[PATCH] D17321: DIEData, DIEWriter: introduce and begin migration.

[Peter Collingbourne via llvm-commits]

On Sat, Feb 20, 2016 at 02:39:10AM +0000, Eric Christopher wrote:
> On Fri, Feb 19, 2016 at 5:45 PM Peter Collingbourne via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
> 
> > On Fri, Feb 19, 2016 at 03:47:22PM -0800, Adrian Prantl wrote:
> > >
> > > > On Feb 19, 2016, at 12:31 PM, Peter Collingbourne via llvm-commits <
> > llvm-commits at lists.llvm.org> wrote:
> > > >
> > > > them incorrectly, see DwarfDebug::makeTypeSignature). Since the page at
> > > > https://gcc.gnu.org/wiki/DebugFission does not specify how the hash
> > is to be
> > > > computed, could we maybe do something simpler (e.g. MD5 hash the DIE
> > bytes
> > > > themselves) and avoid needing to use something like the DWARF type
> > hashing
> > > > algorithm for this?
> > >
> > > Where it can LLVM uses an MD5 sum over the mangled name of the type to
> > avoid the expensive DWARF type hashing.
> > >
> > > Just a side node: Debug info fission is in the process of being
> > standardized in DWARF 5. The progress can be tracked here:
> > >   http://dwarfstd.org/Issues.php?type=closed4 (search for “fission” and
> > “split DWARF”)
> > > The DWARF specification may deviate from the description on the GCC wiki
> > a little, but we probably want to follow the DWARF spec where that makes
> > sense.
> >
> > I see. From http://dwarfstd.org/ShowIssue.php?issue=130313.4 :
> >
> > >   5.  A DW_AT_dwo_id attribute whose value is an 8-byte
> > >       unsigned hash of the full compilation unit.  This hash
> > >       value is computed by the method described in Section 7.27
> > >       ("Type Signature Computation").
> >
> > That's unfortunate. Maybe I'm missing something, but I don't really see why
> > the attribute value needs to be specified like that, as the hash is only
> > used by consumers to match skeleton units with full units, and it should be
> > up to the producer to use a good enough hash. Is there any possibility of
> > changing the specification before DWARF 5 is standardised?
> >
> 
> It has been, but it's not really important as any two compilers aren't
> going to agree on debug info output anyhow so they're unlikely to match. I
> raised this in committee at the time as a "may" rather than "is". That
> said, recent text (I don't have an absolutely updated version in front of
> me) reads:
> 
> A DW_AT_dwo_id attribute whose implementation-defined integer constant
> value, known as the compilation unit ID, provides unique identification of
> this compilation unit as well as the associated split compilation unit in
> the object file named in the DW_AT_dwo_name attribute. For simplicity, the
> DW_AT_dwo_id attributes in the skeleton compilation unit and the
> corresponding split full compilation unit (see Section 3.1.3 on the
> following page) must use the same form to encode this identification value.
> 
> The means of determining a compilation unit ID does not need to be similar
> or related to the means of determining a type unit signature.

Thanks Eric, that wording seems a lot more sensible to me.

Thanks,
-- 
Peter