[glandium]

AFAIK, the only notable difference is with arguments spilled on the stack being packed on macOS (except variadic arguments). If all your arguments are int or larger, that means essentially no difference. (source: I worked on making Firefox work on M1. We didn't need changes to XPCOM, for instance, although theoretically, we'd need some. I don't think the JIT needed changes either. Funnily enough, I had to fix libffi for variadic arguments, and the libffi shipped in macOS is broken)

[saagarjha]

varargs in general is significantly different, I think it's even encoded as a char * in C++ name mangling.