On Wed, Aug 29, 2018 at 3:37 PM, Jason Merrill <ja...@redhat.com> wrote: > On Wed, Aug 29, 2018 at 5:43 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >> 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? > > 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.
