It's "required" by Apple's ABI that the frame pointer always point to a valid stack frame; just the same as on armv7. It's for debugging and ensuring backtraces

It's "required" by Apple's ABI that the frame pointer always point to a valid stack frame; just the same as on armv7. It's for debugging and ensuring backtraces are always valid. Not modifying the current frame pointer complies with this, which is what -fomit-leaf-frame-pointer does. But it's an ABI violation to use it as a general-purpose register. It sounds like you're partially confusing this with r9, which is a completely different story - r9 was globally reserved by the system until iOS 3.0, then allowed. This equivalent in arm64 is x18, which again is globally reserved by the system.