[llvm-dev] [Proposal][Debuginfo] dsymutil-like tool for ELF.
Jonas Devlieghere via llvm-dev
llvm-dev at lists.llvm.org
Wed Aug 26 17:57:48 PDT 2020
On Wed, Aug 26, 2020 at 7:01 AM Alexey <avl.lapshin at gmail.com> wrote:
>
> On 26.08.2020 10:58, James Henderson wrote:
>
> In principle, this sounds reasonable to me. I don't know enough about
> dsymutil's interface to know whether it makes sense to try to make it
> multi-format compatible or not. If it doesn't I'm perfectly happy for a new
> tool to be added using the DWARFLinker library.
>
> Some more general thoughts:
> 1) Assuming the proposal is accepted, this should be introduced piecemeal
> into LLVM from the beginning as it is developed, rather than having a
> separate step 4 in the roadmap.
> 2) The default tombstone values used for dead debug data should be those
> produced by LLD, in my opinion. In an ideal world, we'd factor them into
> some shared constant. Note that at the time of writing, I believe LLD is
> currently using BFD-style tombstones, not the new -1/-2.
>
> agreed.
>
> 3) Does the DWARFLinker library already support multi-threading? If not,
> it might be a lot of work making things thread-safe.
>
> It does, but in a limited way. It can parallelize analyzing and cloning
> stages. i.e. the maximal speedup is two times.
>
> To have a greater performance impact it could probably be parallelized per
> compilation unit basis.
>
I want to elaborate on this a bit as it's been coming up several times now.
With the current design you cannot process CUs in parallel. There are two
reasons for that:
1. When uniquing types, the first time a new type is encountered, it is
marked
as canonical. Every subsequent encounter of that type is replaced by a
reference to the canonical type which is going to be coming from another
CU.
So to process CUs in parallel, you have to guarantee the reproducibility
of
what that canonical DIE is going to be, or postpone this to a sequential
step, which would potentially defeat the purpose of the parallel
analysis.
2. During emission, we emit the offset of the canonical type in the output
directly which allows us to stream out the DWARF. That means we need to
know
the offset when processing the places where the uniquing is removing the
full type, which implies there’s a sequencing between cloning the CU with
the canonical type and processing further uses. Without this property,
we'd
also need to keep all the output DIEs in memory until the offset can be
computed, or we need to do another iteration to patch up the offsets.
I'm not saying this to discourage you, quite the opposite actually. I'd
love to
be able to speed-up dsymutil. I'm just sharing this based on my experience
when
adding the current concurrency which starts analyzing the next CU when we
finished processing the current one and are emitting it.
I'm sure we could design the uniquing algorithm in a way that would be able
to
process CUs in parallel and gather the types, then synchronize to make a
uniquing decision, then clone all CUs in parallel and finally relocate all
the
offsets to the canonical DIE. In the current algorithm it’s just not that
simple, because you don’t know whether a type is going to be kept in a
particular CU before having processed it completely.
> Another thing is that dsymutil currently loads all DIEs from source object
> file into the memory. And releases them after object file is processed. For
> non-linked binary this works OK(big binaries usually compiled from several
> object files). For linked binary that means all DIEs are loaded into the
> memory. In the result it requires a lot of memory resources. The solution
> for this problem could be changing splitting of source data from the file
> to the compilation unit basis.
>
> yes, making dsymutil/dwarfutil to work on compilation unit basis
> supporting multi-threading is a quite a big piece of work. It looks like it
> would be good for both dsymutil and dwarfutil.
>
> 4) Given that DWARF v6 doesn't exist yet, I wouldn't include that as an
> option name just yet...!
>
> Would "maxpc" be OK? --tombstone=maxpc ?
>
>
> Thanks for looking at this! Please keep me involved in any related reviews
> etc.
>
> sure. Thank you for the comments.
>
> Alexey.
>
>
>
> James
>
> On Tue, 25 Aug 2020 at 15:29, Alexey via llvm-dev <llvm-dev at lists.llvm.org>
> wrote:
>
>> Hi,
>>
>> We propose llvm-dwarfutil - a dsymutil-like tool for ELF.
