[Dwarf-discuss] [DWARF5] .debug_names + fdebug-types-sections

[David Blaikie]

On Mon, Oct 16, 2023 at 9:12 AM David Blaikie <dblaikie@gmail.com> wrote:

>
>
> On Mon, Oct 16, 2023 at 8:57 AM Alexander Yermolovich <ayermolo@meta.com>
> wrote:
>
>> For background llvm discussion on how to implement it:
>>
>> https://discourse.llvm.org/t/debuginfo-dwarfv5-lld-debug-names-with-fdebug-type-sections/73445
>>
>> Thanks for explaining the issue, and proposing spec change. 🙂
>> The question I have. Is non-bit identical TUs with the same hash a
>> fundamental issue that needs to be addressed somehow in the next version of
>> the spec? If we could have such guarantee that should simplify things quite
>> a bit. The linker can just follow the same path as for functions. Compiler
>> can generate symbol name unique for the type unit hash. So, when linker
>> comdats TU sections entries in TU list will point to correct address and no
>> special logic is needed for tombstone. I guess there is a hashing mechanism
>> in DWARF spec, but LLVM is not using it. Should we go back to it, is it
>> enough?
>>
>
> The hashing mechanism in the spec doesn't guarantee bit-identicality, I
> believe. It's structural equivalence (eg: if you produce the main type DIE
> followed by an int DIE that the main type needs, or you emit the int DIE
> first, followed by the main type DIE - these hash to the same value
> (because you start from the type DIE and hash outwards/to what it can
> reach, and has structural equivalence - int is int, no matter what offset
> it's at)) not bit identical. For a bunch of reasons this is preferable.
>
> (yes, clang takes this further and hashes based on the C++ ODR - which is
> off-spec, but workable in our experience)
>
> I was thinking another direction we could go is that, I think, the only
> things in a type unit that can be referenced is the type (I think?) then
> perhaps we could modify how types defined in type units are referenced.
>
> If only the type can be referenced in a type unit, we could emit a
> .debug_names entry without a DW_IDX_die_offset - just the DW_IDX_type_unit
> - and the consumer can use the header of the type unit to find the exact
> type unit DIE.
>
> Are there any other things that could be referenced within a type unit?
>

Hmm - this doesn't really relate to the local TU tombstoning issue (since
that's tombstoning the reference to the unit) but would help address the
foreign TU situation - in the DWP case, a consumer could ignore the
DW_IDX_compile_unit alongside the DW_IDx_type_unit and look up that type in
the DWP - then use the type unit's header to find the type DIE.
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