recent troubles with float vectors & bitwise ops

[tim prince]

tbp wrote:
> On 8/23/07, Tim Prince <tprince@computer.org> wrote:
>   
> Note that icc9 has a strong bias for pentium4, which had no stall
> penalty for mistyped fp vectors as for Intel it came with the pentium
> M line, so you see a pxor even if generating code for the core2.
> # cat autoicc.cc
> float foo(const float *a, int n) {
>         float sum = 0.f;
>         for (int i = 0; i <n; ++i)
>                 if (a[i] > 0.f)
>                         sum += a[i];
>         return sum;
> }
> int main() { return 0; }
> # /opt/intel/cce/9.1.051/bin/icpc -O3 -xT autoicc.cc
> autoicc.cc(3) : (col. 2) remark: LOOP WAS VECTORIZED.
>   4007a9:       pxor   %xmm4,%xmm4
>   4007ad:       cmpltps %xmm3,%xmm4
>   4007b1:       andps  %xmm3,%xmm4
> # /opt/intel/cce/10.0.023/bin/icpc -O3 -xT autoicc.cc
> autoicc.cc(3): (col. 2) remark: LOOP WAS VECTORIZED.
>   400b50:       xorps  %xmm3,%xmm3
>   400b53:       cmpltps %xmm4,%xmm3
>   400b57:       andps  %xmm3,%xmm4
>   
For what little it's worth, I found no measurable difference between 
these choices on Core 2 Duo.  People I know prefer to use the Intel -xW 
option, or the gcc default, in the absence of clear evidence that 
another option could improve performance without reducing the range of 
supported targets.

---AV & Spam Filtering by M+Guardian - Risk Free Email (TM)---