[llvm] [NVPTX] Propagate truncate to operands (PR #98666)

[Artem Belevich via llvm-commits]

================
@@ -5541,6 +5541,53 @@ static SDValue PerformREMCombine(SDNode *N,
   return SDValue();
 }
 
+// truncate (logic_op x, y) --> logic_op (truncate x), (truncate y)
+// This will reduce register pressure.
+static SDValue PerformTruncCombine(SDNode *N,
+                                   TargetLowering::DAGCombinerInfo &DCI) {
+  if (!DCI.isBeforeLegalizeOps())
+    return SDValue();
+
+  SDValue LogicalOp = N->getOperand(0);
+  switch (LogicalOp.getOpcode()) {
+  default:
+    break;
+  case ISD::ADD:
+  case ISD::SUB:
+  case ISD::MUL:
+  case ISD::AND:
+  case ISD::OR:
+  case ISD::XOR: {
+    EVT VT = N->getValueType(0);
+    EVT LogicalVT = LogicalOp.getValueType();
+    if (VT != MVT::i32 || LogicalVT != MVT::i64)
+      break;
+    const TargetLowering &TLI = DCI.DAG.getTargetLoweringInfo();
+    if (!VT.isScalarInteger() &&
+        !TLI.isOperationLegal(LogicalOp.getOpcode(), VT))
+      break;
+    if (!all_of(LogicalOp.getNode()->uses(), [](SDNode *U) {
+          return U->isMachineOpcode()
+                     ? U->getMachineOpcode() == NVPTX::CVT_u32_u64
+                     : U->getOpcode() == ISD::TRUNCATE;
+        }))
+      break;
+
+    SDLoc DL(N);
+    SDValue CVTNone =
+        DCI.DAG.getTargetConstant(NVPTX::PTXCvtMode::NONE, DL, MVT::i32);
+    SDNode *NarrowL = DCI.DAG.getMachineNode(NVPTX::CVT_u32_u64, DL, VT,
+                                             LogicalOp.getOperand(0), CVTNone);
+    SDNode *NarrowR = DCI.DAG.getMachineNode(NVPTX::CVT_u32_u64, DL, VT,
+                                             LogicalOp.getOperand(1), CVTNone);
----------------
Artem-B wrote:

Machine nodes are usually dealt with in NVPTXISelDAGToDAG.cpp
Moving it there may simplify things as you would only need to deal with `NVPTX::CVT_u32_u64` only.
Is there any particular reason this optimization must be done here?

On a side note, It actually looks simple enough that we may even be able to pattern-match it in tablegen.


https://github.com/llvm/llvm-project/pull/98666