Both optimizations need the -fwhole-program flag. Transposing is enabled only if profiling information is available.

-ftree-sink

Perform forward store motion on trees. This flag is enabled by default at -O and higher.

-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.

-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-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 builtin functions that may set "errno" but are otherwise side-effect free. 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 which will later be 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

Perform linear loop transformations on tree. This flag can improve cache performance and allow further loop optimizations to take place.

-floop-interchange

Perform loop interchange transformations on loops. Interchanging two nested loops switches the inner and outer loops. For example, given a loop like:

        DO J = 1, M
          DO I = 1, N
            A(J, I) = A(J, I) * C
          ENDDO
        ENDDO

loop interchange will transform the loop as if the user had written:

        DO I = 1, N
          DO J = 1, M
            A(J, I) = A(J, I) * C
          ENDDO
        ENDDO

which can be beneficial when "N" is larger than the caches, because in Fortran, the elements of an array are stored in memory contiguously by column, and the original loop iterates over rows, potentially creating at each access a cache miss. This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation infrastructure.

-floop-strip-mine

Perform loop strip mining transformations on loops. Strip mining splits a loop into two nested loops. The outer loop has strides equal to the strip size and the inner loop has strides of the original loop within a strip. For example, given a loop like:

        DO I = 1, N
          A(I) = A(I) + C
        ENDDO

loop strip mining will transform the loop as if the user had written:

        DO II = 1, N, 4
          DO I = II, min (II + 3, N)
            A(I) = A(I) + C
          ENDDO
        ENDDO

This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation infrastructure.

-floop-block

Perform loop blocking transformations on loops. Blocking strip mines each loop in the loop nest such that the memory accesses of the element loops fit inside caches. For example, given a loop like:

        DO I = 1, N
          DO J = 1, M
            A(J, I) = B(I) + C(J)
          ENDDO
        ENDDO

loop blocking will transform the loop as if the user had written:

        DO II = 1, N, 64
          DO JJ = 1, M, 64
            DO I = II, min (II + 63, N)
              DO J = JJ, min (JJ + 63, M)
                A(J, I) = B(I) + C(J)
              ENDDO
            ENDDO
          ENDDO
        ENDDO

which can be beneficial when "M" is larger than the caches, because the innermost loop will iterate over a smaller amount of data that can be kept in the caches. This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation infrastructure.

-fcheck-data-deps

Compare the results of several data dependence analyzers. This option is used for debugging the data dependence analyzers.

-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-im

Perform loop invariant motion on trees. This pass moves only invariants that would be 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 the loop for that 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-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.

-ftree-copyrename

Perform copy renaming on trees. This pass attempts to rename compiler temporaries to other variables at copy locations, usually resulting in variable names which more closely resemble the original variables. This flag is enabled by default at -O and higher.

-ftree-coalesce-inlined-vars

Permit the copyrename pass to subject inlined variables to coalescing into other variables. This may harm debug information of such inlined variables, but it will keep variables of the main function apart from each other, such that they are more likely to contain the expected values in a debugging session.

-ftree-coalesce-vars

Permit the copyrename pass to subject all variables to SSA coalescing. This may severely limit the ability to debug an optimized program compiled without -fvar-tracking-assignments. In the negated form, this flag prevents SSA coalescing of user variables, including inlined ones.

-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-vectorize

Perform loop vectorization on trees. This flag is enabled by default at -O3.

-ftree-vect-loop-version

Perform loop versioning when doing loop vectorization on trees. When a loop appears to be vectorizable except that data alignment or data dependence cannot be determined at compile time then vectorized and non-vectorized versions of the loop are generated along with runtime checks for alignment or dependence to control which version is executed. This option is enabled by default except at level -Os where it is disabled.

-fvect-cost-model

Enable cost model for vectorization.

-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.

-ftracer

Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do better job.

-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. This option makes code larger, and may or may not make it run faster.

-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,

-fsplit-ivs-in-unroller

Enables expressing 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.

