[llvm] [VectorCombine] Combine scalar fneg with insert/extract to vector fneg when length is different (PR #115209)

[Simon Pilgrim via llvm-commits]

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@@ -697,14 +696,34 @@ bool VectorCombine::foldInsExtFNeg(Instruction &I) {
       TTI.getArithmeticInstrCost(Instruction::FNeg, VecTy, CostKind) +
       TTI.getShuffleCost(TargetTransformInfo::SK_Select, VecTy, Mask, CostKind);
 
+  bool NeedLenChg = SrcVecTy->getNumElements() != NumElts;
+  // If the lengths of the two vectors are not equal,
+  // we need to add a length-change vector. Add this cost.
+  SmallVector<int> SrcMask;
+  if (NeedLenChg) {
+    SrcMask.assign(NumElts, PoisonMaskElem);
+    NewCost += TTI.getShuffleCost(TargetTransformInfo::SK_PermuteSingleSrc,
+                                  SrcVecTy, SrcMask, CostKind);
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RKSimon wrote:

Yes, a shuffle mask of all poison is likely to be assumed to be free

https://github.com/llvm/llvm-project/pull/115209