reorder you alias form when must deb off cif

四種編譯優化類型的解釋:

`-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.

還有個常用的優化選項: -Os

它相當於-O2.5。是使用了所有-O2的優化選項，但又不縮減代碼尺寸的方法。

參考文檔: https://www.linuxjournal.com/article/7269

https://www.cnblogs.com/dylancao/p/9528432.html

GCC 優化選項 -O1 -O2 -O3 -OS 優先級，-FOMIT-FRAME-POINTER（O3的優化很小，只增加了幾條優化而已）