[llvm] [SelectionDAG] Let ComputeKnownSignBits handle (shl (ext X), C) (PR #97695)

[Simon Pilgrim via llvm-commits]

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@@ -4615,12 +4615,30 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
       Tmp = std::min<uint64_t>(Tmp + *ShAmt, VTBits);
     return Tmp;
   case ISD::SHL:
-    if (std::optional<uint64_t> ShAmt =
+    if (std::optional<uint64_t> MaxShAmt =
             getValidMaximumShiftAmount(Op, DemandedElts, Depth + 1)) {
+      if (Op.getOperand(0).getOpcode() == ISD::ANY_EXTEND ||
+          Op.getOperand(0).getOpcode() == ISD::ZERO_EXTEND ||
+          Op.getOperand(0).getOpcode() == ISD::SIGN_EXTEND)
+        if (std::optional<uint64_t> MinShAmt =
+                getValidMinimumShiftAmount(Op, DemandedElts, Depth + 1)) {
+          SDValue Src = Op.getOperand(0);
+          EVT SrcVT = Src.getValueType();
+          SDValue ExtendedOp = Op.getOperand(0).getOperand(0);
+          EVT ExtendedOpVT = ExtendedOp.getValueType();
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RKSimon wrote:

(pedantic) ExtendedOp isn't extended - it's the pre-extended source value

https://github.com/llvm/llvm-project/pull/97695