[all-commits] [llvm/llvm-project] 8760ea: DAG: Avoid forming shufflevector from a single ext...

[Matt Arsenault via All-commits]

  Branch: refs/heads/users/arsenm/dag-do-not-form-shuffle-for-single-extract-element
  Home:   https://github.com/llvm/llvm-project
  Commit: 8760ea5965a338568048293f67faa502e05bbfa7
      https://github.com/llvm/llvm-project/commit/8760ea5965a338568048293f67faa502e05bbfa7
  Author: Matt Arsenault <Matthew.Arsenault at amd.com>
  Date:   2025-01-13 (Mon, 13 Jan 2025)

  Changed paths:
    M llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    M llvm/test/CodeGen/AMDGPU/insert_vector_dynelt.ll
    M llvm/test/CodeGen/X86/sse41.ll

  Log Message:
  -----------
  DAG: Avoid forming shufflevector from a single extract_vector_elt

This avoids regressions in a future AMDGPU commit. Previously we
would have a build_vector (extract_vector_elt x), undef with free
access to the elements bloated into a shuffle of one element + undef,
which has much worse combine support than the extract.

Alternatively could check aggressivelyPreferBuildVectorSources, but
I'm not sure it's really different than isExtractVecEltCheap.



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