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

[Simon Pilgrim via llvm-commits]

================
@@ -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);
+    SrcMask[Index] = Index;
+    NewCost += TTI.getShuffleCost(TargetTransformInfo::SK_PermuteSingleSrc,
+                                  SrcVecTy, SrcMask, CostKind);
+  }
+
   if (NewCost > OldCost)
     return false;
 
-  // insertelt DestVec, (fneg (extractelt SrcVec, Index)), Index -->
-  // shuffle DestVec, (fneg SrcVec), Mask
+  Value *NewShuf;
+  // insertelt DestVec, (fneg (extractelt SrcVec, Index)), Index
   Value *VecFNeg = Builder.CreateFNegFMF(SrcVec, FNeg);
-  Value *Shuf = Builder.CreateShuffleVector(DestVec, VecFNeg, Mask);
-  replaceValue(I, *Shuf);
+  if (NeedLenChg) {
+    // shuffle DestVec, (shuffle (fneg SrcVec), poison, SrcMask), Mask
+    Value *LenChgShuf = Builder.CreateShuffleVector(
+        SrcVec, PoisonValue::get(SrcVecTy), SrcMask);
----------------
RKSimon wrote:

You can use the shorter 1 op version of CreateShuffleVector here:
```
Value *LenChgShuf = Builder.CreateShuffleVector(SrcVec, SrcMask)
```

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