Duplicate .debug_lines (Was: [PATCH 5/5] Add --gdwarf-5 to ASM_SPEC)

[Mark Wielaard <mark@klomp.org>]

Hi gcc hackers, binutils hackers,

Nick, Jakub and I were discussing the gcc patch below and all the ways
it is wrong. Most things can be fixed in the spec. Like only passing
-gdwarf if we are generating DWARF and passing the right DWARF version
as -gdwarf-N for the version that gcc itself creates. But whether or
not we want gas to generate .debug_line info is a bit tricky. But when
giving -gdwarf-N gas will always try to generate a .debug_line section
and error out when there is already one.

Would it be possible to have something like the following in gas, so
that it doesn't try generating a .debug_line section if there already
is one, even when -gdwarf-N is given (unless the assembly also
contains .loc directives because that shows the user is really
confused)?

diff --git a/gas/dwarf2dbg.c b/gas/dwarf2dbg.c
index e4ba56d82ba..c0c09f4e9d0 100644
--- a/gas/dwarf2dbg.c
+++ b/gas/dwarf2dbg.c
@@ -2626,7 +2626,7 @@ dwarf2_init (void)
 
 
 /* Finish the dwarf2 debug sections.  We emit .debug.line if there
-   were any .file/.loc directives, or --gdwarf2 was given, or if the
+   were any .file/.loc directives, or --gdwarf2 was given, and if the
    file has a non-empty .debug_info section and an empty .debug_line
    section.  If we emit .debug_line, and the .debug_info section is
    empty, we also emit .debug_info, .debug_aranges and .debug_abbrev.
@@ -2650,9 +2650,16 @@ dwarf2_finish (void)
   empty_debug_line = line_seg == NULL || !seg_not_empty_p (line_seg);
 
   /* We can't construct a new debug_line section if we already have one.
-     Give an error.  */
+     Give an error if we have seen any .loc, otherwise trust the user
+     knows what they are doing and want to generate the .debug_line
+     (and all other debug sections) themselves.  */
   if (all_segs && !empty_debug_line)
-    as_fatal ("duplicate .debug_line sections");
+    {
+      if (dwarf2_loc_directive_seen)
+	as_fatal ("duplicate .debug_line sections");
+      else
+	return;
+    }
 
   if ((!all_segs && emit_other_sections)
       || (!emit_other_sections && !empty_debug_line))

On Mon, Aug 24, 2020 at 02:56:58PM +0200, Mark Wielaard wrote:
> This is needed to get DWARF version 5 .debug_line tables.
> It is also obviously wrong. It needs a check for whether as supports
> --gdwarf-<version> for all versions we support and it should only
> be added when generating DWARF debug information for the specific
> DWARF version we are generating.
> 
> It also needs some fixes to binutils, to make sure the line table is
> generated correctly:
> https://sourceware.org/pipermail/binutils/2020-August/112685.html
> And to make sure it can read the generated line table itself:
> https://sourceware.org/pipermail/binutils/2020-August/112966.html
> ---
>  gcc/gcc.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/gcc/gcc.c b/gcc/gcc.c
> index 10bc9881aed3..98b10e7cd154 100644
> --- a/gcc/gcc.c
> +++ b/gcc/gcc.c
> @@ -1882,6 +1882,11 @@ init_spec (void)
>    }
>  #endif
>  
> +  static const char dv[] = "--gdwarf-5 ";
> +  obstack_grow (&obstack, dv, sizeof (dv) - 1);
> +  obstack_grow0 (&obstack, asm_spec, strlen (asm_spec));
> +  asm_spec = XOBFINISH (&obstack, const char *);
> +
>  #if defined LINK_EH_SPEC || defined LINK_BUILDID_SPEC || \
>      defined LINKER_HASH_STYLE
>  # ifdef LINK_BUILDID_SPEC
> -- 
> 2.18.4
>