Uros Bizjak <ubiz...@gmail.com> writes:
> The target that I would like to support has 8-bit registers, so for
> any sane compilation, stack pointer, frame pointer and hard frame
> pointer all need to be constructed from at least two registers, to
> form 16-bit register pair {rA, rB}.
>
> The stack pointer is defined as a fixed register, so it is not
> reachable to register allocator.
> The frame pointer is actually a fake register (with a regnum above
> last hard register regnum) that eliminates either to SP or HW_FP, so
> RA ignores it.
>
> The problem is with HW_FP. Looking at the register allocator, it seems
> that only HARD_FRAME_POINTER_REGNO is marked as live in the allocator,
> although hard_frame_pointer_rtx shows itself as a HImode register with
> H_F_P_R register number. Since H_F_P_R+1 is marked "empty", RA puts
> various QImode values there, clobbering high part of hard frame
> pointer.
>
> There is nothing in the documentation that would describe this
> limitation, and I found no solution browsing the web archives. Also, I
> believe that AVR should have the same problems.
>
> Is this a known limitation in the RA? If it is, is there a solution
> for multi-reg [HARD]_FRAME_POINTER?
I think the AVR does it by treating the soft frame pointer as a pair
of registers, and eliminating both registers, one to
HARD_FRAME_POINTER_REGNO and the other to HARD_FRAME_POINTER_REGNO+1.
Make the elimination conditions the same, and the compiler should
reliably eliminate either both or neither.
Ian
