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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
https://gcc.gnu.org/onlinedocs/gcc-3.4.4/gcc/Optimize-Options.html

Summary

-O0

Do not optimize. This is the default. 

-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. 

-Os

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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+1 vote

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arm-none-eabi-gcc -Wall -mfpu=fpv4-sp-d16 -mfloat-abi=softfp -fshort-double-mcpu=cortex-m4 -mthumb -Qn -Os -finstrument-functions -mlong-calls -c temp.c -o temp.o
so on for temp1, temp2... Etc

I am compiling multiple C files. and linker settings are

ld -r temp.o -o one.o
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I am linking multiple .O files. Also I am using linker script i.e link.txt (invoked externally as below)

ld -cref -Map map.txt -S -T link.txt -temp.o -lm -lc -lgcc
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Now, At output I get .abs file of 140KB.

My Questions are
1. How to optimize (reduce size of .abs) by using compiler or linker specific options?
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3. Please Elaborate as I am new to this, I have referred sites, gcc help and tried some "lto" options but NO reduction is size.
4. I also tried using -fdata-sections -ffunction-sections
But It has INCREASED my firmware.abs file size from KB to MB (yes it is MB!!!!) I also wondered WHY?
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+5 votes

I have a C code like this:

int foo(void)
{ 
 int phase;
 . . .
 phase = 1;
 phase = 2;
 phase = 3;
 . . .
}

In case of -O0 gcc generates machine instructions for every assignment 'phase = ...'. But in case of -O2 gcc does not generate instructions for some assignments. Of course, this is correct. However, is there any way to tell gcc that 'phase' object is inspected by another thread, so it should not remove such statements?

+2 votes

I have very high and limited knowledge of compiler's functionality.
As per my knowledge, Compiler does it's job using following steps:
1. Lexical analysis
2. Syntactic analysis
3. Semantic analysis
4. Pre-optimization of internal representation.
5. Code generation
6. Post optimization.

Can some one please explain about 4, 5 and 6 steps ? What they signify ?

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I want to use some loop optimizations. I have build GCC-4.8.1 using gmp 5.1.3, mpfr 3.1.2, mpc 1.0.1, cloog 0.18.0 and isl 0.11.1. I tried optimization flags -floop-interchange and -floop-block. I don't find any changes in the optimized code.

Are graphite loop transforms implemented in GCC-4.8.1. Should I configure and build GCC in a different way.

+1 vote

In general, how a C language written program is converted into a binary by a C compiler ?

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