Using and Porting GNU CC

16.5.2: Order of Allocation of Registers

REG_ALLOC_ORDER

If defined, an initializer for a vector of integers, containing the numbers of hard registers in the order in which GNU CC should prefer to use them (from most preferred to least).

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 machines, define REG_ALLOC_ORDER to be an initializer that lists the highest numbered allocatable register first.

ORDER_REGS_FOR_LOCAL_ALLOC

A C statement (sans semicolon) to choose the order in which to allocate hard registers for pseudo-registers local to a basic block.

Store the desired register order in the array reg_alloc_order. 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.