[llvm-dev] Remove obsolete debug info while garbage collecting
Alexey Lapshin via llvm-dev
llvm-dev at lists.llvm.org
Wed Sep 25 14:56:02 PDT 2019
25.09.2019 9:21, Rui Ueyama пишет:
> 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.
Calling dsymutil as a separate invocation would have significant
overhead. Because linker would create a not optimized binary, later
dsymutil would load it again, parse, write optimized info out. Thus,
there would be additional disk space usage and performance loss. When I
said that dsymutil is not usable at
link-time, I meant a more tight integration case. It is not possible to
reuse the implementation of dsymutil inside linker at the current
moment. It is not possible to say(somewhere after markLive<ELFT>()) :
DwarfLinker.link(DebugMap) // DwarfLinker is a dsymutil class
and have the benefit of already loaded object files, already created
DebugMap and the stripped debug info sections by DwarfLinker.link(),
which later would be written out by lld.
Presented PoC implementation, in contrast, uses benefits of already
loaded object files, created liveness information, reduced output
binary. It allows achieving minimal disk usage and maximal execution speed.
>
> On Wed, Sep 25, 2019 at 7:05 AM Alexey Lapshin <a.v.lapshin at mail.ru
> <mailto: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 <mailto: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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