[llvm] Nvptx port LowerBITCAST to SelectionDAG (PR #120903)

[Simon Pilgrim via llvm-commits]

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@@ -910,6 +910,38 @@ SDValue DAGTypeLegalizer::CreateStackStoreLoad(SDValue Op,
   return DAG.getLoad(DestVT, dl, Store, StackPtr, MachinePointerInfo(), Align);
 }
 
+static SDValue MaybeBitcast(SelectionDAG &DAG, SDLoc DL, EVT VT,
+                            SDValue Value) {
+  if (Value->getValueType(0) == VT)
+    return Value;
+  return DAG.getNode(ISD::BITCAST, DL, VT, Value);
+}
+
+SDValue DAGTypeLegalizer::LowerBitcast(SDNode *Node) const {
+  assert(Node->getOpcode() == ISD::BITCAST && "Unexpected opcode!");
+  // Handle bitcasting from v2i8 without hitting the default promotion
+  // strategy which goes through stack memory.
+  EVT FromVT = Node->getOperand(0)->getValueType(0);
+  if (FromVT != MVT::v2i8) {
+    return SDValue();
+  }
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RKSimon wrote:

Are we gaining anything from having PackBitcastInRegister/UnpackBitcastInRegister vs merging them inside LowerBitcastInRegister? 

https://github.com/llvm/llvm-project/pull/120903