GCC 4.0, Fast Math, and Acovea

[Uros Bizjak]

Hello Scott!

> Specifically, the -funsafe-math-optimizations flag doesn't work 
> correctly on AMD64 because the default on that platform is 
> -mfpmath=sse. Without specifying -mfpmath=387, 
> -funsafe-math-optimizations does not generate inline processor 
> instructions for most floating-point functions.
>
> Let's put it another way: Manually selecting -mfpmath=387 cuts 
> run-times by 50% for programs dependent on functions like sin() and 
> sqrt(), as compared to -funsafe-math-optimizations by itself.


It was found that moving data from SSE registers to X87 registers (and 
back) only to call an x87 builtin degrades performance. Because of this, 
x87 builtins are disabled for -mfpmath=sse and a normal libcall is 
issued for sin(), etc functions. If someone wants to use x87 builtins, 
then _all_ math operations should be done in x87 registers to avoid 
costly SSE->x87 moves.

BTW: Does adding -D__NO_MATH_INLINES improve performance for 
-mfpmath=sse? That would be PR19602.

Uros.