[all-commits] [llvm/llvm-project] 79891a: [VPlan] Fix find last reductions not using frozen ...

[Luke Lau via All-commits]

  Branch: refs/heads/users/lukel97/loop-vectorize/handleFindLastReductions-freeze-cond
  Home:   https://github.com/llvm/llvm-project
  Commit: 79891a162031c931431b2953d4560cc72284aede
      https://github.com/llvm/llvm-project/commit/79891a162031c931431b2953d4560cc72284aede
  Author: Luke Lau <luke at igalia.com>
  Date:   2026-09-21 (Mon, 21 Sep 2026)

  Changed paths:
    M llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp
    M llvm/test/Transforms/LoopVectorize/AArch64/conditional-scalar-assignment-fold-tail.ll
    M llvm/test/Transforms/LoopVectorize/AArch64/conditional-scalar-assignment.ll
    M llvm/test/Transforms/LoopVectorize/AArch64/early_exit_costs.ll
    M llvm/test/Transforms/LoopVectorize/AArch64/findlast-epilogue-loop.ll
    M llvm/test/Transforms/LoopVectorize/AArch64/sve-select-cmp.ll
    M llvm/test/Transforms/LoopVectorize/RISCV/conditional-scalar-assignment-fold-tail.ll
    M llvm/test/Transforms/LoopVectorize/RISCV/conditional-scalar-assignment.ll
    M llvm/test/Transforms/LoopVectorize/RISCV/iv-select-cmp.ll
    M llvm/test/Transforms/LoopVectorize/RISCV/select-cmp-reduction.ll
    M llvm/test/Transforms/LoopVectorize/VPlan/conditional-scalar-assignment-vplan.ll
    M llvm/test/Transforms/LoopVectorize/VPlan/interleave-conditional-scalar-assignment-vplan.ll
    M llvm/test/Transforms/LoopVectorize/X86/conditional-scalar-assignment.ll
    M llvm/test/Transforms/LoopVectorize/alias-mask.ll
    M llvm/test/Transforms/LoopVectorize/conditional-scalar-assignment-interleave-only.ll
    M llvm/test/Transforms/LoopVectorize/find-last-iv-sinkable-expr-tail-folding.ll
    M llvm/test/Transforms/LoopVectorize/find-last-iv-sinkable-expr.ll
    M llvm/test/Transforms/LoopVectorize/find-last-iv-sinkable-load.ll
    M llvm/test/Transforms/LoopVectorize/find-last-ptr-induction.ll
    M llvm/test/Transforms/LoopVectorize/find-last.ll
    M llvm/test/Transforms/LoopVectorize/iv-select-cmp-fold-tail.ll
    M llvm/test/Transforms/LoopVectorize/iv-select-cmp-non-const-iv-start.ll
    M llvm/test/Transforms/LoopVectorize/iv-select-cmp-trunc.ll
    M llvm/test/Transforms/LoopVectorize/iv-select-cmp.ll
    M llvm/test/Transforms/LoopVectorize/select-cmp.ll

  Log Message:
  -----------
  [VPlan] Fix find last reductions not using frozen condition in select

A find last reduction is something like:

    (select c[n], x[n], (select c[n-1], x[n-1], (select ...)))

So even if previous iterations had poison for c[x], the top-level select blocks it.

MaskSelect uses the non-frozen Cond, and that in turn is fed into VPInstruction::ExtractLastActive, which in turn gets expanded to vector.reduce.umax, which returns poison if any of the lanes were poison.

So in the case e.g. c[n] = 1, c[n-1] = poison and VF=2, the scalar loop returns x[n] whilst vector loop returns poison.

We're also introducing multiple uses of Cond, so for these reasons we need to use the frozen Cond.



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