[llvm] Port NVPTXTargetLowering::LowerCONCAT_VECTORS to llvm/lib/CodeGen/SelectionDAG (PR #120030)

[Matt Arsenault via llvm-commits]

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@@ -1517,10 +1518,27 @@ SDValue SelectionDAGLegalize::ExpandInsertToVectorThroughStack(SDValue Op) {
                      BaseVecAlignment);
 }
 
+SDValue SelectionDAGLegalize::ExpandConcatVectors(SDNode *Node) {
+  assert(Node->getOpcode() == ISD::CONCAT_VECTORS && "Unexpected opcode!");
+  SDLoc Dl(Node);
+  SmallVector<SDValue, 0> Ops;
+  unsigned NumOperands = Node->getNumOperands();
+  for (unsigned I = 0; I < NumOperands; ++I) {
+    SDValue SubOp = Node->getOperand(I);
+    EVT VectorValueType =
+        SubOp->getValueType(0);
+    EVT ElementValueType = VectorValueType.getVectorElementType();
+    unsigned NumSubElem = VectorValueType.getVectorNumElements();
+    for (unsigned J = 0; J < NumSubElem; ++J) {
+      Ops.push_back(DAG.getNode(ISD::EXTRACT_VECTOR_ELT, Dl, ElementValueType,
+                                SubOp, DAG.getIntPtrConstant(J, Dl)));
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arsenm wrote:

This should be getVectorIdxTy (and hoisted out of the loop)

https://github.com/llvm/llvm-project/pull/120030