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

[Michael Kuperstein via llvm-dev]

I have a slight preference towards shufflevector, because it makes
sequences of shuffles, where only some of the shuffles can be converted
into selects (because the input and output vector sizes of the others don't
match) simpler to reason about.

I'm not sure this is a particularly good reason, though.

On Mon, Aug 29, 2016 at 8:19 AM, Philip Reames via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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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