[PATCH] D121354: [SLP] Fix lookahead operand reordering for splat loads.

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:1121
+      if (V1 == V2) {
+        if (isa<LoadInst>(V1) && NumLanes == 2)
+          return VLOperands::ScoreSplatLoads;
----------------
vporpo wrote:
> lebedev.ri wrote:
> > This should be a lot more principled than this (i.e, query the actual costmodel).
> > E.g., `movddup` is only for `double`, but i think this will also trigger for `float`?
> > Also, what about broadcasting loads that are available in AVX/AVX2/AVX512?
> I agree, this needs to be target specific. I could add a new `ShuffleKind`, something like `SK_BroadcastLoad`. But this is not strictly a shuffle pattern, it is a combined Load + Shuffle. So this might need its own function, like `TargetTransformInfo::getBroadcastLoadCost(VectorType *VecTy)`. What do you think?
> 
What about adding a IsLoad bool flag to the TTI::getShuffleCost? Or a TTI::IsLegalBroadcastLoad (similar to the existing IsLegal*Load ops)?


Repository:
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  https://reviews.llvm.org/D121354/new/

https://reviews.llvm.org/D121354