[PATCH] D135876: [VectorCombine] Scalarize binops with insertelt nested inside splats

[Sanjay Patel via Phabricator via llvm-commits]

spatel requested changes to this revision.
spatel added a comment.
This revision now requires changes to proceed.

If the motivating case always ends in a splat, could we do this in instcombine instead?

This is an improvement for the specific patterns where it is allowed, but it needs to account for possibly several other patterns where it is changing code in ways that the original transform did not anticipate. For example, this can currently miscompile (tested with aarch64 and x86 targets):

  define <4 x i32> @insert_splat_sub(i32 %x, i32 %y) {
    %x111 = insertelement <4 x i32> <i32 1, i32 1, i32 1, i32 1>, i32 %x, i64 0
    %y567 = insertelement <4 x i32> <i32 4, i32 5, i32 6, i32 7>, i32 %y, i64 0
    %yyyy = shufflevector <4 x i32> %y567, <4 x i32> poison, <4 x i32> zeroinitializer
    %r = sub <4 x i32> %x111, %yyyy
    ret <4 x i32> %r
  }


Repository:
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  https://reviews.llvm.org/D135876/new/

https://reviews.llvm.org/D135876