On Wed, Aug 29, 2018 at 11:33 AM, Jason Merrill <ja...@redhat.com> wrote:
> On Wed, Aug 29, 2018 at 8:47 AM, H.J. Lu <hjl.to...@gmail.com> wrote:
>> On Wed, Aug 8, 2018 at 6:32 AM, H.J. Lu <hjl.to...@gmail.com> wrote:
>>> Assert for SUPPORTS_STACK_ALIGNMENT was added for dynamic stack
>>> alignment. At the time, arg_pointer_rtx would only be eliminated
>>> by either hard_frame_pointer_rtx or stack_pointer_rtx only when
>>> dynamic stack alignment is supported. With
>>>
>>> commit cd557ff63f388ad27c376d0a225e74d3594a6f9d
>>> Author: hjl <hjl@138bc75d-0d04-0410-961f-82ee72b054a4>
>>> Date: Thu Aug 10 15:29:05 2017 +0000
>>>
>>> i386: Don't use frame pointer without stack access
>>>
>>> When there is no stack access, there is no need to use frame pointer
>>> even if -fno-omit-frame-pointer is used and caller's frame pointer is
>>> unchanged.
>>>
>>> this can happen when there is no dynamic stack alignment. This patch
>>> relaxes SUPPORTS_STACK_ALIGNMENT with !crtl->stack_realign_tried to
>>> allow arg_pointer_rtx to be eliminated by either hard_frame_pointer_rtx
>>> or stack_pointer_rtx when there is no dynamic stack alignment at all.
>>>
>>> gcc/
>>>
>>> PR debug/86593
>>> * dwarf2out.c (based_loc_descr): Replace SUPPORTS_STACK_ALIGNMENT
>>> with (SUPPORTS_STACK_ALIGNMENT || !crtl->stack_realign_tried).
>>> (compute_frame_pointer_to_fb_displacement): Likewise.
>>>
>>> gcc/testsuite/
>>>
>>> PR debug/86593
>>> * g++.dg/pr86593.C: New test.
>>>
>>
>> PING:
>>
>> https://gcc.gnu.org/ml/gcc-patches/2018-08/msg00559.html
>
> It looks like crtl->stack_realign_tried is only ever set if
> SUPPORTS_STACK_ALIGNMENT, so (SUPPORTS_STACK_ALIGNMENT ||
> !crtl->stack_realign_tried) is always true.
>
> If you don't need to use the frame pointer, then frame_pointer_needed
> should be false, so the assert should already allow elimination to the
frame_pointer_needed is false:
(gdb) p elim
$1 = (rtx) 0x7fffeadd0390
(gdb) call debug_rtx (elim)
(reg/f:DI 6 bp)
(gdb) call debug_rtx (reg)
(reg/f:DI 16 argp)
(gdb) p x_rtl.frame_pointer_needed
$2 = false
(gdb)
> stack pointer. Are we trying to eliminate to the hard frame pointer
> even though we've decided we don't need it? Why?
>
In this case, we are trying to eliminate argp to the hard frame pointer.
Since SUPPORTS_STACK_ALIGNMENT is false.
(SUPPORTS_STACK_ALIGNMENT
&& (elim == hard_frame_pointer_rtx
|| elim == stack_pointer_rtx))
is false when elim == hard_frame_pointer_rtx is true.
--
H.J.
