Re: [RFA/dwarf-2] Fix for the null record problem

[Daniel Jacobowitz <drow@false.org>]

On Wed, Apr 14, 2004 at 12:21:44PM -0500, Jim Blandy wrote:
> 
> Joel Brobecker <brobecker@gnat.com> writes:
> > The patch is quite tiny, seems almost obvious, and Elena seemed happy
> > with it. But it didn't get in because she asked that a testcase be added
> > first (on Feb 19):
> > <<
> > the patch looks sensible, but I would like to see the testcase go in
> > at the same time, or we'll forget.
> > >>
> > 
> > The new testcase has been checked in now, so I was wondering if somebody
> > had a moment to have a look at it, and confirm Elena's first review?
> 
> I think it looks fine; I've just one question before you commit.
> 
> As it stands, the code sets TYPE_FLAG_STUB if the type die has no
> children, or if die_is_declaration (die, cu) is true.  Your patch
> correctly ditches the first criterion; no problems there.
> 
> But it also modifies the second criterion as well, without comment.
> In particular, die_is_declaration checks for both DW_AT_declaration
> and DW_AT_specification, but your patch only tests for
> DW_AT_declaration.  I think this is correct: in section 5.6.1, the
> Dwarf 2 spec says that the definition of the type has a
> DW_AT_specification attribute pointing to the declaration.  Since it's
> the definition of the type that actually lists the fields,
> DW_AT_specification should not cause GDB to mark the type as a stub.
> Just the opposite: the referent of that attribute is the stub.
> 
> The following C++ code produces Dwarf 2 info where the definition of
> struct s::t has a DW_AT_specification attribute, but GDB doesn't skip
> it, and I don't really understand why:

Did you misread die_is_declaration?

  return (dwarf2_attr (die, DW_AT_declaration, cu)
          && ! dwarf2_attr (die, DW_AT_specification, cu));

I don't even know what that DW_AT_specification test is doing there -
the idea of a declaration with a specification is pretty peculiar.
But the important bit is that it's not going to report that something
is a declartion, if it has a DW_AT_specification.

If the check is important, then Joel should probably use
!die_is_declaration.  This was added by Jason Merrill in 2000, without
much of an explanation; here it is:
 http://sources.redhat.com/ml/gdb-patches/2000-q1/msg00015.html

It's not clear why the specification check is there; the important bit
was presumably:
 > !   if (die->has_children)
 > !   if (die->has_children && ! die_is_declaration (die))

i.e. the point of the patch was to add the DW_AT_declaration check.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer