[Dwarf-Discuss] Split Dwarf vs. CU DW_AT_ranges / DW_AT_low_pc placement

[David Blaikie]

On Thu, Mar 11, 2021 at 12:32 AM Jakub Jelinek <jakub at redhat.com> wrote:

> On Wed, Mar 10, 2021 at 10:07:27PM -0800, David Blaikie wrote:
> > On Wed, Mar 10, 2021 at 9:38 PM Jakub Jelinek <jakub at redhat.com> wrote:
> >
> > > On Wed, Mar 10, 2021 at 04:12:57PM -0800, David Blaikie via
> Dwarf-Discuss
> > > wrote:
> > > > On Wed, Mar 10, 2021 at 4:02 PM Cary Coutant <ccoutant at gmail.com>
> wrote:
> > > >
> > > > > > > So in the end the logical thing to do when encountering a
> > > > > > > DW_FORM_rnglistx in a split-unit, in order to support
> everybody, is
> > > > > > > probably to go to the .debug_rnglists.dwo section, if there's
> one,
> > > > > > > disregarding the (inherited) DW_AT_rnglists_base.  If there
> isn't,
> > > then
> > > > > > > try the linked file's .debug_rnglists section, using
> > > > > > > DW_AT_rnglists_base.  If there isn't, then something is
> malformed.
> > > > >
> > > > > Looks reasonable to me. I think we need a new issue to clarify
> this in
> > > > > DWARF 6.
> > > > >
> > > >
> > > > Given that DWARFv5 isn't on by default in GCC yet & I think has a few
> > > more
> > >
> > > It is on by default.  But -gstrict-dwarf is not on by default.
> > >
> >
> > Oh, it is - in a released version of the compiler, or only in
> development?
>
> Still in development, but the prerelease already widely deployed by
> multiple
> Linux distributions.
>
> > & you're proposing changing the behavior only under -gstrict-dwarf,
> rather
> > than in general? Any particular reason?
>
> Just a typo, sorry, meant -gsplit-dwarf.
>

Ah, right right - I'm with you.

What's your take on:

1) Fixing GDB to handle GCC's current output.
2) Fixing GCC to produce something maybe more standards conforming (to my
mind, ideally: ranges on the skeleton CU (using either
rnglists_base+rnglistx (like LLVM), or sec_offset (actually more
compact/better than LLVM anyway, and avoids the ambiguous situation), and
rnglistx in child DIEs the split full unit using using .debug_rnglists.dwo)
3) both? (so GDB can handle old GCC's output and the newer/more correct
output)

Personally, I'd have thought it'd be enough to move forward, change GCC and
be done - but if folks would like GDB (& GDB folks are cool with it) to
handle the old/weird GCC output, that's cool/up to GDB folks. Though I hope
that sort of DWARF doesn't stick around long/need lots of long-lived
support (I hope we don't need to add it to llvm's symbolizer for instance).

- Dave
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