[saagarjha]

There are already two different ABIs for Apple silicon, although one (arm64e) is mostly reserved for Apple’s use at the moment. Apple could choose to finalize the ABI for that with x18 being available to applications, although this is quite unlikely.

[olliej]

No there aren’t and you know that.

Arm64e has the exact same abi as arm64, but in arm64 mode the pac instructions are no ops.

Otherwise as the parent says apple would need multiple copies of the system libraries.

[saagarjha]

Yeah, I understand that they are currently the same, I'm just saying that as Apple hasn't locked down the ABI yet so they could in the future choose to ship a new copy of all the libraries (as they do with x86_64h) for arm64e (well, split out arm64 slices) and for that ABI allow the use of x18. It would be pretty dumb, but it's possible.