>> Any thoughts on this?
>> Thanks in advance, Alexey.
>>
>> ======================================================================
>>
>> llvm-dwarfutil(Apndx A) - is a tool that is used for processing debug
>> info(DWARF)
>> located in built binary files to improve debug info quality,
>> reduce debug info size and accelerate debug info processing.
>> Supported object files formats: ELF, MachO(Apndx B), COFF(Apndx C),
>> WASM(Apndx C).
>>
>> ======================================================================
>>
>> Specifically, the tool would do:
>>
>> - Remove obsolete debug info which refers to code deleted by the linker
>> doing the garbage collection (gc-sections).
>>
>> - Deduplicate debug type definitions for reducing resulting size of
>> binary.
>>
>> - Build accelerator/index tables.
>> = .debug_aranges, .debug_names, .gdb_index, .debug_pubnames,
>> .debug_pubtypes.
>>
>> - Strip unneeded tables.
>> = .debug_aranges, .debug_names, .gdb_index, .debug_pubnames,
>> .debug_pubtypes.
>>
>> - Compress or decompress debug info as requested.
>>
>> Possible feature:
>>
>> - Join split dwarf .dwo files in a single file containing all debug
>> info
>> (convert split DWARF into monolithic DWARF).
>>
>> ======================================================================
>>
>> User interface:
>>
>> OVERVIEW: A tool for optimizing debug info located in the built binary.
>>
>> USAGE: llvm-dwarfutil [options] input output
>>
>> OPTIONS: (Apndx E)
>>
>> ======================================================================
>>
>> Implementation notes:
>>
>> 1. Removing obsolete debug info would be done using DWARFLinker llvm
>> library.
>>
>> 2. Data types deduplication would be done using DWARFLinker llvm library.
>>
>> 3. Accelerator/index tables would be generated using DWARFLinker llvm
>> library.
>>
>> 4. Interface of DWARFLinker library would be changed in such way that it
>> would be possible to switch on/off various stages:
>>
>> class DWARFLinker {
>> setDoRemoveObsoleteInfo ( bool DoRemoveObsoleteInfo = false);
>>
>> setDoAppleNames ( bool DoAppleNames = false );
>> setDoAppleNamespaces ( bool DoAppleNamespaces = false );
>> setDoAppleTypes ( bool DoAppleTypes = false );
>> setDoObjC ( bool DoObjC = false );
>> setDoDebugPubNames ( bool DoDebugPubNames = false );
>> setDoDebugPubTypes ( bool DoDebugPubTypes = false );
>>
>> setDoDebugNames (bool DoDebugNames = false);
>> setDoGDBIndex (bool DoGDBIndex = false);
>> }
>>
>> 5. Copying source file contents, stripping tables,
>> compressing/decompressing tables
>> would be done by ObjCopy llvm library(extracted from llvm-objcopy):
>>
>> Error executeObjcopyOnBinary(const CopyConfig &Config,
>> object::COFFObjectFile &In, Buffer &Out);
>> Error executeObjcopyOnBinary(const CopyConfig &Config,
>> object::ELFObjectFileBase &In, Buffer &Out);
>> Error executeObjcopyOnBinary(const CopyConfig &Config,
>> object::MachOObjectFile &In, Buffer &Out);
>> Error executeObjcopyOnBinary(const CopyConfig &Config,
>> object::WasmObjectFile &In, Buffer &Out);
>>
>> 6. Address ranges and single addresses pointing to removed code should
>> be marked
>> with tombstone value in the input file:
>>
>> -2 for .debug_ranges and .debug_loc.
>> -1 for other .debug* tables.
>>
>> 7. Prototype implementation - https://reviews.llvm.org/D86539.
>>
>> ======================================================================
>>
>> Roadmap:
>>
>> 1. Refactor llvm-objcopy to extract it`s implementation into separate
>> library
>> ObjCopy(in LLVM tree).
>>
>> 2. Create a command line utility using existed DWARFLinker and ObjCopy
>> implementation. First version is supposed to work with only ELF
>> input object files.
