[llvm] [InstSimplify] Add constant folding support for `ucmp`/`scmp` intrinsics (PR #93730)

[Nikita Popov via llvm-commits]

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@@ -2764,6 +2766,33 @@ static Constant *ConstantFoldIntrinsicCall2(Intrinsic::ID IntrinsicID, Type *Ty,
                   ? *C0
                   : *C1);
 
+    case Intrinsic::scmp:
+    case Intrinsic::ucmp:
+      if (isa<PoisonValue>(Operands[0]) || isa<PoisonValue>(Operands[1]))
+        return PoisonValue::get(Ty);
+
+      if (!C0 || !C1)
+        return UndefValue::get(Ty);
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nikic wrote:

This is incorrect. This would mean that the return value can be *any* value, while only -1, 1 and 0 are legal. You can return one of them instead (0 would be the preferred choice).

https://github.com/llvm/llvm-project/pull/93730