[llvm-dev] Remove obsolete debug info while garbage collecting

Rui Ueyama via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 27 01:46:11 PDT 2019


I'm a bit worried to teach lld about DWARF, as this is something we've been
carefully avoid to do. Linkers are mostly agnostic about the contents of
sections. Sections are basically just bags of bytes, and linkers generally
don't attempt to parse their contents. That being said, we've already
taught lld how to parse (some part of) DWARF to implement --gdb-index and
other features, and because of the nature of DWARF file format it is
unavoidable. So it may be OK to add more code for DWARF dedup, if the
additional complexity is not too much, and the new code is nicely isolated
from existing code. I think I agree with you that linker is perhaps the
best place to drop dead DWARF info. Let me start code review to see how the
code works. Thanks!

On Thu, Sep 26, 2019 at 7:12 AM Alexey Lapshin <a.v.lapshin at mail.ru> wrote:

> 25.09.2019 18:49, David Blaikie пишет:
> On Tue, Sep 24, 2019 at 11:22 PM Rui Ueyama via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> Alexay,
>> Thank you for the detailed explanation. The other question I have is, as
>> discussed above, about dsymutil. You said that dsymutil is not usable at
>> link-time. What does that mean? If we only have to emit an output file in
>> the usual way and then automatically invoke dsymutils on the file that the
>> linker just created, that's easy to do, and lld and dsymutil can live in
>> the same process so that you can keep the linker being not depend on an
>> external command.
> dsymutil isn't really (to my knowledge) setup for that sort of operation
> at the moment - it's currently very tied to the Apple/OSX/MachO debug info
> distribution model (it's for creating dsym debug info bundles from a set of
> object files and an output of addresses from the linker).
> If it was generalized as a post-processing step, that would be good for
> archival purposes (reducing the size of debug info in binaries in the
> long-term) but wouldn't address what are probably the more significant
> drawbacks for some users (including Google) - the sheer number of bytes
> copied from input to output during linking - reducing the amount of linker
> output written in the first place would be significantly beneficial.
> I would like to note that PoC implementation does exactly this. it reduces
> number of bytes copied from input to output during linking, It reduces the
> amount of linker output.
> Additionally, I measured memory usage of PoC implementation. Following
> table shows memory usage for linking clang :
> -----------------------------------------------
> |   |       CL options         |    Memory    |
> -----------------------------------------------
> | A | (default set of options*)|  9145880 kb |
> |   |                          |              |
> | B | A +gc-dbginfo            | 11881960 kb |
> |   |                          |              |
> | C | A +gc-dbginfo+gc-dbgtypes| 10690388 kb |
> |   |                          |              |
> | D | A +fdebug-types-section  |  8006032 kb |
> |   |                          |              |
> | E | D +gc-dbginfo            |  9000872 kb |
> |   |                          |              |
> | F | D +gc-dbginfo+gc-dbgtypes|  8994156 kb |
> -----------------------------------------------
> (though I do think/hope dsymutil's implementation could be
> adapted/generalized to be used in this situation - and I do have concerns
> that doing such non-trivial work at link time might not be a great tradeoff
> because the complexity and memory usage might be more than the savings,
> though I've no certainty one way or the other there)
>> On Wed, Sep 25, 2019 at 7:05 AM Alexey Lapshin <a.v.lapshin at mail.ru>
>> wrote:
>>> 24.09.2019 8:26, Rui Ueyama пишет:
>>> Hi Alexey,
>>> Thank you for sharing this proposal. Reducing the size of debug info is
>>> generally a good thing, and I believe you'd see more debug info size
>>> reduction in Rust programs than in C++ programs, because I heard that the
>>> Rust compiler driver passes a lot of object files to the linker, expecting
>>> that the linker would remove most of them, which leaves dead debug info.
>>> Hi Rui, Thanks!
>>> On Thu, Sep 12, 2019 at 7:32 AM Alexey Lapshin via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>> Debuginfo and linker folks, we (AccessSoftek) would like to suggest a
>>>> proposal for removing obsolete debug info. If you find it useful we will be
>>>> happy to work on improving it. Thank you for any opinions and suggestions.
>>>> Alexey.
>>>>     Currently when the linker does garbage collection a lot of
>>>> abandoned debug info is left behind (see Appendix A for documentation).
>>>> Besides inflated debug info size, we ended up with overlapping address
>>>> ranges and no way to say valid vs garbage ranges. We propose removing debug
>>>> info along with removing code. This would reduce debug info size and make
>>>> sure debug info accuracy.
>>>> There are several approaches which could be used to solve that problem:
>>>> 1.  Require dwarf producers to generate fragmented debug data according
>>>> to DWARF5 specification: "E.3.3 Single-function-per-DWARF-compilation-unit"
>>>> page 388. That approach assumes fragmenting the whole debug info per
>>>> function basis and glue fragmented sections at the link time using section
>>>> groups.
>>>> 2.  Use an additional tool, which would optimize out unnecessary debug
>>>> data, something similar to dwz (dwarf compressor tool), dsymutil (links the
>>>> DWARF debug information). This approach assumes additional post-link
>>>> binaries processing.
>>>> 3.  Teach the linker to parse debug data and let it remove unused debug
>>>> data.
>>>> In this proposal, we focus on approach #3. We show that this approach
>>>> is viable and discuss some preliminary results, leaving particular
>>>> implementation out of the scope. We attach the Proof of Concept (PoC)
>>>> implementation(https://reviews.llvm.org/D67469) for illustrative
>>>> purposes. Please keep in mind that it is not final, and there is room for
>>>> improvements (see Appendix B). However, the achieved results look quite
>>>> promising and demonstrate up to 2 times size reduction and performance
>>>> overhead is 30% of linking time (which is in the same ballpark as the
>>>> already done section compressing (see table 2 point F)).
>>> I believe #1 was added to DWARF5 to make link-time debug info GC
>>> possible, so could you tell me a little bit about why you chose to do #3?
>>> Is this because you want to do this for DWARF4?
>>>> No, that proposal is not DWARF-4 specific. The proposal is for DWARF-5
>>> also.  The solution added to DWARF-5("E.3.3
>>> Single-function-per-DWARF-compilation-unit" page 388.) is not a complete
>>> solution. This is a recommendation which needs to have an additional
>>> specification.
>>> There is -fdebug-types-section implementation which follows that
>>> recommendation.  Other cases(other than type units) do not easily fit into
>>> this recommendation. There are tables which have a common header. F.e.
>>> .debug_line, .debug_rnglists, .debug_addr. It is not clear how these tables
>>> could be separated between section groups.
>>> The more important thing is the fragmentation itself. Dividing debug
>>> tables into pieces would increase debug info size.
>>> It also would significantly complicate code working with debug info.
>>> F.e. include/llvm/DebugInfo/DWARF/DWARFObject.h has interface for class
>>> DWARFObject. It currently is not ready for the case when there could be
>>> multiple tables. Patch introducing support for multiple tables would be
>>> massive change affected many places in llvm codebase.
>>> Another thing is that not only the llvm code base but all other DWARF
>>> consumers should be changed to support fragmented debug info.
>>> Shortly, if all debug tables would be fragmented then working with debug
>>> info would be significantly complicated.
>>> Thus the reasons to select #3 are :
>>> 1. It could be done in a single place, not affecting other parts of the
>>> llvm code base.
>>> 2. It does not require other DWARF consumers to implement support for it.
>>> 3. Avoiding fragmentation would save space.
>>> 4. Processing of not fragmented debug info is faster.
>>> 5. No need to adapt DWARF tables for fragmentation. They could be
>>> handled with their current state.
>>> Alexey
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