gcc优化等级-O0 -O1 -O2 -Os -O3

gcc原文网址：https://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Optimize-Options.html#Optimize-Options

-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:

          -fdefer-pop -fmerge-constants -fthread-jumps -floop-optimize -fif-conversion -fif-conversion2 -fdelayed-branch -fguess-branch-probability -fcprop-registers

-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. The compiler does not perform loop unrolling or function inlining when you specify -O2. 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:

          -fforce-mem -foptimize-sibling-calls -fstrength-reduce -fcse-follow-jumps  -fcse-skip-blocks -frerun-cse-after-loop  -frerun-loop-opt -fgcse  -fgcse-lm  -fgcse-sm  -fgcse-las -fdelete-null-pointer-checks -fexpensive-optimizations -fregmove -fschedule-insns  -fschedule-insns2 -fsched-interblock  -fsched-spec -fcaller-saves -fpeephole2 -freorder-blocks  -freorder-functions -fstrict-aliasing -funit-at-a-time -falign-functions  -falign-jumps -falign-loops  -falign-labels -fcrossjumping

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, -fweb, -frename-registers and -funswitch-loops options.

-O0

Do not optimize. 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  -fprefetch-loop-arrays

If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.

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