[PATCH] D73490: [LV] Remove nondeterminacy by changing LoopVectorizationLegality::Reductions from DenseMap to MapVector

[Ayal Zaks via Phabricator via llvm-commits]

Ayal added a comment.

Culprit is the following from LoopVectorize.cpp:

  // Finally, if tail is folded by masking, introduce selects between the phi
  // and the live-out instruction of each reduction, at the end of the latch.
  if (CM.foldTailByMasking()) {
    Builder.setInsertPoint(VPBB);
    auto *Cond = RecipeBuilder.createBlockInMask(OrigLoop->getHeader(), Plan);
    for (auto &Reduction : *Legal->getReductionVars()) {
      VPValue *Phi = Plan->getVPValue(Reduction.first);
      VPValue *Red = Plan->getVPValue(Reduction.second.getLoopExitInstr());
      Builder.createNaryOp(Instruction::Select, {Cond, Red, Phi});
    }
  }

Hence small estimated trip-count based on branch weights in the above test is used to trigger foldTailByMasking. Another, more direct alternative is to specify -prefer-predicate-over-epilog.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D73490/new/

https://reviews.llvm.org/D73490