Comment by flohofwoe
2 years ago
> Well, not even that (struct returns ... nope).
C compilers actually pack small struct return values into registers:
https://godbolt.org/z/51q5se86s
It's just limited that on x86-64, GCC and Clang use up to two registers while MSVC only uses one.
Also, IMHO there is no such thing as a "C calling convention", there are many different calling conventions that are defined by the various runtime environments (usually the combination of CPU architecture and operating system). C compilers just must adhere to those CPU+OS calling conventions like any other language that wants to interact directly with the operating system.
IMHO the whole performance angle is a bit overblown though, for 'high frequency functions' the compiler should inline the function body anyway. And for situations where that's not possible (e.g. calling into DLLs), the DLL should expose an API that doesn't require such 'high frequency functions' in the first place.
> Also, IMHO there is no such thing as a "C calling convention", there are many different calling conventions [ ... ]
I did not say that. I said "C calling conventions" (plural). Rather aware of the fact that the devil is in the detail here ... heck, if you want it all, back in the bad old days, even the same compiler supported/used multiple ("fastcall" & Co, or on Win 3.x "pascal" for system interfaces, or the various ARM ABIs, ...).
Clang still has some alternative calling conventions via __attribute__((X)) for individual functions with a bunch of options[0], though none just extend the set of arguments passed via GPRs (closest seems to be preserve_none with 12 arguments passed by register, but it also unconditionally gets rid of all callee-saved registers; preserve_most is nice for rarely-taken paths, though until clang-17 it was broken on functions which returned things).
[0]: https://clang.llvm.org/docs/AttributeReference.html#calling-...