[llvm] [VectorCombine] isExtractExtractCheap - specify the extract/insert shuffle mask to improve shuffle costs (PR #114780)

[Alexey Bataev via llvm-commits]

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@@ -482,8 +484,16 @@ bool VectorCombine::isExtractExtractCheap(ExtractElementInst *Ext0,
     // ShufMask = { poison, poison, 0, poison }
     // TODO: The cost model has an option for a "broadcast" shuffle
     //       (splat-from-element-0), but no option for a more general splat.
-    NewCost +=
-        TTI.getShuffleCost(TargetTransformInfo::SK_PermuteSingleSrc, VecTy);
+    if (auto *FixedVecTy = dyn_cast<FixedVectorType>(VecTy)) {
+      SmallVector<int> ShuffleMask;
+      ShuffleMask.append(FixedVecTy->getNumElements(), PoisonMaskElem);
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alexey-bataev wrote:

`SmallVector<int> ShuffleMask(FixedVecTy->getNumElements(), PoisonMaskElem);`

https://github.com/llvm/llvm-project/pull/114780