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

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: efriedma, regehr, aqjune, nlopes, RKSimon, lebedev.ri, liuz.
Herald added subscribers: hiraditya, mcrosier.
Herald added a project: LLVM.

Given a shuffle that includes undef elements in an otherwise identity mask like:

  define <4 x float> @shuffle(<4 x float> %arg) {
    %shuf = shufflevector <4 x float> %arg, <4 x float> undef, <4 x i32> <i32 undef, i32 1, i32 2, i32 3>
    ret <4 x float> %shuf
  }

We were simplifying that to the input operand.

But as discussed in PR43958:
https://bugs.llvm.org/show_bug.cgi?id=43958
...that means that per-vector-element poison that would be stopped by the shuffle can now leak to the result.

For reference, this transform was added long ago with:
rL142671 <https://reviews.llvm.org/rL142671>

Also note that we still have (and there are tests for) the same transform with no undef elements in the mask (a fully-defined identity mask). I don't think there's any controversy about that case - it's a valid transform under any interpretation of shufflevector/undef/poison.

Looking at a few of the diffs into codegen, I don't see any difference in final asm. So depending on your perspective, that's good (no real loss of optimization power) or bad (poison exists in the DAG, so we only partially fixed the bug).


https://reviews.llvm.org/D70246

Files:
  llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
  llvm/test/Transforms/InstCombine/X86/clmulqdq.ll
  llvm/test/Transforms/InstCombine/X86/x86-avx2.ll
  llvm/test/Transforms/InstCombine/X86/x86-avx512.ll
  llvm/test/Transforms/InstCombine/X86/x86-f16c.ll
  llvm/test/Transforms/InstCombine/X86/x86-pack.ll
  llvm/test/Transforms/InstCombine/X86/x86-pshufb.ll
  llvm/test/Transforms/InstCombine/X86/x86-sse.ll
  llvm/test/Transforms/InstCombine/X86/x86-sse41.ll
  llvm/test/Transforms/InstCombine/X86/x86-sse4a.ll
  llvm/test/Transforms/InstCombine/X86/x86-vector-shifts.ll
  llvm/test/Transforms/InstCombine/X86/x86-vpermil.ll
  llvm/test/Transforms/InstCombine/X86/x86-xop.ll
  llvm/test/Transforms/InstCombine/shuffle_select.ll
  llvm/test/Transforms/InstCombine/vec_demanded_elts.ll
  llvm/test/Transforms/InstCombine/vec_shuffle.ll

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