objc_msg_lookup is very emphatically not objc_msgSend.
Lookup operates differently from the nominally tail-calling msgSend in that it cannot support transparent forwarding (where the receiver is switched to another object before dispatch.) Since objc_msgSend is given control over the final call, the destination can be swapped if necessary.
It's not possible[1] to implement objc_msgSend in plain (portable) C, as it needs to pass on an unknowable number of arguments without damaging the ones passed in registers, the stack frame, or the optional hidden return value pointer. Any methods attempting to do so using, say, naked functions and the method's type encoding will be more fragile than a perfect argument forwarding stub written in assembly.
GCC's C is emphatically not "plain" (ANSI/ISO) C, but that does not mean it isn't "portable" C, given that GCC itself targets many platforms, and other compilers (clang, at least) do attempt to support GCC's dialect.
I couldn't find documentation that supports Clang offering the __builtin_apply intrinsics, but their availability would make a C objc_msgSend somewhat more palatable.
Ah. I don't mean to be too argumentative. You've helped me to understand why objc_msgSend() could not be done in standard C, and I thank you for that :)
However, in the case of GCC's ObjC implementation, I don't see why it couldn't be implemented in GCC's dialect, thus obviating any need for assembly.
Pardon me while I reflect on (and lament) the fact that we're still stuck using such crude and ancient tools for systems programming...
FWIW, it appears that Mulle-objc gets around this by actually changing the calling convention for Obj-C methods, so the IMP takes a single parameter (beyond self and _cmd) which is a struct that contains all of the method parameters. This way the compiler can just synthesize the struct for you, and then objc_msgSend() itself can be written in C because it just needs to pass the pointer to the struct along to the IMP rather than messing about with forwarding arbitrary arguments.
Lookup operates differently from the nominally tail-calling msgSend in that it cannot support transparent forwarding (where the receiver is switched to another object before dispatch.) Since objc_msgSend is given control over the final call, the destination can be swapped if necessary.
It's not possible[1] to implement objc_msgSend in plain (portable) C, as it needs to pass on an unknowable number of arguments without damaging the ones passed in registers, the stack frame, or the optional hidden return value pointer. Any methods attempting to do so using, say, naked functions and the method's type encoding will be more fragile than a perfect argument forwarding stub written in assembly.
[1]: __builtin_apply_args, __builtin_apply and __builtin_return (https://gcc.gnu.org/onlinedocs/gcc-5.1.0/gcc/Constructing-Ca...) notwithstanding.