llvm dev - Aug 2013 - [LLVMdev] Failure to optimize vector select

Hi,

I've found a case I would expect would optimize easily, but it doesn't.
A simple implementation of vector select:

float4 simple_select(float4 a, float4 b, int4 c)
{
    float4 result;

    result.x = c.x ? a.x : b.x; 
    result.y = c.y ? a.y : b.y;
    result.z = c.z ? a.z : b.z;
    result.w = c.w ? a.w : b.w;

    return result;
}

I would expect this would be optimized to

%bool = icmp eq <4 x i32> %c, 0
%result = select <4 x i1> %bool, <4 x float> %a, <4x float> %b
ret <4 x float> %result

However, it actually ends up as the 4 separate extractelement/icmp/select
sequence.

Where would be the best place to fix this? Should InstCombine be taking care of
this or the vectorizer?


Thanks

Have you tried running SLP vectorizer pass (-vectorize-slp)?

Eugene


On Mon, Aug 19, 2013 at 9:04 PM, Matt Arsenault <arsenm2 at gmail.com>
wrote:
> Hi,
>
> I've found a case I would expect would optimize easily, but it
doesn't. A
> simple implementation of vector select:
>
> float4 simple_select(float4 a, float4 b, int4 c)
> {
>     float4 result;
>
>     result.x = c.x ? a.x : b.x;
>     result.y = c.y ? a.y : b.y;
>     result.z = c.z ? a.z : b.z;
>     result.w = c.w ? a.w : b.w;
>
>     return result;
> }
>
> I would expect this would be optimized to
>
> %bool = icmp eq <4 x i32> %c, 0
> %result = select <4 x i1> %bool, <4 x float> %a, <4x
float> %b
> ret <4 x float> %result
>
> However, it actually ends up as the 4 separate extractelement/icmp/select
> sequence.
>
> Where would be the best place to fix this? Should InstCombine be taking
> care of this or the vectorizer?
>
>
> Thanks
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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On Aug 19, 2013, at 18:47 , Eugene Toder <eltoder at gmail.com> wrote:
> Have you tried running SLP vectorizer pass (-vectorize-slp)?
Yes. That was the first thing i tried, and it didn't do anything. I was
looking the vectorizer, but then I saw some things that made me wonder if it was
even supposed to do this