Re: [PATCH] x86: Replace hard frame pointer with stack pointer - UNITS_PER_WORD

[Michael Matz <matz at suse dot de>]

Hi,

On Mon, 3 Sep 2018, H.J. Lu wrote:

> > So, what's the testcase testing then?  Before the patch it doesn't ICE,
> > after the patch it doesn't ICE.  What should I look out for so I can see
> > that what the testcase is producing without the patch is wrong?
> 
> Before the patch, debug info is wrong since it uses hard frame pointer
> which isn't set up for the function.  You can do "readelf -w" on .o file to
> verify the debug info.

Yeah, that's what I thought as well, but it's correct:

% ./gcc/cc1plus -quiet -O2 -g -fno-omit-frame-pointer -fvar-tracking x.cc
% gcc -c x.s
% readelf -wfi x.o
...
 <1><8a>: Abbrev Number: 9 (DW_TAG_subprogram)
    <8b>   DW_AT_specification: <0x3a>
    <8f>   DW_AT_decl_line   : 6
    <90>   DW_AT_decl_column : 5
    <91>   DW_AT_object_pointer: <0xa7>
    <95>   DW_AT_low_pc      : 0x0
    <9d>   DW_AT_high_pc     : 0x3
    <a5>   DW_AT_frame_base  : 1 byte block: 9c         (DW_OP_call_frame_cfa)
    <a7>   DW_AT_GNU_all_call_sites: 1
...
 <2><fe>: Abbrev Number: 11 (DW_TAG_formal_parameter)
    <ff>   DW_AT_name        : d
    <101>   DW_AT_decl_file   : 1
    <102>   DW_AT_decl_line   : 6
    <103>   DW_AT_decl_column : 63
    <104>   DW_AT_type        : <0x78>
    <108>   DW_AT_location    : 2 byte block: 91 8      (DW_OP_fbreg: 8)
...
  DW_CFA_def_cfa: r7 (rsp) ofs 8
  DW_CFA_offset: r16 (rip) at cfa-8
  DW_CFA_nop
  DW_CFA_nop
...

So, argument 'd' is supposed to be at DW_AT_frame_base + 8, which is 
%rsp+8+8, aka %rsp+16, which is correct given that it's the eigth argument 
(including the implicit this parameter).

So, can you actually show here what's broken before patch?

> > You talking about this, right:
> >
> >   /* We only use "frame base" when we're sure we're talking about the
> >      post-prologue local stack frame.  We do this by *not* running
> >      register elimination until this point, and recognizing the special
> >      argument pointer and soft frame pointer rtx's.  */
> >   if (reg == arg_pointer_rtx || reg == frame_pointer_rtx)
> >     {
> >       rtx elim = (ira_use_lra_p
> >                   ? lra_eliminate_regs (reg, VOIDmode, NULL_RTX)
> >                   : eliminate_regs (reg, VOIDmode, NULL_RTX));
> >
> >       if (elim != reg)
> >         {
> >          ...
> >
> > So, why would eliminate_regs return hard_frame_pointer_rtx if no frame
> > pointer is desired?
> 
> Frame pointer was skipped at the last minute in 
> x86_finalize_stack_frame_flags. But eliminate_regs uses the info which 
> was computed when frame pointer was available.

Let's assume something needs fixing (though I can't reproduce what right 
now) then I think changing frame_pointer_needed somehow needs to affect 
calls to {lra_,}eliminate_regs that come afterwards (by e.g. recalculating 
its info).  Everything else is just asking for hacks upon hacks.


Ciao,
Michael.