四种编译优化类型的解释:
`-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
-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.
还有个常用的优化选项: -Os
它相当于-O2.5。是使用了所有-O2的优化选项，但又不缩减代码尺寸的方法。
