Re: [PATCH] debug/108772 - ICE with late debug generated with -flto

[Richard Biener <rguenther@suse.de>]

On Wed, 1 Mar 2023, Jason Merrill wrote:

> On 3/1/23 08:09, Jakub Jelinek wrote:
> > On Wed, Mar 01, 2023 at 01:07:02PM +0000, Richard Biener wrote:
> >> When combining -g1 with -flto we run into the DIE location annotation
> >> machinery for globals calling dwarf2out_late_global_decl but not
> >> having any early generated DIE for function scope statics.  In
> >> this process we'd generate a limbo DIE since also the function scope
> >> doesn't have any early generated DIE.  The limbo handling then tries
> >> to force a DIE for the context chain which ultimatively fails and
> >> ICEs at the std namespace decl because at -g1 we don't represent that.
> >>
> >> The following avoids this situation by making sure to never generate
> >> any limbo DIEs from dwarf2out_late_global_decl in the in_lto_p path
> >> but instead for function scope globals rely on DIE generation for
> >> the function to output a DIE for the local static (which doesn't
> >> happen for -g1).
> 
> So the issue is that we're trying to force out a DIE for a decl that we
> wouldn't have generated without -flto?  How is it avoided in the non-LTO case?

When we go rest_of_decl_compilation for this decl we defer to the
containing function to generate an early DIE but that doesn't
(because of -g1).  The call to late_global_decl that's done by
assemble_decl then does nothing because there's no early DIE.  But with
-flto we cannot completely rely on early DIE presence (not even without,
in case of cloning - but we don't clone global variables), esp. because
there's still the "supported" non-early-LTO path for non-ELF targets.

So at this point it seems to be the best thing to mimic what
rest_of_decl_compilation does and defer to dwarf2out of the
containing function to generate the DIE (or not).  For the reason
of the least amount of changes at this point in stage4 I went for
querying the DECL_CONTEXT DIE instead of right-out not handling
local_function_static () decls in this path.

If you'd prefer that, so

      if (! die && in_lto_p
          /* Function scope variables are emitted when emitting the
             DIE for the function.  */
          && ! local_function_static (decl))
        dwarf2out_decl (decl);

then I can test that variant as well which feels a bit more
consistent.

Thanks,
Richard.

> >> I explored a lot of other options to fix this but in the end this
> >> seems to be the most spot-on fix with the least risk of unwanted
> >> effects.
> >>
> >> LTO bootstrapped on x86_64-unknown-linux-gnu (running into PR108984),
> >> bootstrapped and tested on x86_64-unknown-linux-gnu.
> >>
> >> OK?
> >>
> >> Thanks,
> >> Richard.
> >>
> >>  PR debug/108772
> >>  * dwarf2out.cc (dwarf2out_late_global_decl): Do not
> >>  generate a DIE for a function scope static when we do
> >>  not have a DIE for the function already.
> >>
> >>  * g++.dg/lto/pr108772_0.C: New testcase.
> > 
> > LGTM, but please give Jason a day to chime in if he disagrees.
> > 
> >  Jakub
> > 
> 
> 
> 

-- 
Richard Biener <rguenther@suse.de>
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