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

[Alexander Yermolovich]

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?

Alex







________________________________
From: David Blaikie <dblaikie@gmail.com>
Sent: Monday, September 25, 2023 9:02 AM
To: Alexander Yermolovich <ayermolo@meta.com>
Cc: dwarf-discuss@lists.dwarfstd.org <dwarf-discuss@lists.dwarfstd.org>
Subject: Re: [Dwarf-discuss] [DWARF5] .debug_names + fdebug-types-sections

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On Fri, Sep 15, 2023 at 2:45 PM Alexander Yermolovich via
Dwarf-discuss <dwarf-discuss@lists.dwarfstd.org> wrote:
>
> Hello
>
> I am trying to enable debug names acceleration table with fdebug-types-sections in LLVM. One part I am not sure about is the local TU list. It contains an offset into .debug_info section. All the entries have an index entry that points to the local TU list. DIEs within entry offsets are relative to the TU entry.
>
> Linker de-duplicates Type Units using COMDAT. So, the final result will have less type units. As the result Local Type Unit List will be invalid, and all the Entries that point to that TU will not be valid either. Even if we Linker is modified so that somehow when it de-duplicates type sections Local Type Units will get the right offset, that still leaves all the duplicate entries.
> Am I missing something in that linker, specifically LLD, will need to be aware of context of .debug_names sections when it de-duplicates type sections?
>
>
> It seems to me that to fully support it .debug_names need to be created by post build tool (or by linker....).
>
> Thanks.

While DWARF consumers will benefit from a content-aware linking of
.debug_names (using one hash table is more efficient than probing
hundreds/thousands of small hash tables), I don't believe the spec
as-is requires that for correctness.

In the case of type units, I'd expect behavior somewhat similar to how
linkers behave with inline functions - if the two copies of the
function are identical, it's possible that the linker will resolve all
relocations to the function to the single copy that remains after
linking (so two CUs would both describe the inline function "f1" and
both descriptions would have the same start address/length, the two
CUs CU-level DW_AT_ranges would overlap/both contain that function's
addresses - and neither would use the tombstone address). So in that
case, all the duplicate index entries would remain valid (their
TU-relative offsets would be correct - since the TUs were bit-wise
identical, so the offsets still point to the same things).

In the case where a producer produces equivalent but not
bitwise-identical TUs, the linker will choose one, drop the rest, and
use the tombstone value to resolve the relocation used in the local
TUs offset list. A consumer should ignore any entries that reference a
tombstone offset in the local TU list (& probably wouldn't hurt to use
the same code and ignore any tombstoned CUs too - I can't immediately
think of a situation/reason that'd happen, but seems like a good
general idea)

If a consumer does a semantic aware merge of the indexes, then it
should discard (rather than tombstoning) the index entries that
reference dead TUs and the dead TUs in the local TU list itself, and
also discard any duplicate index entries and duplicate elements in the
local TU list.

We could document the use of the tombstone in this context.
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