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 3:37 PM, Jason Merrill <jason@redhat.com> wrote:
> On Wed, Aug 29, 2018 at 5:43 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> 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?
>
> That change (by you, in r138335) looks to have been made to allow
> eliminating to SP even when frame_pointer_needed, which is the
> opposite of the current situation.  And which could be expressed as

r138335 allowed arg_pointer_rtx to be eliminated by either FP or SP,
but only when dynamic stack alignment is supported.  In this case,
arg_pointer_rtx is eliminated by FP even when frame_pointer_needed
is false and there is no dynamic stack alignment at all.

> gcc_assert (elim == stack_pointer_rtx || (frame_pointer_needed && elim
> == hard_frame_pointer_rtx));
>
> so as not to allow eliminating to an uninitialized FP.
>

FP isn't uninitialized.  It is initialized the same way as in the case of
SUPPORTS_STACK_ALIGNMENT is true.

-- 
H.J.