[PATCH] D99719: [SLP] Better estimate cost of no-op extracts on target vectors.

[Florian Hahn via Phabricator via llvm-commits]

fhahn marked an inline comment as done.
fhahn added inline comments.


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Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:3495-3498
+        unsigned MaxVecRegSize = getMaxVecRegSize();
+        unsigned EltSize = getVectorElementSize(VL[0]);
+        unsigned EltsPerVector = MaxVecRegSize / EltSize;
+        unsigned Idx = 0;
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ABataev wrote:
> I think it is better to use `TLI->getTypeLegalizationCost(DL, cast<ExtractElementInst>(V)->getVectorOperandType());` to get the real machine vector type and the number of splits.
I think using `TargetLoweringInfo` would indeed be better, but unfortunately I don't think we can access it here, as it is defined in CodeGen? I tried to see if there are any other such uses in `llvm/lib/Transforms` but couldn't. Perhaps there's a way to use it I am missing?


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Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:3535
+                Cost -= TTI->getVectorInstrCost(Instruction::ExtractElement,
+                                                VecTy, I);
+            }
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ABataev wrote:
> I think you need to use the real extract indices here to be more correct, i.e.
> ```
> for (unsigned I = Idx - EltsPerVector; I <= Idx; ++I)
> Cost -= TTI->getVectorInstrCost(Instruction::ExtractElement,
>                                                 cast<ExtractElementInst>(VL[I])->getVectorOperandType(), *getExtractIndex(cast<Instruction>(VL[I])));
> ```
Thanks, I think that's much better, updated.


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https://reviews.llvm.org/D99719