[llvm] InstSimplify: teach simplifyICmpWithConstant about samesign (PR #125899)

[Ramkumar Ramachandra via llvm-commits]

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@@ -3012,7 +3012,7 @@ static Value *simplifyICmpWithConstant(CmpPredicate Pred, Value *LHS,
   }
 
   // Rule out tautological comparisons (eg., ult 0 or uge 0).
-  ConstantRange RHS_CR = ConstantRange::makeExactICmpRegion(Pred, *C);
+  ConstantRange RHS_CR = ConstantRange::makeAllowedICmpRegion(Pred, *C);
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artagnon wrote:

Thought about this for a bit, and you're right: it is _semantically_ incorrect. There is no bug because we're passing a singleton value to makeAllowedICmpRegion, but I agree that the situation is bad. I'm not sure how to get out of this jam of fundamentally asymmetric samesign ranges: maybe the best thing we can do is to not touch existing functions and create a makeAsymmetricICmpRegion that accepts a CmpPredicate and ConstantRange, and document that it should not be called with a plain predicate? Not sure what else we can possibly do.

https://github.com/llvm/llvm-project/pull/125899