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

[Matt Arsenault via llvm-commits]

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@@ -910,6 +910,35 @@ SDValue DAGTypeLegalizer::CreateStackStoreLoad(SDValue Op,
   return DAG.getLoad(DestVT, dl, Store, StackPtr, MachinePointerInfo(), Align);
 }
 
+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();
+
+  // Pack vector elements into i16 and bitcast to final type
+  SDLoc DL(Node);
+  SDValue Vec0 = DAG.getNode(ISD::EXTRACT_VECTOR_ELT, DL, MVT::i8,
+                             Node->getOperand(0), DAG.getIntPtrConstant(0, DL));
+  SDValue Vec1 = DAG.getNode(ISD::EXTRACT_VECTOR_ELT, DL, MVT::i8,
+                             Node->getOperand(0), DAG.getIntPtrConstant(1, DL));
+
+  SDValue Extend0 = DAG.getNode(ISD::ZERO_EXTEND, DL, MVT::i16, Vec0);
+  SDValue Extend1 = DAG.getNode(ISD::ZERO_EXTEND, DL, MVT::i16, Vec1);
+
+  EVT ShiftAmtTy =
+      TLI.getShiftAmountTy(Extend1.getValueType(), DAG.getDataLayout());
+  SDValue ShiftConst = DAG.getShiftAmountConstant(8, ShiftAmtTy, DL);
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arsenm wrote:

This isn't correct usage. getShiftAmountConstant internally uses getShiftAmountTy. If you use getShiftAmountTy, you use the type directly to getConstant. Or you could use getShiftAmountConstant with Extend1.getValueType()

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