[llvm] [SelectionDAG] Add very basic computeKnownBits support for ISD::CLMUL. (PR #177445)

[Craig Topper via llvm-commits]

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@@ -3913,6 +3913,16 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
     Known.Zero.setBitsFrom(1);
     break;
   }
+  case ISD::CLMUL: {
+    Known = computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
+    Known2 = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
+    // TODO: Trailing zeros, known LSBs.
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topperc wrote:

How? Don't we need to be conservative about unknown bits?

https://github.com/llvm/llvm-project/pull/177445