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

Fri Feb 19 19:45:22 PST 2016

You know I favor the pedantic answer. ☺  The point remains, type unit signatures need to be content-dependent and dwo_ids really don't, regardless of the details, because they have different consistency requirements.
--paulr

From: Eric Christopher [mailto:echristo at gmail.com]
Sent: Friday, February 19, 2016 7:07 PM
To: Robinson, Paul; Peter Collingbourne; Adrian Prantl
Cc: llvm-commits at lists.llvm.org
Subject: Re: [PATCH] D17321: DIEData, DIEWriter: introduce and begin migration.

On Fri, Feb 19, 2016 at 6:52 PM Robinson, Paul <Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>> wrote:
Type units are COMDATs produced across (potentially) many compilations, so the key has to correspond to the content in a predictable/consistent way in order for the linker to throw away duplicates. Therefore the type-unit hash is specified the way it is.

Doesn't matter. The way that we're doing it in clang is perfectly acceptable for this.

dwo_ids are just identifiers matching two lumps of data for the same CU, produced by the same compilation, so they aren't sensitive to content (at least not in the same way as type units).  But it would seem prudent for two compilations of the "same" source to have different IDs, thus hashing the .debug_info content might not be the best choice; a more random-number kind of ID would be more appropriate.

I don't see this either, but I know where you're coming from.

-eric

--paulr

From: llvm-commits [mailto:llvm-commits-bounces at lists.llvm.org<mailto:llvm-commits-bounces at lists.llvm.org>] On Behalf Of Eric Christopher via llvm-commits
Sent: Friday, February 19, 2016 6:39 PM
To: Peter Collingbourne; Adrian Prantl
Cc: llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>
Subject: Re: [PATCH] D17321: DIEData, DIEWriter: introduce and begin migration.

On Fri, Feb 19, 2016 at 5:45 PM Peter Collingbourne via llvm-commits <llvm-commits at lists.llvm.org<mailto: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<mailto: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.

That said, ISTR at the time having a good reason why a hash of the debug_info section wasn't a sufficient method for unique identifiers, but my brain isn't coming up with it right now. Might have been related to not having a finalized section before wanting to splat in the hash, but I'm not sure.

For the type ids we haven't worried because we're only supporting type units for C++ types at the moment, if we wanted to support C types we'd want to come up with another identifier so we could merge similar types across compilation units.

Hope this helps.

-eric
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