[llvm] [DAG] Constant Folding for U/SMUL_LOHI (PR #69437)

[Simon Pilgrim via llvm-commits]

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@@ -9879,6 +9879,27 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, SDVTList VTList,
            VTList.VTs[0] == Ops[0].getValueType() &&
            VTList.VTs[0] == Ops[1].getValueType() &&
            "Binary operator types must match!");
+    // Constant fold.
+    ConstantSDNode *LHS = dyn_cast<ConstantSDNode>(Ops[0]);
+    ConstantSDNode *RHS = dyn_cast<ConstantSDNode>(Ops[1]);
+    if (LHS && RHS) {
+      unsigned Width = VTList.VTs[0].getScalarSizeInBits();
+      unsigned OutWidth = Width * 2;
+      APInt Val = LHS->getAPIntValue();
+      APInt Mul = RHS->getAPIntValue();
+      if (Opcode == ISD::SMUL_LOHI) {
+        Val = Val.sext(OutWidth);
+        Mul = Mul.sext(OutWidth);
+      } else {
+        Val = Val.zext(OutWidth);
+        Mul = Mul.zext(OutWidth);
+      }
+      Val *= Mul;
+
+      SDValue Hi = getConstant(Val.extractBits(32, 32), DL, VTList.VTs[0]);
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RKSimon wrote:

You need to generalize the extractBits args

https://github.com/llvm/llvm-project/pull/69437