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[SLP] Ignore unreachable blocks
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Authored by hvdijk on Jun 1 2021, 11:48 AM.

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Summary

As the existing test unreachable.ll shows, we should be doing more
work to avoid entering unreachable blocks: we should not stop
vectorization just because a PHI incoming value from an unreachable
block cannot be vectorized. We know that particular value will never
be used so we can just replace it with poison.

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Event Timeline

hvdijk created this revision.Jun 1 2021, 11:48 AM
hvdijk requested review of this revision.Jun 1 2021, 11:48 AM
Herald added a project: Restricted Project. · View Herald TranscriptJun 1 2021, 11:48 AM
ABataev added inline comments.Jun 1 2021, 11:57 AM
llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
2833–2834

Better to do something like this:

if (!DT->isReachableFromEntry(PH->getIncomingBlock(I))) {
  ValueList Operands(VL.size(), PoisonValue::get(PH->getType()));
  TE->setOperand(I, Operands);
  OperandsVec.push_back(Operands);
  continue;
}

and remove the change in BoUpSLP::vectorizeTree

hvdijk updated this revision to Diff 349053.Jun 1 2021, 12:15 PM
hvdijk marked an inline comment as done.
hvdijk added inline comments.
llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
2833–2834

Good idea, re-ran tests with exactly what you put here, that works.

hvdijk marked an inline comment as done.Jun 1 2021, 12:17 PM
This revision is now accepted and ready to land.Jun 1 2021, 12:20 PM
hvdijk closed this revision.Jun 1 2021, 12:22 PM

Thanks. Accidentally pushed without updating the commit message, linking and closing manually.