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

[Justin Fargnoli via llvm-commits]

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@@ -15198,6 +15202,25 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
     break;
   }
 
+  if (TLI.shouldReduceRegisterPressure()) {
+    switch (N0.getOpcode()) {
+    case ISD::ADD:
+    case ISD::SUB:
+    case ISD::MUL:
+    case ISD::AND:
+    case ISD::OR:
+    case ISD::XOR:
+      if (!(N0.hasOneUse() && VT.isScalarInteger() &&
+            TLI.isTruncateFree(SrcVT, VT)))
+        break;
+      if (LegalOperations && !TLI.isOperationLegal(N0.getOpcode(), VT))
+        break;
+      SDValue NarrowL = DAG.getNode(ISD::TRUNCATE, DL, VT, N0.getOperand(0));
+      SDValue NarrowR = DAG.getNode(ISD::TRUNCATE, DL, VT, N0.getOperand(1));
+      return DAG.getNode(N0.getOpcode(), DL, VT, NarrowL, NarrowR);
+    }
+  }
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justinfargnoli wrote:

The `switch` above looks for ["a suitable binary operation with a non-opaque constant operand,"](https://github.com/llvm/llvm-project/blob/7b801c19a0ac21b07883ee58ba550865a4e25075/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp#L15150) whereas this `switch` will run regardless of whether an operand is constant. 

Originally, I tried to extend the existing `switch`, but this approach felt cleaner. 

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