[all-commits] [llvm/llvm-project] bb3c16: [SLP]Drop unprofitable splat gather subtrees

[Alexey Bataev via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: bb3c1628511349afbc0a0a82ec690a2da7020b63
      https://github.com/llvm/llvm-project/commit/bb3c1628511349afbc0a0a82ec690a2da7020b63
  Author: Alexey Bataev <a.bataev at outlook.com>
  Date:   2026-09-09 (Wed, 09 Sep 2026)

  Changed paths:
    M llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
    M llvm/test/Transforms/SLPVectorizer/AArch64/splat-gather-subtree-drop.ll
    M llvm/test/Transforms/SLPVectorizer/AArch64/splat-gather-subtree-store-chain.ll
    M llvm/test/Transforms/SLPVectorizer/X86/lookahead.ll
    M llvm/test/Transforms/SLPVectorizer/X86/reorder_phi.ll

  Log Message:
  -----------
  [SLP]Drop unprofitable splat gather subtrees

The keep/drop check ran only when trimming changed something and
ignored the splat emission cost on both sides. Run it in both paths
with the actual costs: keep the subtree if its price plus extracts plus
the reusing gathers' current costs is not worse than re-emitting those
gathers without it. Exclude splat roots from the main trimming loop.

Fixes the perf regression from #220250, reported in https://github.com/llvm/llvm-project/pull/220250?email_source=notifications&email_token=ABI45DSHUYRFN4NA4DILB5T5NZTZTA5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTKNJWG4ZTSMBZGUYKM4TFMFZW63VMON2GC5DFL5RWQYLOM5S2KZLWMVXHJLDGN5XXIZLSL5RWY2LDNM#issuecomment-5567390950.

Reviewers: RKSimon, bababuck

Pull Request: https://github.com/llvm/llvm-project/pull/221717



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