[llvm-dev] Implementing a DWP tool in LLVM

[Alexey Samsonov via llvm-dev]

On Tue, Nov 3, 2015 at 10:01 PM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
> On Tue, Nov 3, 2015 at 5:06 PM, Alexey Samsonov <vonosmas at gmail.com>
> wrote:
>
>> SGTM. This will bring us closer to the point when we can write tests,
>> where we strip out the .dwo files from executables, package them together
>> with llvm-dwp, and then verify that we still get all we need from
>> llvm-symbolizer.
>>
>
> Not quite following here - dwo sections are already stripped from .o files
> and should never appear in executables.
>
> llvm-symbolizer is tested against .o files that contain no dwo contents,
> right? Or is the dwo/dwp information optionally used in some way?
>

Well, technically llvm-symbolizer is able to read the reference from
skeleton compile unit in the executable, load the necessary .dwo file, and
fetch the information from there, but we don't do much testing of this.


>
>
>> I didn't fully understand the part about walking DIEs to patch references
>> from .debug_info.dwo to .debug_loc.dwo: shouldn't their values stay the
>> same, as they will be treated relative to the value of DW_SECT_LOC offset?
>>
>
> Ah, right you are - hadn't spotted that bit. I guess this'll all become
> more clear to me as I implement dumping support for the indexes and can
> start to look at some examples of the behavior of the existing dwp tool.
> (thanks muchly for the pointer there)
>
> - Dave
>
>
>>
>> On Tue, Nov 3, 2015 at 1:52 PM, David Blaikie via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Much like the recent efforts to provide a port of dsymutil in the LLVM
>>> project, I'm looking at providing an implementation of the Fission/Split
>>> DWARF DWP tool ( https://gcc.gnu.org/wiki/DebugFissionDWP ) in LLVM.
>>>
>>> While there's potentially some overlap between the two tools, I'm
>>> thinking of keeping them separate at least initially since much of the
>>> debug info doesn't need to be touched by a DWP tool, unlike dsymutil.
>>>
>>> Basically all the tool needs to do is concatenate (or deduplicate, in
>>> the case of type units) the sections and apply a few domain-specific
>>> relocations, but that doesn't include having to read the DIE tree in
>>> debug_info.dwo (only the headers). The other thing is to build a couple of
>>> indexing data structures to allow fast lookup of CUs and TUs.
>>>
>>> Likely I'll start with:
>>>
>>> * adding llvm-dwarfdump support for the DWP indexes
>>> * basic prototype of llvm-dwp just concatenating sections
>>> * handle each of the domain specific relocations in turn
>>>   * abbr_offset
>>>   * debug_str_offsets.dwo entries
>>>   * type_unit's DW_AT_stmt_list
>>>   * references to debug_loc.dwo from debug_info.dwo
>>>      * this one, at first blush, makes me particularly sad, as it'll
>>> involve actually walking all the DIEs in any CUs (stmt_list isn't great
>>> either, but at least that'd only be the header - same for accessing the
>>> signature for the CU, it's always in the root DIE)
>>> * deduplicate type units
>>> * add CU/TU indexes
>>> * DWP merging (being able to read existing indexes and merge those into
>>> larger indexes)
>>> * possibly support the thin DWP mode
>>>
>>> Does this all seem feasible/plausible/reasonable to do in LLVM? Any
>>> particular points of contention/interest/clarification?
>>>
>>> It's possible at some point in the future it might be nice to share the
>>> type merging logic of dsymutil (type units make sense when you don't have a
>>> debug aware linker - but they do have unfortunate overhead which would be
>>> nice to avoid, if possible), in which case there might be some code sharing
>>> opportunity. But the two tools are still going to be fairly different in
>>> their purpose/handling (dsymutil has to get the address mappings and update
>>> all of that, DWP won't  have to deal with code addresses, etc).
>>>
>>> - Dave
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>>
>> --
>> Alexey Samsonov
>> vonosmas at gmail.com
>>
>
>


-- 
Alexey Samsonov
vonosmas at gmail.com
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