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. Jason
