Re: [PATCH 1/2] Fix debug info for ignored decls at start of assembly

[Richard Biener <rguenther@suse.de>]

On Wed, 28 Jul 2021, Bernd Edlinger wrote:

> On 7/28/21 2:51 PM, Richard Biener wrote:
> > On Mon, 26 Jul 2021, Bernd Edlinger wrote:
> > 
> >> Ignored functions decls that are compiled at the start of
> >> the assembly have bogus line numbers until the first .file
> >> directive, as reported in PR101575.
> >>
> >> The work around for this issue is to emit a dummy .file
> >> directive when the first function is DECL_IGNORED_P, when
> >> that is not already done, mostly for -fdwarf-4.
> > 
> > I wonder if it makes sense to unconditionally announce the
> > TU with a .file directive at the beginning.  ISTR this is
> > what we now do with -gdwarf-5.
> > 
> 
> Yes, that would work, even when the file name is not guessed
> correctly.
> 
> Initially I had "<dummy>" unconditionally here, and it did
> not really hurt, except that it is visible with readelf.

I think I'd prefer that, since if we don't announce a .file
before the first assembler statement but ask gas to produce
line info it might be tempted to create line info referencing
the possibly temporary filename of the assembler file which
is undesirable from a build reproducability point.

Richard.

> > Note get_AT_string (comp_unit_die (), DW_AT_name) doesn't
> > work with LTO, you'll get <dummy> then.
> > 
> 
> Yeah, that's why I wanted to restrict that to the case where
> it's absolutely necessary.
> 
> > Is the dwarf assembler bug reported/fixed?  Can you include
> > a reference please?
> > 
> 
> I've just added a bug report, it's unlikely to be fixed IMHO:
> https://sourceware.org/bugzilla/show_bug.cgi?id=28149
> 
> I will add that to the patch description:
> 
> Ignored functions decls that are compiled at the start of
> the assembly have bogus line numbers until the first .file
> directive, as reported in PR101575.
> 
> The corresponding binutils bug report is
> https://sourceware.org/bugzilla/show_bug.cgi?id=28149
> 
> The work around for this issue is to emit a dummy .file
> directive when the first function is DECL_IGNORED_P, when
> that is not already done, mostly for -fdwarf-4.
> 
> 
> Thanks
> Bernd.
> 
> > Thanks,
> > Richard.
> > 
> >> 2021-07-24  Bernd Edlinger  <bernd.edlinger@hotmail.de>
> >>
> >> 	PR ada/101575
> >> 	* dwarf2out.c (dwarf2out_begin_prologue): Move init
> >> 	of fde->ignored_debug to dwarf2out_set_ignored_loc.
> >> 	(dwarf2out_set_ignored_loc): This is now also called
> >> 	when no .loc statement is to be generated, in that case
> >> 	we emit a dummy .file statement when needed.
> >> 	* final.c (final_start_function_1,
> >> 	final_scan_insn_1): Call debug_hooks->set_ignored_loc
> >> 	for all DECL_IGNORED_P functions.
> >> ---
> >>  gcc/dwarf2out.c | 29 +++++++++++++++++++++++++----
> >>  gcc/final.c     |  5 ++---
> >>  2 files changed, 27 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
> >> index 884f1e1..8de0d6f 100644
> >> --- a/gcc/dwarf2out.c
> >> +++ b/gcc/dwarf2out.c
> >> @@ -1115,7 +1115,6 @@ dwarf2out_begin_prologue (unsigned int line ATTRIBUTE_UNUSED,
> >>    fde->dw_fde_current_label = dup_label;
> >>    fde->in_std_section = (fnsec == text_section
> >>  			 || (cold_text_section && fnsec == cold_text_section));
> >> -  fde->ignored_debug = DECL_IGNORED_P (current_function_decl);
> >>    in_text_section_p = fnsec == text_section;
> >>  
> >>    /* We only want to output line number information for the genuine dwarf2
> >> @@ -28546,10 +28545,32 @@ dwarf2out_set_ignored_loc (unsigned int line, unsigned int column,
> >>  {
> >>    dw_fde_ref fde = cfun->fde;
> >>  
> >> -  fde->ignored_debug = false;
> >> -  set_cur_line_info_table (function_section (fde->decl));
> >> +  if (filename)
> >> +    {
> >> +      set_cur_line_info_table (function_section (fde->decl));
> >> +
> >> +      dwarf2out_source_line (line, column, filename, 0, true);
> >> +    }
> >> +  else
> >> +    {
> >> +      fde->ignored_debug = true;
> >> +
> >> +      /* Work around for PR101575: output a dummy .file directive.  */
> >> +      if (in_first_function_p
> >> +	  && debug_info_level >= DINFO_LEVEL_TERSE
> >> +	  && dwarf_debuginfo_p ()
> >> +#if defined(HAVE_AS_GDWARF_5_DEBUG_FLAG) && defined(HAVE_AS_WORKING_DWARF_N_FLAG)
> >> +	  && dwarf_version <= 4
> >> +#endif
> >> +	  && output_asm_line_debug_info ())
> >> +	{
> >> +	  const char *filename0 = get_AT_string (comp_unit_die (), DW_AT_name);
> >>  
> >> -  dwarf2out_source_line (line, column, filename, 0, true);
> >> +	  if (filename0 == NULL)
> >> +	    filename0 = "<dummy>";
> >> +	  maybe_emit_file (lookup_filename (filename0));
> >> +	}
> >> +    }
> >>  }
> >>  
> >>  /* Record the beginning of a new source file.  */
> >> diff --git a/gcc/final.c b/gcc/final.c
> >> index ac6892d..82a5767 100644
> >> --- a/gcc/final.c
> >> +++ b/gcc/final.c
> >> @@ -1725,7 +1725,7 @@ final_start_function_1 (rtx_insn **firstp, FILE *file, int *seen,
> >>    if (!dwarf2_debug_info_emitted_p (current_function_decl))
> >>      dwarf2out_begin_prologue (0, 0, NULL);
> >>  
> >> -  if (DECL_IGNORED_P (current_function_decl) && last_linenum && last_filename)
> >> +  if (DECL_IGNORED_P (current_function_decl))
> >>      debug_hooks->set_ignored_loc (last_linenum, last_columnnum, last_filename);
> >>  
> >>  #ifdef LEAF_REG_REMAP
> >> @@ -2205,8 +2205,7 @@ final_scan_insn_1 (rtx_insn *insn, FILE *file, int optimize_p ATTRIBUTE_UNUSED,
> >>  	    }
> >>  	  else if (!DECL_IGNORED_P (current_function_decl))
> >>  	    debug_hooks->switch_text_section ();
> >> -	  if (DECL_IGNORED_P (current_function_decl) && last_linenum
> >> -	      && last_filename)
> >> +	  if (DECL_IGNORED_P (current_function_decl))
> >>  	    debug_hooks->set_ignored_loc (last_linenum, last_columnnum,
> >>  					  last_filename);
> >>  
> >>
> > 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)