[PATCH] D127073: [SLP] Treat undef as any other constant

[Alexey Bataev via Phabricator via llvm-commits]

ABataev added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:7598
       if (isConstant(V)) {
         ReuseShuffleIndicies.emplace_back(UniqueValues.size());
         UniqueValues.emplace_back(V);
----------------
hvdijk wrote:
> ABataev wrote:
> > For undefs need to push `UndefMaskElem`:
> > ```
> > ReuseShuffleIndicies.emplace_back(isa<UndefValue>(V) ? UndefMaskElem : UniqueValues.size());
> > ```
> I commented in D126939 that if we use UndefMaskElem when we want undef results, we run into another issue later on where we mistakenly treat a shuffle mask as an identity mask and remove the shuffle.
But here it should be a case? You're pushing original UndefValue to the UniqueValues vector. So, you have undervalue in the vector and can have UndefMaskElem for it in the mask.


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  https://reviews.llvm.org/D127073/new/

https://reviews.llvm.org/D127073