Wednesday, December 28, 2005

Some Optimization Flags on GCC

Yesterday, I was playing around with some optimizations options supplied by GCC (ver 4.0.2) on my AMD64 laptop. I created a very small code to compute sin(x) * cos (x). As we know, Pentium and newer processors support a FP machine code to compute these two functions simulatenously in single mnemonic code: FSINCOS.

the code is as follow:


inline double fsincos(double x)
return sin(x)*cos(x);

First, I compiled it to assembly source with no optimization enabled, just using defaults as follow:

> gcc -c -S fsincos.c -o fsincos.s

and then open the generated assembly code in fsincos.s. Overthere I saw somewhere it called internal library functions for sin and cos trigonometry functions as I expected. I then recompiled it with full optimization flags enabled:

> gcc -c -S -mtune=athlon64 -mfpmath=sse -msse3 -ffast-math -m64 -O3 fsincos.c -o fsincos64.s

Suprisingly, it still called GNU's "sin" and "cos" math functions!. What? I said (not loud, though). I checked again the gcc man page, nothing special about these things. Hm....let's try to add "387" in the mfpmath, I said in my mind.

So, the command is now as follow:

gcc -c -S -mtune=athlon64 -mfpmath=387,sse -msse3 -ffast-math -m64 -O3 fsincos.c -o fsincos64.s

Voila! now the assembly code called inline FP code "fsincos" in it. Since then, I add an environment variable in the enviroment as follow:

export CFLAGS="-mtune=athlon64 -mfpmath=387,sse -msse3 -ffast-math -m64"

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