[llvm-dev] IR canonicalization: vector select or shufflevector?

[Philip Reames via llvm-dev]

I don't have a strong preference, though it is clear we should pick 
one.  I'd mildly prefer the select form for readability. From an 
optimization standpoint, I see reasonable arguments for either.

Philip


On 08/28/2016 12:37 PM, Sanjay Patel via llvm-dev wrote:
> A vector select with a constant vector condition operand:
>
> define <4 x i32> @foo(<4 x i32> %a, <4 x i32> %b) {
>   %sel = select <4 x i1> <i1 true, i1 false, i1 false, i1 true>, <4 x 
> i32> %a, <4 x i32> %b
>   ret <4 x i32> %sel
> }
>
>
> ...is equivalent to a shufflevector:
>
> define <4 x i32> @goo(<4 x i32> %a, <4 x i32> %b) {
>   %shuf = shufflevector <4 x i32> %a, <4 x i32> %b, <4 x i32> <i32 0, 
> i32 5, i32 6, i32 3>
>   ret <4 x i32> %shuf
> }
>
>
> For the goal of canonicalization in IR, which of these should we 
> prefer? Some backend / lowering differences for AArch64 and PPC are 
> noted in:
> https://llvm.org/bugs/show_bug.cgi?id=28530
> https://llvm.org/bugs/show_bug.cgi?id=28531
>
> x86 converts either form optimally in all cases I've looked at.
>
> This question first came up in D22114 ( 
> https://reviews.llvm.org/D22114 ) and is extended in D23886 ( 
> https://reviews.llvm.org/D23886 ) with a constant value example.
>
>
>
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