[llvm] [InstCombine] Fold comparisons of llvm.usub.sat result with its LHS (PR #214108)

[Haram Jeong via llvm-commits]

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@@ -7811,6 +7740,24 @@ Instruction *InstCombinerImpl::foldICmpCommutative(CmpPredicate Pred,
   }
 
   const SimplifyQuery Q = SQ.getWithInstruction(&CxtI);
+
+  {
+    Value *Y;
+    // For Y != 0:
+    // usub.sat(X, Y) == X  --> X == 0
+    // usub.sat(X, Y) != X  --> X != 0
+    // usub.sat(X, Y) <  X  --> X != 0
+    if (match(Op0, m_Intrinsic<Intrinsic::usub_sat>(m_Specific(Op1),
+                                                    m_Value(Y))) &&
+        (CmpInst::isEquality(Pred) || Pred == ICmpInst::ICMP_ULT) &&
+        isKnownNonZero(Y, Q)) {
+      ICmpInst::Predicate NewPred =
+          CmpInst::isEquality(Pred) ? Pred.dropSameSign() : ICmpInst::ICMP_NE;
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haramj wrote:

Added a samesign equality test to cover this.

https://github.com/llvm/llvm-project/pull/214108