-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’ and ‘-frename-registers’
options.
-O0 Do not optimize. This is the default.
对于-O3新增的三个优化选项:
-finline-functions
Integrate all simple functions into their callers. The compiler heuristically decides
which functions are simple enough to be worth integrating in this way.
If all calls to a given function are integrated, and the function is declared
static, then the function is normally not output as assembler code in its own
right.
Enabled at level ‘-O3’.
-fweb
Constructs webs as commonly used for register allocation purposes and assign
each web individual pseudo register. This allows the register allocation pass
to operate on pseudos directly, but also strengthens several other optimization
passes, such as CSE, loop optimizer and trivial dead code remover. It can,
however, make debugging impossible, since variables will no longer stay in a
“home register”.
Enabled at levels ‘-O3’.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers
left over after register allocation. This optimization will most benefit
processors with lots of registers. It can, however, make debugging impossible,
since variables will no longer stay in a “home register”.
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