Using and Porting GNU CC
If this macro is not defined, registers are used lowest numbered first (all else being equal).
One use of this macro is on machines where the highest numbered
registers must always be saved and the save-multiple-registers
instruction supports only sequences of consecutive registers. On such
REG_ALLOC_ORDER to be an initializer that lists
the highest numbered allocatable register first.
Store the desired register order in the array
Element 0 should be the register to allocate first; element 1, the next
register; and so on.
The macro body should not assume anything about the contents of
reg_alloc_order before execution of the macro.
On most machines, it is not necessary to define this macro.