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

[Alexey Bataev via Phabricator via llvm-commits]

ABataev 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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hvdijk wrote:
> 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?
It won't help, there is still a bug, better just to copy VL in the corner case.


Repository:
  rG LLVM Github Monorepo

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

https://reviews.llvm.org/D126939