[llvm] [Instcombine]: Folds`llvm.ucmp` and `llvm.scmp` (PR #168505)

[Nikita Popov via llvm-commits]

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@@ -4461,6 +4461,47 @@ static Value *simplifyWithOpsReplaced(Value *V,
         return Absorber;
     }
 
+    if (auto *CI = dyn_cast<CallInst>(I)) {
+      Function *F = CI->getCalledFunction();
+
+      // `x == y ? 0 : ucmp(x, y)` where under the replacement y -> x, `ucmp(x,
+      // x)` becomes `0`.
+      if (F && F->isIntrinsic() &&
+          (F->getIntrinsicID() == Intrinsic::scmp ||
+           F->getIntrinsicID() == Intrinsic::ucmp)) {
+        // If the call contains (an invalid) range attribute then a replacement
+        // might produce an unexpected poison value.
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nikic wrote:

I'd rather not check the specifics here. Can we just use `hasPoisonGeneratingAnnotations()` here and add to DropFlags (see above)? That way the attribute is just going to be dropped if it's present.

https://github.com/llvm/llvm-project/pull/168505