Re: PING^1 [PATCH] DWARF: Relax SUPPORTS_STACK_ALIGNMENT with !crtl->stack_realign_tried

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Wed, Aug 29, 2018 at 2:17 PM, Jason Merrill <jason@redhat.com> wrote:
> On Wed, Aug 29, 2018 at 3:38 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Wed, Aug 29, 2018 at 12:32 PM, Jason Merrill <jason@redhat.com> wrote:
>>> On Wed, Aug 29, 2018 at 2:59 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>> On Wed, Aug 29, 2018 at 11:33 AM, Jason Merrill <jason@redhat.com> wrote:
>>>>> On Wed, Aug 29, 2018 at 8:47 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>> On Wed, Aug 8, 2018 at 6:32 AM, H.J. Lu <hjl.tools@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.