Does Swift use message dispatch for methods?

Question

I'm sure my terminology is off, so here's an example:

• C/C++ has methods and virtual methods. Both have the opportunity to be inlined at compile time.
• C#'s CIL has call and callvirt instructions (which closely resemble C++ methods and virtual methods). Although almost all method calls in C# become callvirt (due to langauge snafu) the JIT compiler is able to optimize most back to call instructions and then (if worthwhile) also inline them.
• Objective-C method calls are done very differently (and inefficiently); a message object is passed via objc_msgsend every time you call a method, it's a form of dynamic dispatch, and can never be inlined.

Reading up on the language specification for functions for Swift, I don't know if Swift is using the same messaging system as Objective-C or something different.

Answer 1

Sometimes yes, sometimes no. If you have pure swift code, and do not expose your classes/protocols to Objective-C with the @objc decoration, it appears that pure-swift method calls are not dispatched via objc_msgSend, however in other cases they are. If the protocol your swift object adopts is declared in Objective-C, or if the swift protocol is decorated with @objc, then method calls to protocol methods, even from swift objects to other swift objects, are dispatched via objc_msgSend.

The documentation is currently a little thin; I'm sure there are other nuances... but empirically speaking (i.e. I've tried it out) some swift method calls go through objc_msgSend and others don't. I think getting the best performance will be dependent on keeping your code as much pure-swift as possible and crossing the Obj-C/swift boundary as little as possible, and through bottleneck interfaces/protocols, so as to limit the number of swift calls that have to be dispatched dynamically.

I'm sure more detailed docs will emerge sooner or later.