GCC 优化选项（1）

找到英文版的了，很尴尬；

-O'
-O1’
Optimize. Optimizing compilation takes somewhat more time, and a
lot more memory for a la·rge 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       -fdelayed-branch       -fguess-branch-probability       -fcprop-registers       -floop-optimize       -fif-conversion       -fif-conversion2       -ftree-ccp       -ftree-dce       -ftree-dominator-opts       -ftree-dse       -ftree-ter       -ftree-lrs       -ftree-sra       -ftree-copyrename       -ftree-fre       -ftree-ch       -funit-at-a-time       -fmerge-constants `-O' also turns on `-fomit-frame-pointer' on machines where doing so does not interfere with debugging. `-O' doesn't turn on `-ftree-sra' for the Ada compiler.  This option must be explicitly specified on the command line to be enabled for the Ada compiler.

-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:      -fthread-jumps       -fcrossjumping       -foptimize-sibling-calls       -fcse-follow-jumps  -fcse-skip-blocks       -fgcse  -fgcse-lm        -fexpensive-optimizations       -fstrength-reduce       -frerun-cse-after-loop  -frerun-loop-opt       -fcaller-saves       -fpeephole2       -fschedule-insns  -fschedule-insns2       -fsched-interblock  -fsched-spec       -fregmove       -fstrict-aliasing       -fdelete-null-pointer-checks       -freorder-blocks  -freorder-functions       -falign-functions  -falign-jumps       -falign-loops  -falign-labels       -ftree-vrp       -ftree-pre 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' and-fgcse-after-reload’ options.

`-O0’
Do not optimize. This is the default.