Efficient passing of structs with movable fields

[ibukanov]

Consider the following struct that simply wraps an array pointer:

struct Latin1String {
    data: ~[u8]
}

Due to the move semantic one cannot use Latin1String directly as argument type. However, using &Latin1String for arguments currently implies double-indirection when accessing characters even if semantically there would be no difference if internally &Latin1String is implemented as a copy via registers all fields of Latin1String.

It would be nice if Rust implement such optimization for &T arguments effectively turning that into "the most efficient way to pass read-only parameters to a function" .

Similar problem exists when a generic function takes &T argument and is instantiated with int and similar built-in types.

Note that C++ has the same problem when (const T&) leads to inefficient passing of 1-3 word-sized T like int or double forcing to use workarounds like inlined functions in headers, explicit template specialization for primitive types etc.

[thestinger]

LLVM has the argpromotion pass for this and Rust enables it at --opt-level=3. For a dynamically linked library, I think the indirection for the symbol thunk stuff will be significantly more expensive.

[ibukanov]

Nice, so &T as argument is already the most efficient way to pass arguments. As for dynamic linking I suppose requiring that &T is always passed as a copy for small-sized T can lead to binary incompatibility as it would prevent changing the size of T while staying compatible with callers. So it is not an option there.

[thestinger]

I don't think this is possible, because the address of references can be used in safe code. A proposal to change the semantics or introduce a new feature would be necessary, not just an implementation change.