On Wed, Aug 29, 2018 at 2:17 PM, Jason Merrill <ja...@redhat.com> wrote:
> On Wed, Aug 29, 2018 at 3:38 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
>> On Wed, Aug 29, 2018 at 12:32 PM, Jason Merrill <ja...@redhat.com> wrote:
>>> On Wed, Aug 29, 2018 at 2:59 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
>>>> 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.
>>>
>>> Right, but why are we trying to do that when frame_pointer_needed is false?
>>
>> 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.
>>
>> we may skip frame pointer when there is no stack access even if
>> -fno-omit-frame-pointer is used. Here argp is only referenced
>> in debug info, not in the function body. In this case, what else
>> can argp be eliminated to in debug info?
>
> SP or CFA?
>
> If the function body doesn't set the hard frame pointer register, then
> we can't rely on it having a useful value, so we shouldn't refer to it
> in debug info.
There are:
(SUPPORTS_STACK_ALIGNMENT
&& (elim == hard_frame_pointer_rtx
|| elim == stack_pointer_rtx))
When there is no stack realignment, SUPPORTS_STACK_ALIGNMENT
isn't relevant. Why can't elim be hard_frame_pointer_rtx?
--
H.J.
