Já me disseram que eu não deveria usar as opções -O3 ou -Ofast em meus projetos. Bem… elas tendem a oferecer as melhores otimizações possíveis e, na tabela abaixo, mostro a diferença entre elas e a compilação sem opções de otimização (que chamei de “generic”) e a opção de “nenhuma” otimização (-O0). A opção -Os, em teoria, gera código pequeno (‘s’ de smaller), mas essencialmente, ele é a mesma coisa que -O2, exceto pela possível otimização de chamadas à strlen (que, na minha experiência, quase nunca o compilador consegue otimizar):
| generic | -Os | -O0 | -O1 | -O2 | -O3 | -Ofast | |
| -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 | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fearly-inlining | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -ffunction-cse | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fgcse-lm | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fira-hoist-pressure | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fira-share-save-slots | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fira-share-spill-slots | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fivopts | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fkeep-static-consts | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fleading-underscore | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -flifetime-dse | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -flto-odr-type-merging | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fpeephole | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -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 | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fsplit-ivs-in-unroller | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fstack-protector-strong | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fstdarg-opt | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fstrict-volatile-bitfields | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fsync-libcalls | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -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 | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -malign-stringops | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -mavx256-split-unaligned-load | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -mavx256-split-unaligned-store | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -mfancy-math-387 | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -mfp-ret-in-387 | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -mfxsr | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -mglibc | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -mno-sse4 | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -mpush-args | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -mred-zone | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -msse | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -msse2 | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -mtls-direct-seg-refs | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -mvzeroupper | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| -fsigned-zeroes | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | |
| -fdelete-null-pointer-checks | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fident | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -finline-atomics | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftrapping-math | ✓ | ✓ | ✓ | ||||
| -fbranch-count-reg | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fcombine-stack-adjustments | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fcompare_elim | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fcprop-registers | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fdefer-pop | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fforward-propagate | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fguess-branch-probability | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fhoist-adjacent-loads | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fif-conversion | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fif-conversion2 | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -finline | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -finline-functions-called-once | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fipa-profile | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fipa-pure-const | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fipa-reference | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fmerge-constants | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fmove-loop-invariants | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fomit-frame-pointer | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fshrink-wrap | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fsplit-wide-types | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -fssa-phiopt | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftoplevel-reorder | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-bit-ccp | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-ccp | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-ch | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-coalesce-vars | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-copy-prop | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-copyrename | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-cselim | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-dce | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-dominator-opts | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-dse | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-forwprop | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-fre | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-pta | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-sink | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-slsr | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-sra | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -ftree-ter | ✓ | ✓ | ✓ | ✓ | ✓ | ||
| -falign-labels | ✓ | ✓ | ✓ | ✓ | |||
| -fcaller-saves | ✓ | ✓ | ✓ | ✓ | |||
| -fcrossjumping | ✓ | ✓ | ✓ | ✓ | |||
| -fcse-follow-jumps | ✓ | ✓ | ✓ | ✓ | |||
| -fdevirtualize | ✓ | ✓ | ✓ | ✓ | |||
| -fdevirtualize-speculatively | ✓ | ✓ | ✓ | ✓ | |||
| -fexpensive-optimizations | ✓ | ✓ | ✓ | ✓ | |||
| -fgcse | ✓ | ✓ | ✓ | ✓ | |||
| -findirect-inlining | ✓ | ✓ | ✓ | ✓ | |||
| -finline-small-functions | ✓ | ✓ | ✓ | ✓ | |||
| -fipa-cp | ✓ | ✓ | ✓ | ✓ | |||
| -fipa-cp-alignment | ✓ | ✓ | ✓ | ✓ | |||
| -fipa-icf | ✓ | ✓ | ✓ | ✓ | |||
| -fipa-icf-functions | ✓ | ✓ | ✓ | ✓ | |||
| -fipa-icf-variables | ✓ | ✓ | ✓ | ✓ | |||
| -fipa-ra | ✓ | ✓ | ✓ | ✓ | |||
| -fipa-sra | ✓ | ✓ | ✓ | ✓ | |||
| -fisolate-erroneous-paths-dereference | ✓ | ✓ | ✓ | ✓ | |||
| -flra-remat | ✓ | ✓ | ✓ | ✓ | |||
| -foptimize-sibling-calls | ✓ | ✓ | ✓ | ✓ | |||
| -fpartial-inlining | ✓ | ✓ | ✓ | ✓ | |||
| -fpeephole2 | ✓ | ✓ | ✓ | ✓ | |||
| -free | ✓ | ✓ | ✓ | ✓ | |||
| -freorder-blocks | ✓ | ✓ | ✓ | ✓ | |||
| -freorder-blocks-and-partition | ✓ | ✓ | ✓ | ✓ | |||
| -freorder-functions | ✓ | ✓ | ✓ | ✓ | |||
| -frerun-cse-after-loop | ✓ | ✓ | ✓ | ✓ | |||
| -fschedule-insns2 | ✓ | ✓ | ✓ | ✓ | |||
| -fstrict-aliasing | ✓ | ✓ | ✓ | ✓ | |||
| -fstrict-overflow | ✓ | ✓ | ✓ | ✓ | |||
| -fthread-jumps | ✓ | ✓ | ✓ | ✓ | |||
| -ftree-builtin-call-dce | ✓ | ✓ | ✓ | ✓ | |||
| -ftree-switch-conversion | ✓ | ✓ | ✓ | ✓ | |||
| -ftree-tail-merge | ✓ | ✓ | ✓ | ✓ | |||
| -ftree-vrp | ✓ | ✓ | ✓ | ✓ | |||
| -foptimize-strlen | ✓ | ✓ | ✓ | ||||
| -ftree-pre | ✓ | ✓ | ✓ | ||||
| -finline-functions | ✓ | ✓ | ✓ | ||||
| -fgcse-after-reload | ✓ | ✓ | |||||
| -fipa-cp-clone | ✓ | ✓ | |||||
| -fpredictive-commoning | ✓ | ✓ | |||||
| -ftree-loop-distribute-patterns | ✓ | ✓ | |||||
| -ftree-loop-vectorize | ✓ | ✓ | |||||
| -ftree-partial-pre | ✓ | ✓ | |||||
| -ftree-slp-vectorize | ✓ | ✓ | |||||
| -funswitch-loops | ✓ | ✓ | |||||
| -fassociative-math | ✓ | ||||||
| -fcx-limited-range | ✓ | ||||||
| -ffinite-math-only | ✓ | ||||||
| -freciprocal-math | ✓ | ||||||
| -funsafe-math-optimizations | ✓ |
Note que somente com as opções -O3 e -Ofast podemos ter o recurso de vetorização de um loop que, é claro, poderia ser “ligada” usando a chave -ftree-loop-vectorize, mas, de qualquer maneira, Essas duas opções oferencem muitas vantagens, tanto do ponto de vista da performance e do tamanho do código gerado pelo compilador.
Se você usa apenas a opção -O2, note que algumas coisas não são oferecidas, como o recurso de organizar funções de pouco uso como inline, algumas otimizações inter process também são são realizadas e além da vetorização, algumas eliminações de sub expressões globais e da previsão de loops (para auxiliar no branching predition) podem ficar seriamente comprometidas.
Tá certo que a opção -Ofast só é realmente útil em dois pontos: Quando não precisamos da conformidade estrita com IEEE 754 e quanto otimizações ainda mais agressivas (porém “unsafe”) sejam interessantes. Eu evitaria o -Ofast a não ser que você saiba o que faz. Mas adotaria -O3 como padrão.
UPDATE: Eu já tinha observado isso antes mas fiz questão de fazer alguns testes… Ao que parece, -O2 realmente gera código mais rápido que -O3 ou -Ofast!
