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GCC : What are all optimization flags present in gcc compiler ?

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I am looking for detailed description of optimization flags that are provided by GCC compiler ?

posted Jun 23, 2014 by Rupam

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1 Answer

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Check the following Link



Do not optimize. This is the default. 


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:


-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging. 


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:

          -fcse-follow-jumps  -fcse-skip-blocks 
          -frerun-cse-after-loop  -frerun-loop-opt 
          -fgcse  -fgcse-lm  -fgcse-sm  -fgcse-las 
          -fschedule-insns  -fschedule-insns2 
          -fsched-interblock  -fsched-spec 
          -freorder-blocks  -freorder-functions 
          -falign-functions  -falign-jumps 
          -falign-loops  -falign-labels 

Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos. 


Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -fweb and -frename-registers options. 


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
answer Jun 23, 2014 by Tapesh Kulkarni
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 int phase;
 . . .
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