Combination of -fweb and CSE is often sufficient to obtain the same effect. However in cases the loop body is more complicated than a single basic block, this is not reliable. It also does not work at all on some of the architectures due to restrictions in the CSE pass.

This optimization is enabled by default.

-fvariable-expansion-in-unroller

With this option, the compiler will create multiple copies of some local variables when unrolling a loop which can result in superior code.

-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-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 will use 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 will be 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 -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, for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections.

-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 in 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 will work 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.

-fstrict-overflow

Allow the compiler to assume strict signed overflow rules, depending on the language being compiled. For C (and C++) this means that overflow when doing arithmetic with signed numbers is undefined, which means that the compiler may assume that it will not happen. This permits various optimizations. For example, the compiler will assume that an expression like "i + 10 > i" will always be true for signed "i". This assumption is only valid if signed overflow is undefined, as the expression is false if "i + 10" overflows when using twos complement arithmetic. When this option is in effect any attempt to determine whether an operation on signed numbers will overflow must be written carefully to not actually involve overflow.

This option also allows the compiler to assume strict pointer semantics: given a pointer to an object, if adding an offset to that pointer does not produce a pointer to the same object, the addition is undefined. This permits the compiler to conclude that "p + u > p" is always true for a pointer "p" and unsigned integer "u". This assumption is only valid because pointer wraparound is undefined, as the expression is false if "p + u" overflows using twos complement arithmetic.

See also the -fwrapv option. Using -fwrapv means that integer signed overflow is fully defined: it wraps. When -fwrapv is used, there is no difference between -fstrict-overflow and -fno-strict-overflow for integers. With -fwrapv certain types of overflow are permitted. For example, if the compiler gets an overflow when doing arithmetic on constants, the overflowed value can still be used with -fwrapv, but not otherwise.

The -fstrict-overflow 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 would align 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 will not be 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.

Enabled at levels -O2, -O3.

-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 will not be 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.

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. The hope is that the loop will be executed many times, which will make up for any execution of the dummy operations.

-fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned.

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 will not be aligned.

If n is not specified or is zero, use a machine-dependent default.

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 will not be removed. This option is intended to support existing code which relies on a particular ordering. For new code, it is better to use attributes.

Enabled at level -O0. When disabled explicitly, it also imply -fno-section-anchors that 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 will no longer stay in a ``home register''.

Enabled by default with -funroll-loops.

-fwhole-program

Assume that the current compilation unit represents 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 a affect gets more aggressively optimized by interprocedural optimizers. While this option is equivalent to proper use of "static" keyword for programs consisting of single file, in combination with option --combine this flag can be used to compile most of smaller scale C programs since the functions and variables become local for the whole combined compilation unit, not for the single source file itself.

This option is not supported for Fortran programs.

-fcprop-registers

After register allocation and post-register allocation instruction splitting, we 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 will use heuristics to correct or smooth out such inconsistencies. By default, GCC will emit an error message when an inconsistent profile is detected.

-fprofile-dir=path

Set the directory to search 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. By default, GCC will use the current directory as path thus the profile data file will appear 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 will look at the path to find the profile feedback data files. See -fprofile-dir.

-fprofile-use

-fprofile-use=path

Enable profile feedback directed optimizations, and optimizations generally profitable only with profile feedback available.

The following options are enabled: "-fbranch-probabilities", "-fvpt", "-funroll-loops", "-fpeel-loops", "-ftracer"

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 will look at the path to find the profile feedback data files. See -fprofile-dir.

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.

-ffast-math

Sets -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and -fcx-limited-range.

This option causes the preprocessor macro "__FAST_MATH__" to be defined.

This option is not turned on by any -O option since it can result in incorrect output for programs which 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 which 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 which 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 a code which 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.

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 which 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 which 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".

-frtl-abstract-sequences

It is a size optimization method. This option is to find identical sequences of code, which can be turned into pseudo-procedures and then replace all occurrences with calls to the newly created subroutine. It is kind of an opposite of -finline-functions. This optimization runs at RTL level.

-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.

-fsingle-precision-constant

Treat floating point constant as single precision constant instead of implicitly converting it to double precision constant.

