[PATCH] D144689: [SLP]Improve handling gathers/buildvectors with undefs.

[Alexey Bataev via Phabricator via llvm-commits]

ABataev added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:1449
         // Undefs are always profitable for extractelements.
         if (isa<UndefValue>(V2))
+          return (isa<PoisonValue>(V2) || isUndefVector(EV1).all())
----------------
vdmitrie wrote:
> ABataev wrote:
> > vdmitrie wrote:
> > > This needs a bit of explanation (a comment). 
> > We can easily combine `poison and extractelement <non-poison>` or `undef and extractelement <poison>`. But combining `undef + extractelement <non-poison-but-my-produce-poison>` requires some extra operations and it is not very effective to combine such elements (to preserve the difference between undefs and poison), rather than extractelement from the same EV1, even in reversed order.
> Thanks for explanation. I just meant to put it into the code as a comment.
Ok, will do


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  https://reviews.llvm.org/D144689/new/

https://reviews.llvm.org/D144689