[llvm] [SelectionDAG] Fold subvector inserts into concat operands (PR #200937)

[Simon Pilgrim via llvm-commits]

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@@ -29503,16 +29503,38 @@ SDValue DAGCombiner::visitINSERT_SUBVECTOR(SDNode *N) {
     }
   }
 
-  // If the input vector is a concatenation, and the insert replaces
-  // one of the pieces, we can optimize into a single concat_vectors.
-  if (N0.getOpcode() == ISD::CONCAT_VECTORS && N0.hasOneUse() &&
-      N0.getOperand(0).getValueType() == N1.getValueType() &&
-      N0.getOperand(0).getValueType().isScalableVector() ==
-          N1.getValueType().isScalableVector()) {
-    unsigned Factor = N1.getValueType().getVectorMinNumElements();
-    SmallVector<SDValue, 8> Ops(N0->ops());
-    Ops[InsIdx / Factor] = N1;
-    return DAG.getNode(ISD::CONCAT_VECTORS, SDLoc(N), VT, Ops);
+  // If the input vector is a concatenation and the insert is wholly contained
+  // in one of its operands, push the insertion into that operand.
+  if (N0.getOpcode() == ISD::CONCAT_VECTORS && N0.hasOneUse()) {
+    EVT ConcatOpVT = N0.getOperand(0).getValueType();
+    EVT InsVT = N1.getValueType();
+    unsigned Factor = ConcatOpVT.getVectorMinNumElements();
+    unsigned ConcatOpIdx = InsIdx / Factor;
+    unsigned RelativeIdx = InsIdx - ConcatOpIdx * Factor;
+    assert(ConcatOpIdx < N0.getNumOperands() && "have enoug concat operands");
+
+    // If the insert replaces a whole concat operand, optimize into a single
+    // concat_vectors.
+    if (RelativeIdx == 0 && ConcatOpVT == InsVT &&
+        ConcatOpVT.isScalableVector() == InsVT.isScalableVector()) {
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RKSimon wrote:

why test for ConcatOpVT.isScalableVector() == InsVT.isScalableVector() is you've already tested ConcatOpVT == InsVT ?

https://github.com/llvm/llvm-project/pull/200937