[Dwarf-Discuss] EXTERNAL: Corner-cases with bitfields

Todd Allen todd.allen at concurrent-rt.com
Mon May 9 16:00:36 PDT 2022

On Mon, May 09, 2022 at 09:41:03PM +0000, Dwarf Discussion wrote:
> > Pedro Alves wrote:
> > On 2022-05-09 16:48, Ron Brender via Dwarf-Discuss wrote:
> > > So my suggestion is to file a bug report with CLANG, requesting they
> > correct their DWARF output to reflect all details needed
> > > by your language.
> > 
> > An issue here is that DWARF does say this, in (DWARF 5, 5.7.6 Data Member
> > Entries, page 119):
> > 
> >  "If the size of a data member is not the same as the size of the type
> > given for the
> > 
> > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > ^^^^^^^
> >  data member, the data member has either a DW_AT_byte_size or a
> >  ^^^^^^^^^^^
> >  DW_AT_bit_size attribute whose integer constant value (see Section 2.19
> > on
> >  page 55) is the amount of storage needed to hold the value of the data
> > member."
> > 
> > Note the part I underlined.  In Lancelot's case, the size of the data
> > member
> > IS the same as the size of the type given for the data member.  So Clang
> > could well pedantically
> > claim that they _are_ following the spec.  Shouldn't the spec be clarified
> > here?
> What the spec says is that a producer isn't _required_ to emit the
> DW_AT_bit_size attribute.  But, given that DWARF is a permissive
> standard, the producer is certainly _allowed_ to emit the attribute.  
> If this is a hint that the target debugger will understand, regarding
> the ABI, it seems okay to me for the producer to do that.
> > This then raises the question of whether a debugger can assume that the
> > presence of a DW_AT_bit_size
> > attribute indicates that the field is a bit field at the C/C++ source
> > level.  GDB is assuming that
> > today, as there's really no other way to tell, but I don't think the spec
> > explicitly says so.
> GDB is choosing to make that interpretation, which it's allowed to do.
> The DWARF spec just doesn't promise that interpretation is correct.
> You can propose to standardize that interpretation by filing an issue
> with the DWARF committee at https://dwarfstd.org/Comment.php and it might
> or might not become part of DWARF v6.  It might be tricky because you'd
> be generalizing something very specific to your environment.
> You can also, separately, try to get Clang to emit the DW_AT_bit_size
> attribute in these cases for the AMDGPU target(s).  This seems more
> likely to work, especially as there's an ABI requirement involved, and
> (given that GDB makes this interpretation) I assume gcc already does this.


I suppose, if you didn't want to submit an issue, another solution would be to
require the necessary tags & attributes in the ABI itself.  We already expect
ABI documents to provide things like register values, CFI initial values, and
some more esoteric stuff (augmentations, non-standard endianity & isa).  An ABI
that required descriptions in ABI-specific situations like these two seems
reasonable to me.  And it places no burden on compilers for other ABI's.

Todd Allen
Concurrent Real-Time

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