두근두근 울렁울렁 가슴 뛰지만 무섭고도 두려워서 겁이 나지만 생각난 김에 이것도 써두는 것이 좋겠네요 :)

Gcc 를 사용하다 보면 가끔 미리 정의된 매크로가 무엇인지 알고 싶은 때가 있습니다. 이럴 때,
gcc -dM -E - < /dev/null
라는 명령을 내려서 확인하는 것은 널리 알려진 사용법입니다. 만일, 미리 정의된 옵션이 어떤 것인지 알고 싶을때는 어떻게 할까요? 예를 들어, -O0 옵션과 -O2 옵션의 차이는 무엇일까요? 이렇게 명령을 내립니다. (예에서는 -O2 옵션을 봅니다.)
echo 'int main(){return 0;}' | gcc -v -Q -O2 -x c -
C++ 에 적용되는 옵션을 알고싶다면 -x c 대신 -x c++ 이라고 합니다. 화면에 여러 줄이 지나갈텐데요... 제 환경에서는 다음과 같은 차이가 있습니다.

	-O0	-O2
GGC heuristics:	--param ggc-min-expand=100 --param ggc-min-heapsize=131072	--param ggc-min-expand=100 --param ggc-min-heapsize=131072
options passed:	-v - -march=core2 -mmmx -mno-3dnow -msse -msse2 -msse3 -mssse3 -mno-sse4a -mcx16 -msahf -mno-movbe -mno-aes -mno-sha -mno-pclmul -mno-popcnt -mno-abm -mno-lwp -mno-fma -mno-fma4 -mno-xop -mno-bmi -mno-sgx -mno-bmi2 -mno-tbm -mno-avx -mno-avx2 -mno-sse4.2 -msse4.1 -mno-lzcnt -mno-rtm -mno-hle -mno-rdrnd -mno-f16c -mno-fsgsbase -mno-rdseed -mno-prfchw -mno-adx -mfxsr -mno-xsave -mno-xsaveopt -mno-avx512f -mno-avx512er -mno-avx512cd -mno-avx512pf -mno-prefetchwt1 -mno-clflushopt -mno-xsavec -mno-xsaves -mno-avx512dq -mno-avx512bw -mno-avx512vl -mno-avx512ifma -mno-avx512vbmi -mno-avx5124fmaps -mno-avx5124vnniw -mno-clwb -mno-mwaitx -mno-clzero -mno-pku -mno-rdpid --param l1-cache-size=32 --param l1-cache-line-size=64 --param l2-cache-size=3072 -mtune=core2 -O0	-v - -march=core2 -mmmx -mno-3dnow -msse -msse2 -msse3 -mssse3 -mno-sse4a -mcx16 -msahf -mno-movbe -mno-aes -mno-sha -mno-pclmul -mno-popcnt -mno-abm -mno-lwp -mno-fma -mno-fma4 -mno-xop -mno-bmi -mno-sgx -mno-bmi2 -mno-tbm -mno-avx -mno-avx2 -mno-sse4.2 -msse4.1 -mno-lzcnt -mno-rtm -mno-hle -mno-rdrnd -mno-f16c -mno-fsgsbase -mno-rdseed -mno-prfchw -mno-adx -mfxsr -mno-xsave -mno-xsaveopt -mno-avx512f -mno-avx512er -mno-avx512cd -mno-avx512pf -mno-prefetchwt1 -mno-clflushopt -mno-xsavec -mno-xsaves -mno-avx512dq -mno-avx512bw -mno-avx512vl -mno-avx512ifma -mno-avx512vbmi -mno-avx5124fmaps -mno-avx5124vnniw -mno-clwb -mno-mwaitx -mno-clzero -mno-pku -mno-rdpid --param l1-cache-size=32 --param l1-cache-line-size=64 --param l2-cache-size=3072 -mtune=core2 -O2
options enabled:	-fPIC -fPIE -faggressive-loop-optimizations -fasynchronous-unwind-tables -fauto-inc-dec -fchkp-check-incomplete-type -fchkp-check-read -fchkp-check-write -fchkp-instrument-calls -fchkp-narrow-bounds -fchkp-optimize -fchkp-store-bounds -fchkp-use-static-bounds -fchkp-use-static-const-bounds -fchkp-use-wrappers -fcommon -fdelete-null-pointer-checks -fdwarf2-cfi-asm -fearly-inlining -feliminate-unused-debug-types -ffp-int-builtin-inexact -ffunction-cse -fgcse-lm -fgnu-runtime -fgnu-unique -fident -finline-atomics -fira-hoist-pressure -fira-share-save-slots -fira-share-spill-slots -fivopts -fkeep-static-consts -fleading-underscore -flifetime-dse -flto-odr-type-merging -fmath-errno -fmerge-debug-strings -fpeephole -fplt -fprefetch-loop-arrays -freg-struct-return -fsched-critical-path-heuristic -fsched-dep-count-heuristic -fsched-group-heuristic -fsched-interblock -fsched-last-insn-heuristic -fsched-rank-heuristic -fsched-spec -fsched-spec-insn-heuristic -fsched-stalled-insns-dep -fschedule-fusion -fsemantic-interposition -fshow-column -fshrink-wrap-separate -fsigned-zeros -fsplit-ivs-in-unroller -fssa-backprop -fstack-protector-strong -fstdarg-opt -fstrict-volatile-bitfields -fsync-libcalls -ftrapping-math -ftree-cselim -ftree-forwprop -ftree-loop-if-convert -ftree-loop-im -ftree-loop-ivcanon -ftree-loop-optimize -ftree-parallelize-loops= -ftree-phiprop -ftree-reassoc -ftree-scev-cprop -funit-at-a-time -funwind-tables -fvar-tracking -fvar-tracking-assignments -fzero-initialized-in-bss -m128bit-long-double -m64 -m80387 -malign-stringops -mcx16 -mfancy-math-387 -mfp-ret-in-387 -mfxsr -mglibc -mieee-fp -mlong-double-80 -mmmx -mpush-args -mred-zone -msahf -msse -msse2 -msse3 -msse4.1 -mssse3 -mstv -mtls-direct-seg-refs -mvzeroupper	-fPIC -fPIE -faggressive-loop-optimizations -falign-labels -fasynchronous-unwind-tables -fauto-inc-dec -fbranch-count-reg -fcaller-saves -fchkp-check-incomplete-type -fchkp-check-read -fchkp-check-write -fchkp-instrument-calls -fchkp-narrow-bounds -fchkp-optimize -fchkp-store-bounds -fchkp-use-static-bounds -fchkp-use-static-const-bounds -fchkp-use-wrappers -fcode-hoisting -fcombine-stack-adjustments -fcommon -fcompare-elim -fcprop-registers -fcrossjumping -fcse-follow-jumps -fdefer-pop -fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively -fdwarf2-cfi-asm -fearly-inlining -feliminate-unused-debug-types -fexpensive-optimizations -fforward-propagate -ffp-int-builtin-inexact -ffunction-cse -fgcse -fgcse-lm -fgnu-runtime -fgnu-unique -fguess-branch-probability -fhoist-adjacent-loads -fident -fif-conversion -fif-conversion2 -findirect-inlining -finline -finline-atomics -finline-functions-called-once -finline-small-functions -fipa-bit-cp -fipa-cp -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-profile -fipa-pure-const -fipa-ra -fipa-reference -fipa-sra -fipa-vrp -fira-hoist-pressure -fira-share-save-slots -fira-share-spill-slots -fisolate-erroneous-paths-dereference -fivopts -fkeep-static-consts -fleading-underscore -flifetime-dse -flra-remat -flto-odr-type-merging -fmath-errno -fmerge-constants -fmerge-debug-strings -fmove-loop-invariants -fomit-frame-pointer -foptimize-sibling-calls -foptimize-strlen -fpartial-inlining -fpeephole -fpeephole2 -fplt -fprefetch-loop-arrays -free -freg-struct-return -freorder-blocks -freorder-functions -frerun-cse-after-loop -fsched-critical-path-heuristic -fsched-dep-count-heuristic -fsched-group-heuristic -fsched-interblock -fsched-last-insn-heuristic -fsched-rank-heuristic -fsched-spec -fsched-spec-insn-heuristic -fsched-stalled-insns-dep -fschedule-fusion -fschedule-insns2 -fsemantic-interposition -fshow-column -fshrink-wrap -fshrink-wrap-separate -fsigned-zeros -fsplit-ivs-in-unroller -fsplit-wide-types -fssa-backprop -fssa-phiopt -fstack-protector-strong -fstdarg-opt -fstore-merging -fstrict-aliasing -fstrict-volatile-bitfields -fsync-libcalls -fthread-jumps -ftoplevel-reorder -ftrapping-math -ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-cselim -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-loop-if-convert -ftree-loop-im -ftree-loop-ivcanon -ftree-loop-optimize -ftree-parallelize-loops= -ftree-phiprop -ftree-pre -ftree-pta -ftree-reassoc -ftree-scev-cprop -ftree-sink -ftree-slsr -ftree-sra -ftree-switch-conversion -ftree-tail-merge -ftree-ter -ftree-vrp -funit-at-a-time -funwind-tables -fvar-tracking -fvar-tracking-assignments -fzero-initialized-in-bss -m128bit-long-double -m64 -m80387 -malign-stringops -mcx16 -mfancy-math-387 -mfp-ret-in-387 -mfxsr -mglibc -mieee-fp -mlong-double-80 -mmmx -mpush-args -mred-zone -msahf -msse -msse2 -msse3 -msse4.1 -mssse3 -mstv -mtls-direct-seg-refs -mvzeroupper

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덧붙임: 위에서 최적화 레벨에 따라 차이가 나는 것은 options enabled 부분입니다. Options passed 부분은 큰 차이가 나지 않는데요... CPU 에 따라 왜 이런 옵션들이 선택되었는지 보려면 다음과 같은 명령을 내립니다.
gcc -c -Q -march=native --help=target