[all-commits] [llvm/llvm-project] f8d909: [DAGCombiner] `combineShuffleOfSplatVal()`: try to...

[Roman Lebedev via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: f8d9097168b73e3befd94ac4e16cbc5f944afcec
      https://github.com/llvm/llvm-project/commit/f8d9097168b73e3befd94ac4e16cbc5f944afcec
  Author: Roman Lebedev <lebedev.ri at gmail.com>
  Date:   2023-01-15 (Sun, 15 Jan 2023)

  Changed paths:
    M llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    M llvm/test/CodeGen/PowerPC/v4i32_scalar_to_vector_shuffle.ll
    M llvm/test/CodeGen/X86/srem-seteq-vec-nonsplat.ll
    M llvm/test/CodeGen/X86/urem-seteq-illegal-types.ll
    M llvm/test/CodeGen/X86/urem-seteq-vec-nonsplat.ll
    M llvm/test/CodeGen/X86/vec_smulo.ll
    M llvm/test/CodeGen/X86/vec_umulo.ll

  Log Message:
  -----------
  [DAGCombiner] `combineShuffleOfSplatVal()`: try to canonicalize to a splat shuffle

As noted in https://reviews.llvm.org/D141778#inline-1369900,
we fail to produce splat shuffles from certain sequences
of shuffles, that may have non-shuffles in the middle of seq.

There is a big pitfail to avoid here: just because `isSplatValue()`
says that all demanded elements are splat, we can't pick any random
one of them, because some of them could be undef! We must ignore those!