> Apple changed some things that impact PyPy, like the register uses and ffi calling conventions.

I thought everyone who used 64-bit ARM used ARM's AAPCS64 (https://github.com/ARM-software/abi-aa/blob/master/aapcs64/a...), so the register usage and FFI calling convention should be the same as on Linux and Windows. What did Apple do differently that would affect the PyPy JIT?

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)