-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 the 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 mostly 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 and adds REG_VALUE_PROFILE notes to instructions for their later usage in optimizations.

Enabled with -fprofile-generate and -fprofile-use.

-fvpt

If combined with -fprofile-arcs, it instructs the compiler to add a 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 operation 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 will most benefit processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables will no longer stay in a ``home register''.

Enabled by default with -funroll-loops.

-ftracer

Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do 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 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 the loops for that there is enough information that they do not roll much (from profile feedback). It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations).

Enabled with -fprofile-use.

-fmove-loop-invariants

Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1

-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 and SPARC processors running Solaris 2 have linkers with such optimizations. AIX may have these optimizations in the future.

Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker will create larger object and executable files and will also be slower. You will not be able to use "gprof" on all systems if you specify this option and you may have problems with debugging if you specify both this option and -g.

-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 in within any basic block.

-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.

-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; }

would usually calculate the addresses of all three variables, but if you compile it with -fsection-anchors, it will access 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 will not inline functions that contain more that 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 given in the following table:

sra-max-structure-size

The maximum structure size, in bytes, at which the scalar replacement of aggregates (SRA) optimization will perform block copies. The default value, 0, implies that GCC will select the most appropriate size itself.

sra-field-structure-ratio

The threshold ratio (as a percentage) between instantiated fields and the complete structure size. We say that if the ratio of the number of bytes in instantiated fields to the number of bytes in the complete structure exceeds this parameter, then block copies are not used. The default is 75.

struct-reorg-cold-struct-ratio

The threshold ratio (as a percentage) between a structure frequency and the frequency of the hottest structure in the program. This parameter is used by struct-reorg optimization enabled by -fipa-struct-reorg. We say that if the ratio of a structure frequency, calculated by profiling, to the hottest structure frequency in the program is less than this parameter, then structure reorganization is not applied to this structure. The default is 10.

predictable-branch-cost-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-crossjump-edges

The maximum number of incoming edges to consider for crossjumping. 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 compile time increase with probably small improvement in executable size.

min-crossjump-insns

The minimum number of instructions which must be matched at the end of two blocks before crossjumping will be performed on them. This value is ignored in the case where all instructions in the block being crossjumped 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 is searched, the time savings from filling the delay slot will be minimal so stop searching. Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in executable run 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 compile 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 will be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization will not be done.

max-gcse-passes

The maximum number of passes of GCSE to run. The default is 1.

max-pending-list-length

The maximum number of pending dependencies scheduling will allow 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-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 will consider for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++). The default value is 450.

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 will be investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied. The default value is 90.

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 backend. 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 (consider unit consisting of function A that is inline and B that just calls A three time. 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 30 which limits unit growth to 1.3 times the original 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. 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 maximum number of instructions out-of-line copy of self recursive inline function can grow into by performing recursive inlining.

For functions declared inline --param max-inline-insns-recursive is taken into account. For function not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled and --param max-inline-insns-recursive-auto is used. The default value is 450.

max-inline-recursive-depth

max-inline-recursive-depth-auto

Specifies maximum recursion depth used by the recursive inlining.

For functions declared inline --param max-inline-recursive-depth is taken into account. For function not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled and --param max-inline-recursive-depth-auto is used. 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 probability that function will recurse via given call expression. This parameter limits inlining only to call expression whose probability exceeds given threshold (in percents). The default value is 10.

inline-call-cost

Specify cost of call instruction relative to simple arithmetics operations (having cost of 1). Increasing this cost disqualifies inlining of non-leaf functions and at the same time increases size of leaf function that is believed to reduce function size by being inlined. In effect it increases amount of inlining for code having large abstraction penalty (many functions that just pass the arguments to other functions) and decrease inlining for code with low abstraction penalty. The default value is 12.

min-vect-loop-bound

The minimum number of iterations under which a loop will not get 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.

max-unrolled-insns

The maximum number of instructions that a loop should have if that loop is unrolled, and if the loop is unrolled, it 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 should have if that loop is unrolled, and if the loop is unrolled, it 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 should have if that loop is peeled, and if the loop is peeled, it determines how many times the loop code is peeled.

max-peel-times

The maximum number of peelings of a single loop.

