GDExtension: PtrToArg::convert() creates unnecessary copies

[Bromeon]

Godot version

4.2-dev (branched off 75f9c97)

System information

Windows 10

Issue description

Stumbled upon this while debugging equality between StringName instances (for unrelated issue #78580).

The GDExtension function pointer for equality calls OperatorEvaluatorEqual<StringName, StringName>::ptr_evaluate().
This one checks PtrToArg<A>::convert(left) == PtrToArg<B>::convert(right).

The function PtrToArg::convert() is defined as follows (substituted m_type with StringName):

template <>                                                  
struct PtrToArg<m_type> {                                    
    _FORCE_INLINE_ static StringName convert(const void *p_ptr) {
        return *reinterpret_cast<const StringName *>(p_ptr);     
    }
}

reinterpret_cast returns an expression of type const StringName*.
Dereferencing that yields const StringName& (and not StringName).

The return type of convert() however is StringName (by value).
Unless I'm missing something, the C++ compiler cannot apply RVO here because we don't have a value, only a const-reference.

So this calls the copy-constructor StringName::StringName(const StringName &p_name) -- I verified this via debugger.

This copy is unnecessary for many operations which only have read access, like equality here. Even if StringName is cheap to copy, this still comes with refcount inc/dec + locking, when it would mostly need to compare two hashes.

Steps to reproduce

Equality-compare two StringName objects in a GDExtension binding and step in via debugger. Copy constructor of StringName is invoked.

Minimal reproduction project

N/A