Re: [PATCH V5] Fix issues with reading rnglists, especially from dwo files, for DWARF v5

[Caroline Tice <cmtice@google.com>]

On Thu, Jul 16, 2020 at 8:41 AM Simon Marchi <simark@simark.ca> wrote:
>
> On 2020-07-16 12:46 a.m., Caroline Tice wrote:
> > The attached patch has an updated  commit message that explains how to
> > run the test and what version of clang was used.  Other than that it
> > is identical  to the previous patch.
>
> Thanks, I've now merged it.

Thank you!  :-)

>
> >> Just wondering, doesn't gcc emit at least *some* DWARF 5 by now?  If so,
> >> can't the GNU assembler deal with it?
> >>
> >
> > Not that it matters for this patch, but I believe you are
> > misunderstanding the situation. GCC *does* emit some  DWARF 5, but at
> > least the GCC version I'm using (Debian 9.3.0-8) does not seem to be
> > generating DWARF v5 line table sections; the line table section it
> > generates (when I compile with -gdwarf-5) is a DWARF v3 .debug_line
> > sections.
> >
> > In fact my gcc-generated binary has the following debug sections,
> > which are a mix of various dwarf versions:
> >
> > .debug_aranges               (dwarf v2)
> > .debug_info                      (dwarf v5)
> > .debug_abbrev                 (dwarf v5)
> > .debug_line                      (dwarf v3)
> > .debug_str
> > .debug_addr                    (dwarf v5)
> > .debug_gnu_pubnames  (dwarf v2)
> > .debug_gnu_pubtypes    (dwarf v2)
> > .debug_rnglists               (dwarf v5)
>
> I see.  But if we generate a synthetic test case (using the DWARF assembler
> in the GDB testsuite) for the current issue, that deals with range lists,
> I don't think we even need a .debug_line section, do we?  And if we do, we
> always have the option of generating a v3 one.

We do in fact need the .debug_line section, because the test relies on
setting a breakpoint at a particular line, and then seeing whether or
not we can find the value of a variable in the lexical block there;
without line table information I don't think GDB will be able to do
that.

>
> > In contrast, here are the sections my clang compiler, when passed
> > -gdwarf-5 is generating dwarf v5 versions
> > of nearly all the sections:
> >
> > .debug_info                      (dwarf v5)
> > .debug_abbrev                 (dwarf v5)
> > .debug_line                      (dwarf v5)
> > .debug_str
> > .debug_addr                     (dwarf v5)
> > .debug_rnglists                (dwarf v5)
> > .debug_str_offsets           (dwarf v5)
> > .debug_gnu_pubnames   (dwarf v2)
> > .debug_gnu_pubtypes     (dwarf v2)
> > .debug_line_str                (dwarf v5)
> >
> > So either the GNU assembler has not been updated to handle DWARF v5
> > .debug_line file indices because GCC is not yet generating them; or
> > GCC is not yet generating them because the GNU assembler has not yet
> > been
> > updated to handle them.  Take your pick.
> >
> > In any case, I hope my patch is now acceptable! :-)
>
> Simon
>