[PATCH] D70246: [InstCombine] remove identity shuffle simplification for mask with undefs

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune added a comment.

The optimization can be kept live by adding freeze (`shufflevector <4 x float> %arg, ...` -> `freeze %arg`), but existing analyzers are not aware of freeze yet (which will block following optimizations) and the underlying problem is more related with interaction of undef/poison (which disappears if undef is removed), so I'm fine with this patch's direction.

BTW, does the current semantics of shufflevector imply that

  %shuf = shufflevector <4 x float> %arg, <4 x float> undef, <4 x i32> <i32 undef, i32 1, i32 2, i32 3>
  ->
  %shuf = shufflevector <4 x float> %arg, <4 x float> undef, <4 x i32> <i32 0, i32 1, i32 2, i32 3>

is incorrect?
If the reason why shufflevector returns undef is simply there was no poison constant, why don't we move toward adding poison constant?


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  https://reviews.llvm.org/D70246/new/

https://reviews.llvm.org/D70246