That's nifty, but I don't think it quite gets you there:
"It is not always simple to compute the proper value for size. The value is used by __builtin_apply to compute the amount of data that should be pushed on the stack and copied from the incoming argument area."
If there was a variant that didn't need this size argument (would probably require being a tail call) then that would do it.
Sort of. If you did it that way, then the message dispatch code would get compiled into the calling code rather than being a separate function. That would work, but it would greatly increase code size, and would also mean Apple couldn't make incompatible changes to how messaging works without breaking old code. This last part is fairly important: Apple does make such changes, and the fact that objc_msgSend is part of the system means that old programs just keep on working. They've introduced non-pointer isas and tagged pointers this way.
That requires knowing types at compile time, and expands to just the code with the specific types used in the instantiation. That doesn't work with a virtual method dispatch kind of scenario where the types in the target method are not known to the runtime.
"It is not always simple to compute the proper value for size. The value is used by __builtin_apply to compute the amount of data that should be pushed on the stack and copied from the incoming argument area."
If there was a variant that didn't need this size argument (would probably require being a tail call) then that would do it.