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


On Mon, Sep 3, 2018 at 9:44 AM, Michael Matz <matz@suse.de> wrote:
> 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).

Can we use DW_AT_frame_base when the frame pointer isn't available?
If yes,

         gcc_assert ((SUPPORTS_STACK_ALIGNMENT
                       && (elim == hard_frame_pointer_rtx
                           || elim == stack_pointer_rtx))
                      || elim == (frame_pointer_needed
                                  ? hard_frame_pointer_rtx
                                  : stack_pointer_rtx));

should be changed to

          gcc_assert (elim == hard_frame_pointer_rtx
                      || elim == stack_pointer_rtx);

This will also fix:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86593

> 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.
>

The only reference to hard frame pointer is in debug info.  Is recomputing
eliminate info really necessary?

-- 
H.J.


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