[llvm] [SelectionDAG] WidenVecOp_INSERT_SUBVECTOR - Replace `INSERT_SUBVECTOR` with series of `INSERT_VECTOR_ELT` (PR #124420)

[Simon Pilgrim via llvm-commits]

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@@ -7070,8 +7073,24 @@ SDValue DAGTypeLegalizer::WidenVecOp_INSERT_SUBVECTOR(SDNode *N) {
     return DAG.getNode(ISD::INSERT_SUBVECTOR, SDLoc(N), VT, InVec, SubVec,
                        N->getOperand(2));
 
-  report_fatal_error("Don't know how to widen the operands for "
-                     "INSERT_SUBVECTOR");
+  // If the operands can't be widened legally, just replace the INSERT_SUBVECTOR
+  // with a series of INSERT_VECTOR_ELT
+  EVT OrigVT = OrigSubVec.getValueType();
+  unsigned Idx = N->getConstantOperandVal(2);
+
+  SDValue InsertElt;
+  SDLoc DL(N);
+  EVT VectorIdxTy = TLI.getVectorIdxTy(DAG.getDataLayout());
+  for (unsigned I = 0; I < OrigVT.getVectorNumElements(); ++I) {
+    SDValue ExtractElt =
+        DAG.getNode(ISD::EXTRACT_VECTOR_ELT, DL, VT.getScalarType(), SubVec,
+                    DAG.getConstant(I, DL, VectorIdxTy));
+    InsertElt =
+        DAG.getNode(ISD::INSERT_VECTOR_ELT, DL, VT, I != 0 ? InsertElt : InVec,
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RKSimon wrote:

Just initialize InsertElt to InVec and remove the `I !=0` handling

https://github.com/llvm/llvm-project/pull/124420