>> It would take input ELF file with unoptimized debug info and create
>> output
>> ELF file with optimized debug info. That version would be done out
>> of the llvm tree.
>>
>> 3. Make a tool to be able to work in multi-thread mode.
>>
>> 4. Consider it to be included into LLVM tree.
>>
>> 5. Support DWARF5 tables.
>>
>> ======================================================================
>>
>> Appendix A. Should this tool be implemented as a new tool or as an
>> extension
>> to dsymutil/llvm-objcopy?
>>
>> There already exists a tool which removes obsolete debug info on
>> darwin - dsymutil.
>> Why create another tool instead of extending the already existed
>> dsymutil/llvm-objcopy?
>>
>> The main functionality of dsymutil is located in a separate library
>> - DWARFLinker.
>> Thus, dsymutil utility is a command-line interface for DWARFLinker.
>> dsymutil has
>> another type of input/output data: it takes several object files and
>> address map
>> as input and creates a .dSYM bundle with linked debug info as
>> output. llvm-dwarfutil
>> would take a built executable as input and create an optimized
>> executable as output.
>> Additionally, there would be many command-line options specific for
>> only one utility.
>> This means that these utilities(implementing command line interface)
>> would significantly
>> differ. It makes sense not to put another command-line utility
>> inside existing dsymutil,
>> but make it as a separate utility. That is the reason why
>> llvm-dwarfutil suggested to be
>> implemented not as sub-part of dsymutil but as a separate tool.
>>
>> Please share your preference: whether llvm-dwarfutil should be
>> separate utility, or a variant of dsymutil compiled for ELF?
>>
>> ======================================================================
>>
>> Appendix B. The machO object file format is already supported by dsymutil.
>> Depending on the decision whether llvm-dwarfutil would be done as a
>> subproject
>> of dsymutil or as a separate utility - machO would be supported or
>> not.
>>
>> ======================================================================
>>
>> Appendix C. Support for the COFF and WASM object file formats presented as
>> possible future improvement. It would be quite easy to add them
>> assuming
>> that llvm-objcopy already supports these formats. It also would
>> require
>> supporting DWARF6-suggested tombstone values(-1/-2).
>>
>> ======================================================================
>>
>> Appendix D. Documentation.
>>
>> - proposal for DWARF6 which suggested -1/-2 values for marking bad
>> addresses
>> http://www.dwarfstd.org/ShowIssue.php?issue=200609.1
>> - dsymutil tool https://llvm.org/docs/CommandGuide/dsymutil.html.
>> - proposal "Remove obsolete debug info in lld."
>> http://lists.llvm.org/pipermail/llvm-dev/2020-May/141468.html
>>
>> ======================================================================
>>
>> Appendix E. Possible command line options:
>>
>> DwarfUtil Options:
>>
>> --build-aranges - generate .debug_aranges table.
>> --build-debug-names - generate .debug_names table.
>> --build-debug-pubnames - generate .debug_pubnames table.
>> --build-debug-pubtypes - generate .debug_pubtypes table.
>> --build-gdb-index - generate .gdb_index table.
>> --compress - Compress debug tables.
>> --decompress - Decompress debug tables.
>> --deduplicate-types - Do ODR deduplication for debug types.
>> --garbage-collect - Do garbage collecting for debug info.
>> --num-threads=<n> - Specify the maximum number (n) of
>> simultaneous threads
>> to use when optimizing input file.
>> Defaults to the number of cores on the
>> current machine.
>> --strip-all - Strip all debug tables.
>> --strip=<name1,name2> - Strip specified debug info tables.
>> --strip-unoptimized-debug - Strip all unoptimized debug tables.
>> --tombstone=<value> - Tombstone value used as a marker of
>> invalid address.
>> =bfd - BFD default value
>> =dwarf6 - Dwarf v6.
>> --verbose - Enable verbose logging and encoding
>> details.
>>
>> Generic Options:
>>
>> --help - Display available options (--help-hidden
>> for more)
>> --version - Display the version of this program
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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