Comment by dgrunwald

I don't know why I wrote the comment about parameters earlier -- (N)RVO is about return values. Those have a different reason for being passed behind a hidden pointer: the class type might have self-referencing pointers, so there must be an explicit move/copy constructor call whenever it changes address, to give the class an opportunity to update those pointers. This cannot work when returning in a register: the callee doesn't know the target address, and the caller doesn't the source address, so neither can call the move constructor. Thus, all ABIs must pass a pointer (or let caller+callee agree on a memory location in some other way) for types that aren't trivially copyable.