[llvm-dev] unified debug information despite function/data sections flags

[James Henderson via llvm-dev]

Yep, I took a look at this last year/early this year, but never really came
up with a fully functioning prototype that was actually efficient enough,
and have since switched teams, so haven't had the time to work on it
further.

You can see my lightning talk from last year on the topic here:
https://www.youtube.com/watch?v=0y6TlfFhCsU, and a mailing thread where I
discussed it further here:
https://lists.llvm.org/pipermail/llvm-dev/2020-November/146469.html. The
main issue I ran into was the number of hard-coded relative references
within DWARF. Every single one of these needs to be updated at link time,
if any of the data is dropped, or the DWARF will end up invalid. To do
this, I had to add relocations to the DWARF which patched the relevant
fields at link time, based on the final computed offset, but this had a
serious performance cost in the linker (not to mention any potential cost
in the assembler). This approach is certainly possible for the most part,
at least for .debug_line and .debug_info (it's not necessarily clear
whether it can be done with some of the other DWARF sections, although the
benefits in most of them aren't particularly clear), but the difficulty is
getting it to be fast.

I'd be happy to discuss this further, and provide any feedback on other
ideas, if you have any, but currently have no plans to continue this work
at this time myself.

By the way, if you are using the DWARF for stack usage analysis, have you
considered the .stack_sizes section? This emits a section that contains the
stack size of every function in the output, and can be dumped using
llvm-readobj. It is split up so that the linker can strip bits that
reference dead data, so you should only end up with the actually useful
information in the output.

James


On Thu, 30 Sept 2021 at 07:51, David Blaikie <dblaikie at gmail.com> wrote:

> You can differentiate dead function descriptions from others on most
> platforms by checking if the low_pc == 0. If 0 is a valid instruction
> address on your architecture, you can use an lld feature to set a more
> authoritative/unambiguous tombstone value for dead code addresses, passing
> something like:
>
> * -z 'dead-reloc-in-nonalloc=.debug_ranges=0xfffffffffffffffe'
> ** -z 'dead-reloc-in-nonalloc=.debug_loc=0xfffffffffffffffe'
> *
> * -z 'dead-reloc-in-nonalloc=.debug_*=0xffffffffffffffff'*
>
> to the linker.
>
> As for reducing debug info size by omitting debug info descriptions of
> dead code - Apple/MachO's dsymutil does this, and I believe Alexey Lapshin
> is working on trying to get similar behavior into lld, possibly (or as a
> post-link tool).
>
> There's also the possibility of using comdats to make the linker's job
> easier - I think there might be ways to structure the DWARF into chunks
> that could be deduplicated and dropped naturally by a linker's existing
> comdat support, but I haven't fully prototyped it. I think there was a
> thread a while back with JHenderson and others discussing this possibility
> further.
>
> - Dave
>
> On Wed, Sep 29, 2021 at 12:50 PM Youssefi, Anna via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi,
>>
>>
>>
>> I was wondering if there are any plans to separate debug information into
>> distinct sections accordingly when the compiler flags -ffunction-sections
>> and/or -fdata-sections are used.  If an unreferenced function is removed
>> from the link, it makes no sense for its associated debug information to
>> still be included.  As we rely on the debug information for stack usage
>> analysis, we wind up displaying stack usage statistics for unreferenced
>> functions that were eliminated from the link if debug information for any
>> other referenced functions is in the same debug section.  It seems that
>> others have run into this problem previously so I wanted to check whether
>> there are any plans to change the behavior.
>>
>>
>>
>> Thanks,
>>
>> Anna Youssefi
>>
>> Texas Instruments, Codegen group
>>
>>
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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