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.

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 that all candidates are considered for each use in induction variable optimizations. Only the most relevant candidates are considered if there are more candidates, 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 number of candidates in the set is smaller than this value, we always try to remove unnecessary ivs from the set during its optimization when a new iv is added to the set.

scev-max-expr-size

Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer.

omega-max-vars

The maximum number of variables in an Omega constraint system. The default value is 128.

omega-max-geqs

The maximum number of inequalities in an Omega constraint system. The default value is 256.

omega-max-eqs

The maximum number of equalities in an Omega constraint system. The default value is 128.

omega-max-wild-cards

The maximum number of wildcard variables that the Omega solver will be able to insert. The default value is 18.

omega-hash-table-size

The size of the hash table in the Omega solver. The default value is 550.

omega-max-keys

The maximal number of keys used by the Omega solver. The default value is 500.

omega-eliminate-redundant-constraints

When set to 1, use expensive methods to eliminate all redundant constraints. The default value is 0.

vect-max-version-for-alignment-checks

The maximum number of runtime checks that can be performed when doing loop versioning for alignment in the vectorizer. See option ftree-vect-loop-version for more information.

vect-max-version-for-alias-checks

The maximum number of runtime checks that can be performed when doing loop versioning for alias in the vectorizer. See option ftree-vect-loop-version for more information.

max-iterations-to-track

The maximum number of iterations of a loop the brute force algorithm for analysis of # of iterations of the loop tries to evaluate.

hot-bb-count-fraction

Select fraction of the maximal count of repetitions of basic block in program given basic block needs to have to be considered hot.

hot-bb-frequency-fraction

Select fraction of the maximal 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 function contain single loop with known bound and other loop with unknown. We predict the known number of iterations correctly, while the unknown number of iterations average to roughly 10. This means that the loop without bounds would appear artificially cold relative to the other one.

align-threshold

Select fraction of the maximal frequency of executions of basic block in function given basic block will get aligned.

align-loop-iterations

A loop expected to iterate at lest the selected number of iterations will get 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 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 rather hokey argument, as most of the duplicates will be 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-ratio

tracer-min-branch-ratio-feedback

Stop forward growth if the best edge do have probability lower than this threshold.

Similarly to tracer-dynamic-coverage two values are present, one for compilation for profile feedback and one for compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.

max-cse-path-length

Maximum number of basic blocks on path that cse considers. The default is 10.

max-cse-insns

The maximum instructions CSE process before flushing. The default is 1000.

max-aliased-vops

Maximum number of virtual operands per function allowed to represent aliases before triggering the alias partitioning heuristic. Alias partitioning reduces compile times and memory consumption needed for aliasing at the expense of precision loss in alias information. The default value for this parameter is 100 for -O1, 500 for -O2 and 1000 for -O3.

Notice that if a function contains more memory statements than the value of this parameter, it is not really possible to achieve this reduction. In this case, the compiler will use the number of memory statements as the value for max-aliased-vops.

avg-aliased-vops

Average number of virtual operands per statement allowed to represent aliases before triggering the alias partitioning heuristic. This works in conjunction with max-aliased-vops. If a function contains more than max-aliased-vops virtual operators, then memory symbols will be grouped into memory partitions until either the total number of virtual operators is below max-aliased-vops or the average number of virtual operators per memory statement is below avg-aliased-vops. The default value for this parameter is 1 for -O1 and -O2, and 3 for -O3.

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 which 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 compile 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 compile time increase with probably slightly better performance. The default value is 500.

reorder-blocks-duplicate

reorder-blocks-duplicate-feedback

Used by basic block reordering pass to decide whether to use unconditional branch or duplicate the code on its destination. Code is duplicated when its estimated size is smaller than this value multiplied by the estimated size of unconditional jump in the hot spots of the program.

The reorder-block-duplicate-feedback is used only when profile feedback is available and may be set to higher values than reorder-block-duplicate since information about the hot spots is more accurate.

