[llvm] [VectorCombine] Combine extract/insert from vector (PR #115213)

[Simon Pilgrim via llvm-commits]

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@@ -2678,6 +2679,49 @@ bool VectorCombine::shrinkType(llvm::Instruction &I) {
   return true;
 }
 
+/// insert (DstVec, (extract SrcVec, ExtIdx), InsIdx) -->
+/// shuffle (DstVec, SrcVec, Mask)
+bool VectorCombine::foldInsExtVectorToShuffle(Instruction &I) {
+  Value *DstVec, *SrcVec;
+  uint64_t ExtIdx, InsIdx;
+  if (!match(&I, m_InsertElt(m_Value(DstVec),
+                             m_OneUse(m_ExtractElt(m_Value(SrcVec),
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RKSimon wrote:

We should be able to remove the m_OneUse if we can account for it in the NewCost below by adding the ExtractElementInst cost back if its has multiple uses.

https://github.com/llvm/llvm-project/pull/115213