[llvm] [SelectionDAG] Fold extracts spanning concat operands (PR #200936)

[Matt Arsenault via llvm-commits]

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@@ -27537,6 +27537,64 @@ static SDValue foldExtractSubvectorFromShuffleVector(EVT NarrowVT, SDValue Src,
   return DAG.getVectorShuffle(NarrowVT, DL, NewOps[0], NewOps[1], NewMask);
 }
 
+static SDValue foldExtractSubvectorFromConcatVectors(EVT VT, SDValue V,
+                                                     uint64_t ExtIdx,
+                                                     const SDLoc &DL,
+                                                     SelectionDAG &DAG,
+                                                     bool LegalOperations) {
+  if (V.getOpcode() != ISD::CONCAT_VECTORS)
+    return SDValue();
+
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arsenm wrote:

Should this depend on hasOneUse || isExtractSubvectorCheap?

https://github.com/llvm/llvm-project/pull/200936