Not certain what you mean by "virtual method cache", the Ruby VM has multiple layers of method cache (or call cache) but I've never heard any of them referred t

Not certain what you mean by "virtual method cache", the Ruby VM has multiple layers of method cache (or call cache) but I've never heard any of them referred to as the "virtual method cache". For the inline call caches in the interpreter loop, they are monomorphic, so if you call the same codepath with the decorator and the actual object, they will indeed flip flop. The second layer of cache is class based, so after the inline cache is defeated you will end up doing a single hash-table lookup on the class. As for YJIT, IIRC it does handle polymorphic call caches, so it won't mind such situation at all unless you have more than a handful of different implementations of that method being called at a given callsite.