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. 0 - disable region extension, N - do at most N iterations. The default value is 0.

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 insn will be scheduled. The default value is 40.

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 will be 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-max-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.

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.

integer-share-limit

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.

min-virtual-mappings

Specifies the minimum number of virtual mappings in the incremental SSA updater that should be registered to trigger the virtual mappings heuristic defined by virtual-mappings-ratio. The default value is 100.

virtual-mappings-ratio

If the number of virtual mappings is virtual-mappings-ratio bigger than the number of virtual symbols to be updated, then the incremental SSA updater switches to a full update for those symbols. The default ratio is 3.

ssp-buffer-size

The minimum size of buffers (i.e. arrays) that will receive stack smashing protection when -fstack-protection is used.

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 we will treat 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 we prefetch 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.

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 will refuse 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 will allow 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 will not be done and optimizations depending on it will be disabled. The default maximum SCC size is 10000.

ira-max-loops-num

IRA uses a regional register allocation by default. If a function contains loops more than number given by the parameter, only at most given number of the most frequently executed loops will form regions for the regional register allocation. The default value of the parameter is 100.

ira-max-conflict-table-size

Although IRA uses a sophisticated algorithm of compression conflict table, the table can be still big for huge functions. If the conflict table for a function could be more than size in MB given by the parameter, the conflict table is not built and faster, simpler, and lower quality register allocation algorithm will be used. The algorithm do not use pseudo-register conflicts. The default value of the parameter is 2000.

loop-invariant-max-bbs-in-loop

Loop invariant motion can be very expensive, both in compile 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.

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.

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.

Options Controlling the Preprocessor

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.

-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 which GCC does not know how to 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.

-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 will be 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 will need to 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.

-undef

Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.

-I dir

Add the directory dir to the list of directories to be searched for header files. Directories named by -I are searched before the standard system include directories. If the directory dir is a standard system include directory, the option is ignored to ensure that the default search order for system directories and the special treatment of system headers are not defeated . If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.

-o file

Write output to file. This is the same as specifying file as the second non-option argument to cpp. gcc has a different interpretation of a second non-option argument, so you must use -o to specify the output file.

-Wall

Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment, -Wtrigraphs, -Wmultichar and a warning about integer promotion causing a change of sign in "#if" expressions. Note that many of the preprocessor's warnings are on by default and have no options to control them.

-Wcomment

-Wcomments

Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a // comment. (Both forms have the same effect.)

-Wtrigraphs

Most trigraphs in comments cannot affect the meaning of the program. However, a trigraph that would form an escaped newline (??/ at the end of a line) can, by changing where the comment begins or ends. Therefore, only trigraphs that would form escaped newlines produce warnings inside a comment.

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.

-Wtraditional

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 problematic constructs which should be avoided.

-Wundef

Warn whenever an identifier which is not a macro is encountered in an #if directive, outside of defined. Such identifiers are replaced with zero.

-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 will also warn 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 CPP will report 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

-Wendif-labels

Warn whenever an #else or an #endif are followed by text. This usually happens in code of the form

        #if FOO
        ...
        #else FOO
        ...
        #endif FOO

The second and third "FOO" should be in comments, but often are not in older programs. This warning is on by default.

-Werror

Make all warnings into hard errors. Source code which triggers warnings will be rejected.

-Wsystem-headers

Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your own code, therefore suppressed. If you are responsible for the system library, you may want to see them.

-w

Suppress all warnings, including those which GNU CPP issues by default.

-pedantic

Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default, since they trigger frequently on harmless code.

-pedantic-errors

Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This includes mandatory diagnostics that GCC issues without -pedantic but treats as warnings.

-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 will still be 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 will appear in -MM dependency output. This is a slight change in semantics from GCC versions 3.0 and earlier.

-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 have sent preprocessed output.

When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.

-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 will set 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.

-fpch-deps

When using precompiled headers, this flag will cause the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified only the precompiled header would be 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.

