abi-aa

Here is ARM's explanation in the AAPCS64 (ARM 64-bit Architecture Procedure Call Standard): > Software developers creating platform-independent code are advised to avoid using r18 if at all possible. Most compilers provide a mechanism to prevent specific registers from being used for general allocation; portable hand-coded assembler should avoid it entirely. It should not be assumed that treating the register as callee-saved will be sufficient to satisfy the requirements of the platform. Virtualization code must, of course, treat the register as they would any other resource provided to the virtual machine. From: https://github.com/ARM-software/abi-aa/blob/2bcab1e3b22d5517...

You can also pass structs of floating point or SIMD types by register. They have to be 2-4 elements and all of the same type. These are called HFA (Homogeneous Floating-point Aggregate) and HVA (Homogeneous Short-Vector Aggregate). Source: the Procedure Call Standard for the Arm 64-bit Architecture https://github.com/ARM-software/abi-aa/blob/2021Q1/aapcs64/a...

My guess is that it's some effort to maintain some kind of compatibility with ARM32. Back when I used to write ARM asm, it was the ARM 26/32 days, and it was rXX for all the register named in the APCS. It doesn't seem to be as confusing as the x86 situation anyway: none of the registers are _really_ special purpose on ARM. Here's a link to the details in the ARM64 APCS: https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aap...

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/aap...