Re: AIX DWARF debugging sections

Joel Brobecker
2015-10-10T00:04:38Z

> >>> That's what I have been trying to do, but I don't see any evidence
> >>> that GCC emits it, or that binutils handles it. So the answer must
> >>> lie somewhere on the system side. David?
> >>
> >> The official answer from IBM XLC is the section is for both
> >> .debug_macinfo and .debug_macro
> >>
> >> For DWARF4 and below, the dwmac section corresponds to .debug_macinfo.
> >> For DWARF5 and above, the dwmac section corresponds to .debug_macro.
> >>
> >> XLC currently does not generate any macro debugging information for DWARF4.
> >
> > Huh.
> > Let's make sure this is documented in the code.
>
> Is the following sufficient or you would like more exposition?
>
> diff --git a/gdb/xcoffread.c b/gdb/xcoffread.c
> index 0d49751..86d4ca4 100644
> --- a/gdb/xcoffread.c
> +++ b/gdb/xcoffread.c
> @@ -159,11 +159,11 @@ static const struct dwarf2_debug_sections
> dwarf2_xcoff_names = {
> { ".dwabrev", NULL },
> { ".dwline", NULL },
> { ".dwloc", NULL },
> - { NULL, NULL }, /* debug_macinfo */
> - { NULL, NULL }, /* debug_macro */
> + { ".dwmac", NULL }, /* debug_macinfo for DWARF4 and below */
> + { ".dwmac", NULL }, /* debug_macro for DWARF5 and above */

Looking at the code in GDB reading the DWARF sections, I am a little
uncomfortable having two entries for 2 sections sharing the same name.
I don't have a full grasp of that code, so maybe things would be fine,
but I'm not sure. Doug has more experience with this code, so might
know better.

In the meantime, I started exploring a bit on the GCC side. I have
been using GCC 4.9, which I think is an interesting reference,
because not super recent. What I have found is that, unless you
use -gstrict-dwarf, GCC generates a .debug_macro section. You have
to use -gstrict-dwarf to get the older .debug_macinfo format.

The bad new is: once assembled using GNU as, the section name
in the object file is .dwmacro, not .dwmac. As for the .debug_macinfo
section, it gets renamed to .dwmacif.

I'm not really sure how to best proceed, here. On the one hand,
IBM can be considered the authority on these kinds of questions.
On the other hand, GDB being a GNU tool, we should probably make
it easier to debug code produced by GNU tools rather than by
propriatery ones... It's particularly tempting since GNU as
has chosen different section names, which seems to be a better
choice to me, and also happens to make it easier on the code.

Perhaps what we could do is handle it at the binutils level,
and pretend that ".dwmac" is actually ".dwmacro" or ".dwmacif"
depending on the compilation unit's DWARF version. Then GDB
would be modified to handle the names chosen on the GNU side.

> { ".dwstr", NULL },
> { ".dwrnges", NULL },
> - { NULL, NULL }, /* debug_types */
> + { ".dwpbtyp", NULL },
> { NULL, NULL }, /* debug_addr */
> { ".dwframe", NULL },
> { NULL, NULL }, /* eh_frame */
>
> Thanks, David

--
Joel