[llvm-dev] [Proposal][Debuginfo] dsymutil-like tool for ELF.
James Henderson via llvm-dev
llvm-dev at lists.llvm.org
Wed Aug 26 00:58:53 PDT 2020
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.
3) Does the DWARFLinker library already support multi-threading? If not, it
might be a lot of work making things thread-safe.
4) Given that DWARF v6 doesn't exist yet, I wouldn't include that as an
option name just yet...!
Thanks for looking at this! Please keep me involved in any related reviews
etc.
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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