This might be true in the abstract but it's not true of actual compilers dealing with real world calling conventions. Absent inlining or whole program optimization, calling conventions across translation units don't leave much room for flexibility.
The semantics of pass by const reference are also not exactly the same as pass by value in C++. The compiler can't in general assume a const reference doesn't alias other arguments or global variables and so has to be more conservative with certain optimizations than with pass by value.
The semantics of pass by const reference are also not exactly the same as pass by value in C++. The compiler can't in general assume a const reference doesn't alias other arguments or global variables and so has to be more conservative with certain optimizations than with pass by value.