x16, x17, and x18 are all different env pointers not really available for general use per se in the main ABI. x16 and x17 are used for plt linkage there, x18 i

x16, x17, and x18 are all different env pointers not really available for general use per se in the main ABI. x16 and x17 are used for plt linkage there, x18 is reserved for the system to make use of as it sees fit, at it's leisure. x18's use there seems to have survived into Apple's slightly modified ABI.