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Re: [PATCH,rs6000] fix interrupt safety issue on E500 targets
On 10/3/07, Nathan Froyd <firstname.lastname@example.org> wrote:
> Any objection to simply calling rs6000_emit_stack_tie unconditionally
> before the stack adjustment, then? (Patch below--I don't know what
> AIX/Darwin requires here, but I can't imagine it would hurt performance
> that much.)
Inside rs6000_stack_info, we have the following comment:
/* Determine if we need to allocate any stack frame:
For AIX we need to push the stack if a frame pointer is needed
(because the stack might be dynamically adjusted), if we are
debugging, if we make calls, or if the sum of fp_save, gp_save,
and local variables are more than the space needed to save all
non-volatile registers: 32-bit: 18*8 + 19*4 = 220 or 64-bit: 18*8
+ 18*8 = 288 (GPR13 reserved).
For V.4 we don't have the stack cushion that AIX uses, but assume
that the debugger can handle stackless frames. */
So only SVS V.4 ABI have this requirement which means PPC64-linux,
darwin or AIX have a red zone. And yes this can worse code for OS's
with a red zone as the main reason why the movement is happening is so
the processor can handle the loads better.