[llvm] Port `NVPTXTargetLowering::LowerCONCAT_VECTORS` to 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, 16> Ops;
+  unsigned NumOperands = Node->getNumOperands();
+  MVT VectorIdxType = TLI.getVectorIdxTy(DAG.getDataLayout());
+  for (unsigned I = 0; I < NumOperands; ++I) {
+    SDValue SubOp = Node->getOperand(I);
+    EVT VectorValueType = SubOp.getValueType();
+    EVT ElementValueType = VectorValueType.getVectorElementType();
+    unsigned NumSubElem = VectorValueType.getVectorNumElements();
+    for (unsigned Idx = 0; Idx < NumSubElem; ++Idx) {
+      Ops.push_back(DAG.getNode(ISD::EXTRACT_VECTOR_ELT, DL, ElementValueType,
+                                SubOp, DAG.getConstant(Idx, DL, VectorIdxType)));
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arsenm wrote:

EXTRACT_VECTOR_ELT has the weird property of supporting some implicit casts from the source vector to the result element, you'll probably need to adjust the input.

But yes this is a forever DAG design flaw. GlobalISel does not have this problem 

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