[PATCH, middle-end, i386]: reciprocal rsqrt pass + full recip x86 backend support

[Bradley Lucier]

You wrote:

> gcc -O2 -msse2 -mfpmath=sse -ffast-math equal.c
> ./a.out
> 0x1p+0
> 0x1p+0 0x1p+0 1
>
> gcc -O2 -msse2 -mfpmath=sse -ffast-math -mrecip equal.c
> ./a.out
> 0x1.fffffep-1
> 0x1p+0 0x1.fffffep-1 0
>
> Actually, the recip pass is OK, it is just a precision problem. This
> is why -mrecip should stay separate option, not included in
> -ffast-math...

-ffast-math does other things to code that sometimes cause even  
bigger errors than -mrecip.

I think -ffast-math should turn on -mrecip (and similar  
optimizations) if available, and then they can be turned off with

-ffast-math -mno-recip

if needed.

The original reporter really shouldn't have been using -ffast-math or  
-mrecip if he was comparing floating-point values for equality.   
(There are many valid reasons to compare floating-point values for  
equality, but then one should try to have precisely-defined floating- 
point arithmetic.)

I was impressed with the results of this optimization.

I'm CC'ing Roger Sayle in the hope that he weighs in with his opinion.

Brad