[llvm] [SelectionDAG] Detect impossible conditions using known bits analysis (PR #150715)

[Simon Pilgrim via llvm-commits]

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@@ -13529,6 +13529,78 @@ SDValue DAGCombiner::visitSETCC(SDNode *N) {
   SDValue N0 = N->getOperand(0), N1 = N->getOperand(1);
   SDLoc DL(N);
 
+  // Detect impossible conditions using known bits analysis.
+  if (ConstantSDNode *N1C = dyn_cast<ConstantSDNode>(N1)) {
+    const APInt &C1 = N1C->getAPIntValue();
+    KnownBits KnownRHS = KnownBits::makeConstant(C1);
+
+    // Bail out early if RHS is unknown (shouldn't happen for constants)
+    if (KnownRHS.isUnknown())
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RKSimon wrote:

How can KnownBits::makeConstant ever create an unknown? At the very best this should just be an assert but really I don't think it needs to be here at all.

https://github.com/llvm/llvm-project/pull/150715