There are 31 GPRs. The zero register shares and encoding with SP, and is it’s own separate thing. From the 31 GPRs, x30 is the link register and x29 is the fr

There are 31 GPRs. The zero register shares and encoding with SP, and is it’s own separate thing. From the 31 GPRs, x30 is the link register and x29 is the frame pointer. x30 can be used for other purposes within a routine, but x29 must always hold a valid frame record in the iOS ABI. Additionally, iOS reserves x18 (“The platform register”) for all use. So there are really 28 GPRs, or 29 if you include x30/lr, which is something of a hybrid.