[_red]

Disclaimer: I may be completely wrong, but I don't think OP is saying that while writing normal ObjC that you need to write the message passing in assembly. Instead, that you could recreate ObjC in regular C but you would have to use assembly to construct the message passing portion of the language.

[slrz]

Sure, but what's the property of ObjC-style message passing that makes it hard to implement in C?

Like when you implement coroutines/threads in your language runtime, you're going to implement the context switching part in assembly because you just don't have access to the relevant information (registers/stack/...) from portable C (aside from using setjmp/longjmp maybe).

[favorited]

It's really the functionality of a method taking an unspecified number of arguments and jumping directly to the implementation's function pointer. Though, now that you mention it, there's really no reason you need to do that.

You could have your own calling convention where arguments aren't passed in registers or on the stack, but are boxed into some container. That way every method's implementation function could have the same signature, taking a pointer to the receiver ("self"), the method name, a box of arguments, and a place to pass the return value back out:

    typedef void (*IMP)(id, SEL, struct context *, void **); // arguments are `self`, the method name (`const char *`), a pointer to a structure containing the arguments, and a place to put your return value

I know of at least one Objective-C runtime implementation that does this. It won't be nearly as efficient – you have to pack/unpack the arguments, it pushes a stack frame instead of just jumping to the implementation, etc. – but it would certainly be portable.

[exikyut]

A parent comment says it's due to variable number of arguments.

I thought it was about speed, heh.

[saagarjha]

To be honest, it's probably both. The runtime gurus at Apple do a great job hand-optimizing objc_msgSend for each platform since it's such an important factor in the overall speed of an Objective-C program.

[favorited]

It's (if not mostly, these days) about speed, but it is also to retain the notion that Objective-C methods are just C functions with 2 extra parameters, `self` and `_cmd`.

But yes – when objc_msgSend is invoked tens of thousands of times/second, you'll write it in ASM to get the performance boost anyway. When you're a compiler or runtime engineer working in a language with as few opportunities for static optimization as Objective-C, you bet your ass you'll squeeze every drop possible from message dispatch.