objc_msgSend would actually be impossible to implement in C.
Its role is to look up the function pointer that implements a method, then call that function pointer with the arguments passed to objc_msgSend, returning the result of the implementation.
Implementing objc_msgSend in asm guarantees that the argument & return registers aren't touched by the method lookup. Similarly to how ffi libraries and implementations of setjmp/longjmp need to be implemented in asm, it operates at a lower level than C's stack & function abstractions.
To elaborate on this, a C implementation of objc_msgSend would look something like this:
id objc_msgSend(id self, SEL _cmd, args...) {
IMP methodImplementation = ...; // look up the method however
return methodImplementation(self, _cmd, args...);
}
But there is no facility in C that lets you take arbitrary additional arguments and then pass them all to another function unchanged.
(In theory you could accomplish this using variadic functions. But then every method would have to take a va_list for its parameters instead of just taking a regular parameter list, breaking the idea that ObjC methods are just C functions with two implicit parameters. It would also be substantially slower.)
Its role is to look up the function pointer that implements a method, then call that function pointer with the arguments passed to objc_msgSend, returning the result of the implementation.
Implementing objc_msgSend in asm guarantees that the argument & return registers aren't touched by the method lookup. Similarly to how ffi libraries and implementations of setjmp/longjmp need to be implemented in asm, it operates at a lower level than C's stack & function abstractions.