On 22/05/13 21:19, Richard Henderson wrote:
On 05/22/2013 12:43 AM, Jakub Jelinek wrote:
Changing frame grows upward into frame grows downward shouldn't be that
hard, see e.g. rs6000 port, where
#define FRAME_GROWS_DOWNWARD (flag_stack_protect != 0 || flag_asan != 0)
and grep the port where it uses FRAME_GROWS_DOWNWARD.
Basically you need to tweak initial elimination offset computation for it,
and that might be it, or perhaps one or two extra spots.
FWIW, I would actually recommend against conditionalizing FRAME_GROWS_DOWNWARD
for a new port. Just make it _always_ grow down and save yourself the
additional code bloat in the backend.
Doing that would add significantly to the cost of setting up the frame.
FRAME_GROWS_DOWNWARD
Define this macro to nonzero value if the addresses of local variable
slots are at negative offsets from the frame pointer.
The optimal frame establishment sequence (for small frames, less than
128 bytes) is generated by doing
stp fp, lr, [sp, #-frame-size]!
mov fp, sp
Job done, along with stack allocation. Larger frames can be generated
with an initial subtraction from SP to allocate the entire frame. Total
cost of maintaining the frame structure comes out to about 1.5
instructions in the prologue and 0.5 instructions in the epilogue (50%
of functions end up with one additional store and one additional load
instruction).
Any other sequence requires modifying the stack pointer, then saving the
registers (potentially having to generate a further temporary value for
the large offset from SP to them), then adding another value back into FP.
Furthermore, load/store immediate operations have a significantly larger
positive offset from the base than negative; so you really hurt
performance by having the frame record above the local variables for
large functions.
R.
