[PATCH] D126939: [SLP] Avoid converting undef to poison when gathering.

[Harald van Dijk via Phabricator via llvm-commits]

hvdijk added inline comments.


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Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:7530
         std::distance(VL.begin(), find_if(reverse(VL), [](Value *V) {
                                     return !isa<UndefValue>(V);
                                   }).base());
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What if we try this the other way around? We have the initialisation of `NumValues` here to find the last non-undef value and then assume we do not need to look beyond that. What if we look for the last non-poison value there, by changing this to `return !isa<PoisonValue>(V);` and treat `undef` as just another constant that needs to be preserved? (The `isa<UndefValue>` inside the next loop would also need to be changed to `isa<PoisonValue>`.) That appears to fix your test just as well, and the impact on other existing SLPVectorizer tests is far less great. Does that also fix your original test?


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  https://reviews.llvm.org/D126939/new/

https://reviews.llvm.org/D126939