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

[Jason Merrill <jason at redhat dot com>]

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?

Jason