For more context, look at the standard ARM ABI spec. Apple’s ABI diverges from the standard one in some places, IIRC, but in this case it’s complying with it:

For more context, look at the standard ARM ABI spec. Apple’s ABI diverges from the standard one in some places, IIRC, but in this case it’s complying with it: the spec says that X18 (which it calls r18) can be reserved by platforms for platform-specific use. The spec also shows how the registers are divided into several categories, which helps explain the choice of X18: https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#general-purpose-registers https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aap...