[llvm] [Inline][Cloning] Drop incompatible attributes from `NewFunc` before `instSimplify` (PR #90489)

[Nikita Popov via llvm-commits]

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@@ -823,54 +820,48 @@ void llvm::CloneAndPruneIntoFromInst(Function *NewFunc, const Function *OldFunc,
     }
   }
 
-  // Make a second pass over the PHINodes now that all of them have been
-  // remapped into the new function, simplifying the PHINode and performing any
-  // recursive simplifications exposed. This will transparently update the
-  // WeakTrackingVH in the VMap. Notably, we rely on that so that if we coalesce
-  // two PHINodes, the iteration over the old PHIs remains valid, and the
-  // mapping will just map us to the new node (which may not even be a PHI
-  // node).
+  // Drop all incompatible return attributes that cannot be applied to NewFunc
+  // during cloning, so as to allow instruction simplification to reason on the
+  // old state of the function. The original attributes are restored later.
+  AttributeMask IncompatibleAttrs =
+      AttributeFuncs::typeIncompatible(OldFunc->getReturnType());
+  AttrBuilder RetAttrs(NewFunc->getContext(),
+                       NewFunc->getAttributes().getRetAttrs());
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nikic wrote:

This is a bit of a roundabout way to restore the attributes. It would be better to do `AttributeList Attrs = NewFunc->getAttributes();` here and then `NewFunc->setAttributes(Attrs)` below, instead of going through AttrBuilder etc.

https://github.com/llvm/llvm-project/pull/90489