[LLVMdev] Failure to optimize vector select

[Eugene Toder]

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
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