[llvm] [VectorCombine] Add special handling for truncating shuffles (PR #70013)

[Alexey Bataev via llvm-commits]

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@@ -1472,21 +1472,27 @@ bool VectorCombine::foldShuffleFromReductions(Instruction &I) {
       dyn_cast<FixedVectorType>(Shuffle->getOperand(0)->getType());
   if (!ShuffleInputType)
     return false;
-  int NumInputElts = ShuffleInputType->getNumElements();
+  unsigned int NumInputElts = ShuffleInputType->getNumElements();
 
   // Find the mask from sorting the lanes into order. This is most likely to
   // become a identity or concat mask. Undef elements are pushed to the end.
   SmallVector<int> ConcatMask;
   Shuffle->getShuffleMask(ConcatMask);
   sort(ConcatMask, [](int X, int Y) { return (unsigned)X < (unsigned)Y; });
+  // In the case of a truncating shuffle it's possible for the mask
+  // to have an index greater than the size of the resulting vector.
+  // This requires special handling.
+  bool IsTruncatingShuffle = VecType->getNumElements() < NumInputElts;
   bool UsesSecondVec =
-      any_of(ConcatMask, [&](int M) { return M >= NumInputElts; });
+      any_of(ConcatMask, [&](int M) { return M >= (int)NumInputElts; });
   InstructionCost OldCost = TTI.getShuffleCost(
       UsesSecondVec ? TTI::SK_PermuteTwoSrc : TTI::SK_PermuteSingleSrc,
-      UsesSecondVec ? VecType : ShuffleInputType, Shuffle->getShuffleMask());
+      (UsesSecondVec && !IsTruncatingShuffle) ? VecType : ShuffleInputType,
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alexey-bataev wrote:

Outline this logic and store the result in a variable

https://github.com/llvm/llvm-project/pull/70013