-x c

-x c++

-x objective-c

-x assembler-with-cpp

Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with standards conformance or extensions; it merely selects which base syntax to expect. If you give none of these options, cpp will deduce the language from the extension of the source file: .c, .cc, .m, or .S. Some other common extensions for C++ and assembly are also recognized. If cpp does not recognize the extension, it will treat the file as C; this is the most generic mode.

Note: Previous versions of cpp accepted a -lang option which selected both the language and the standards conformance level. This option has been removed, because it conflicts with the -l option.

-std=standard

-ansi

Specify the standard to which the code should conform. Currently CPP knows about C and C++ standards; others may be added in the future.

standard may be one of:

iso9899:1990

c89

The ISO C standard from 1990. c89 is the customary shorthand for this version of the standard.

The -ansi option is equivalent to -std=c89.

iso9899:199409

The 1990 C standard, as amended in 1994.

iso9899:1999

c99

iso9899:199x

c9x

The revised ISO C standard, published in December 1999. Before publication, this was known as C9X.

gnu89

The 1990 C standard plus GNU extensions. This is the default.

gnu99

gnu9x

The 1999 C standard plus GNU extensions.

c++98

The 1998 ISO C++ standard plus amendments.

gnu++98

The same as -std=c++98 plus GNU extensions. This is the default for C++ code.

-I-

Split the include path. 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"". This option has been deprecated.

-nostdinc

Do not search the standard system directories for header files. Only the directories you have specified with -I 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.)

-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.

-idirafter dir

Search dir for header files, but do it after all directories specified with -I and the standard system directories have been exhausted. dir is treated as a system include directory. If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.

-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. See the --sysroot option for more information.

-imultilib dir

Use dir as a subdirectory of the directory containing target-specific C++ headers.

-isystem dir

Search dir for header files, after all directories specified by -I but before the standard system directories. Mark it as a system directory, so that it gets the same special treatment as is applied to the standard system directories. If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.

-iquote dir

Search dir only for header files requested with "#include "file""; they are not searched for "#include <file>", before all directories specified by -I and before the standard system directories. If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.

-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 experimental; in a future version of GCC, it will be enabled by default for C99 and C++.

-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.

-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.

-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.

-fworking-directory

Enable generation of linemarkers in the preprocessor output that will let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor will emit, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC will use 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.

-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.

-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.

-dCHARS

CHARS is a sequence of one or more of the following characters, and must not be preceded by a space. Other characters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify characters whose behavior conflicts, the result is undefined.

M

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

will show all the predefined macros.

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach.

D

Like M 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.

N

Like D, but emit only the macro names, not their expansions.

I

Output #include directives in addition to the result of preprocessing.

U

Like D 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.

-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.

-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.

-traditional-cpp

Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.

-trigraphs

Process trigraph sequences. 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. By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options.

The nine trigraphs and their replacements are

        Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??'  ??!  ??-
        Replacement:      [    ]    {    }    #    \    ^    |    ~

-remap

Enable special code to work around file systems which only permit very short file names, such as MS-DOS.

--help

--target-help

Print text describing all the command line options instead of preprocessing anything.

-v

Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the final form of the include path.

-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 ...! .

-version

--version

Print out GNU CPP's version number. With one dash, proceed to preprocess as normal. With two dashes, exit immediately.

Passing 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 which GCC does not know how to 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

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.

-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 will be passed to the linker. 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 will be passed to the linker. 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 that 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. (For example, __main, used to ensure C++ constructors will be called.)

-pie

Produce a position independent executable on targets which support it. For predictable results, you must also specify the same set of options that were used to generate code (-fpie, -fPIE, or model suboptions) when you specify this option.

-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 prevents linking with the shared libraries. On other systems, this option has no effect.

-shared

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 that were used to generate code (-fpic, -fPIC, or model suboptions) when you specify this option.[1]

-shared-libgcc

-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++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared library or a main executable, because C++ and Java 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 will not always be 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 will link the shared version of libgcc into shared libraries by default. Otherwise, it will take advantage of the linker and optimize 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++ or GCJ driver, as appropriate for the languages used in the program, or using the option -shared-libgcc, such that it is linked with the shared libgcc.

-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 which GCC does not know how to 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.