The AmigaOS ABI was really designed for the m68k, it leverages registers as much as possible. Pass variables in data registers, pointers in address registers. A4 for local data, A6 for the library pointer, results in D0.

That said, most library routines would then store the register contents on the stack and restore on return so not sure it's more efficient for memory access etc.

The m68k is the most CISC of ISAs I've seen, you have plentiful addressing modes and register orthogonality. It had adressing modes to do auto postincrements/predecrements, indirect indexed accesses and all sorts of fun.