[Dwarf-Discuss] debug_aranges use and overhead

[Samy Al Bahra]

Hi David,

Sorry I'm a bit late to the game. On the value of having .debug_aranges and
the performance impact:

Our debugger was designed for performance and does end to end lazy
evaluation (down to the DIE). This is quite old (excuse the formatting) but
numbers are here:
https://engineering.backtrace.io/2014-09-15-bt-lightweight-backtrace-tool/
, search for "Chromium".  This is something other debuggers can take
advantage of if they run in a non-interactive / batch mode (think bulk
processing of millions - billions of dumps a month) and is generally useful
when folks are iterating in development (fast feedback for crashes while
having some background indexing work going on). I'm also happy to run
benchmarks for you with and without .debug_aranges on top of our debugger
if it'll be useful.

One of the crucial optimizations we made is incremental indexing on top of
.debug_aranges based on PC values (+ complexities Greg mentions later in
the thread). In cases where we lack this, we use our own persistent cache
which introduces unnecessary complexity. Now I am considering going as far
as adding a multi-threaded indexer for cases where a persistent cache /
build system modifications aren't an option (work to begin in the next week
or two).

.debug_aranges would provide a lot of value to our users.

On Thu, Mar 11, 2021 at 3:48 PM David Blaikie via Dwarf-Discuss <
dwarf-discuss at lists.dwarfstd.org> wrote:

> On Thu, Mar 11, 2021 at 5:48 AM <paul.robinson at sony.com> wrote:
>
>> Hopefully not to side-track things too much... maybe wants its own
>> thread, if there's more to debate here.
>>
>
> Yeah, how about we spin it off into another thread (done here)
>
>
>> >> For the case you suggested where it would be useful to keep the range
>> >> list for the CU in the .o file, I think .debug_aranges is what you're
>> >> looking for.
>> >
>> > aranges has been off by default in LLVM for a while - it adds a lot of
>> > overhead (doesn't have all the nice rnglist encodings for instance -
>> > nor can it use debug_addr, and if it did it'd still be duplicate with
>> > the CU ranges wherever they were).
>>
>> Did you want to file an issue to improve how .debug_aranges works?
>>
>
> I don't currently understand the value it provides, and I at least don't
> have a use case for it, so I'm not sure I'd be the best person to
> advocate/drive that work.
>
> Complaining that it duplicates CU ranges is missing the point, though;
>> it's an index, like .debug_names, of course it duplicates other info.
>> If you want to suggest an improved index, like we did with .debug_names,
>> that would be great too.
>>
>
> .debug_names is quite different though - it collects information from
> across the DIE tree - information that is expensive to otherwise gather
> (walking the whole DIE tree).
>
> .debug_aranges is not like that for most producers (producers that do
> include the address ranges on the CU DIE) - the data is readily available
> immediately on the CU. That does involve reading some of .debug_abbrev, and
> interpreting a handful of attributes - but at least for the use cases I'm
> aware of, that overhead isn't worth the size increase.
>
> Do you have numbers on the benefits of .debug_aranges compared to parsing
> the ranges from CU DIEs?
>
> (one possible issue: the CU doesn't /have/ to contain low/high/ranges if
> its children DIEs contain addresses - having that as a guarantee, or some
> preferred way of encoding zero length (high/low of 0 would be acceptable, I
> guess) would be nice & make it cheap to skip over CUs that don't have any
> address ranges)
>
> Roughly, a modern debug_aranges to me would look something like:
>
> <length>
> <version>
> <CU sec_offset>
> <addr_base>
> <rnglist sec_offset>
>
> So it could fully re-use the rnglist encoding. If this was going to be as
> compact as possible, it'd need to be configurable which encodings it uses -
> ranges V high/low, addrx V addr - at which point it'd probably look like a
> small DIE with an inline abbrev (similar to the way DWARFv5 encodes the
> file and directory entries now, and how debug_names is self-describing) -
> at which point it looks to me a lot like parsing the CU DIEs.
>
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-- 
Samy Al Bahra [http://repnop.org]
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