Re: FP compares and TARGET_SSE_MATH

[Uros Bizjak <uros at kss-loka dot si>]

Jan Hubicka wrote:

> Do you have any benchmarks that suggest that avoiding the mixture of
> both is a lost?  I briefly benchmarked this on SPECfp at the time I was
> implementing this and using both sets was a win that time, but times
> might've changed.
>  
I have tried some FP benchmark with -mno-sse, but -mno-sse also disabled 
cvtt* patterns, so I don't trust the results. But consider this simple 
testcase:

double test (double a, double b) {
       if (a > b)
        return a;
       else
        return b;
}

with '-O2 -S -march=pentium4 -mfpmath=sse -ffast-math':

test:
       pushl   %ebp
       movl %esp, %ebp
       subl $8, %esp
       fldl 8(%ebp)
       movsd   16(%ebp), %xmm0
       movsd   %xmm0, -8(%ebp)
       fldl -8(%ebp)
       fcomi   %st(1), %st
       fcmovb  %st(1), %st
       fstp %st(1)
       leave
       ret

> I also sent patch to teach regclass to discover the dependencies (ie to
> avoid putting register X in x87 when it is used in comparsion operator
> with register Y that needs to live in SSE).  perhaps we might thing
> about sollution in this dirrection. (I originally gave up mostly because
> new-RA seemed to make progress)
>
>  
Would this patch solve these situations, when -mfpmath=387 is choosen:

80a0f0a:       d9 03                   flds   (%ebx)
80a0f0c:       d9 5c 24 4c             fstps  0x4c(%esp,1)
80a0f10:       f3 0f 10 44 24 4c       movss  0x4c(%esp,1),%xmm0
80a0f16:       0f 2f 00                comiss (%eax),%xmm0
80a0f19:       0f 43 c3                cmovae %ebx,%eax

And similar for -mfpmath=sse:

804d112:       dd 43 28                fldl   0x28(%ebx)
804d115:       f2 0f 11 04 24          movsd  %xmm0,(%esp,1)
804d11a:       dd 04 24                fldl   (%esp,1)
804d11d:       31 c0                   xor    %eax,%eax
804d11f:       df f1                   fcomip %st(1),%st
804d121:       0f 95 c0                setne  %al

These codes are from PovRay, where:

grep fcomi povray.sse | wc -l
   668
grep comis povray.387 | wc -l
   414

Uros.