[llvm-dev] [Proposal][Debuginfo] dsymutil-like tool for ELF.
David Blaikie via llvm-dev
llvm-dev at lists.llvm.org
Mon Aug 31 20:24:15 PDT 2020
On Fri, Aug 28, 2020 at 2:24 PM James Y Knight <jyknight at google.com> wrote:
> If we're designing a new tool and process, it would be wonderful if it did
> not require multiple stages of copying and slightly modifying the binary,
> in order to create final output with separate debug info. It seems to me
> that the variants of this sort of thing which exist today are somewhat
> suboptimal.
>
> With Mach-O and dsymutil:
> 1. Given a collection of object files (which contain debuginfo), link a
> binary with ld. The binary then includes special references to the object
> files that were actually used as part of the link.
> 2. Given the linked binary, and all of the same object files, link the
> debuginfo with dsymutil.
> 3. Strip the references to the object file paths from the binary.
> Finally, you have a binary without debug info, and a dsym debuginfo
> file. But it would be better if the binary created in step 1 didn't need to
> include the extraneous object-file path info, and that was instead emitted
> in a second file. Then we wouldn't need step 3.
>
> With "normal" ELF:
> 1. Given a collection of object files (which contain debuginfo), link a
> binary with ld, which includes linking all the debug info into the binary.
> 2. Given the linked binary, objcopy --only-keep-debug to create a new
> separated debug file.
> 3. Given the linked binary, objcopy --strip-debug to create a copy of
> the binary without debug info.
> Finally you have a binary without debug info, and a separate debug file.
> But it would be better if the linker could just write the debug info into a
> separate file in the first place, then we'd only have the one step. (But,
> downside, the linker needs to manage all the debug info, which can be
> excessively large.)
>
> With "split-dwarf" ELF support:
> 1. Given object files (which exclude *most* but not all of the
> debuginfo), link a binary. The binary will include that smaller set of
> debug info.
> 2. Given the collection of dwo files corresponding to the object
> files, run the "dwp" tool to create a dwp file.
> 3. objcopy --only-keep-debug
> 4. --strip-debug
> And then you need to keep both a debug file *and* a dwp file, which is
> weird.
>
>
> I think, ideally, users would have the following three *good* options:
> Easy option: store debuginfo in the object files, and have the linker
> create a pair of {binary, separated dwarf-optimized debuginfo} files
> directly from the object files.
>
(as discussed by other replies - that was an early proposal, didn't gain a
lot of traction/Eric & Ray weren't super convinced it was worth adding to
lld at this stage, given the link time cost & thus the small expected user
base)
> More scalable option: emit (most of the) debuginfo in separate *.dwo
> files using -gsplit-dwarf, and then,
> 1. run the linker on the object files to create a pair of {binary,
> separated debuginfo} files. In this case the latter file contains the
> minimal debuginfo which was in the object files.
>
Yeah, that ^ is probably a nice feature regardless. Save folks an extra
objcopy, etc. Usable right now for any build that is already running
only-keep-debug/strip-debug.
> 2. run a second tool, which reads the minimal debuginfo from above,
> and all the DWO files, and creates a full optimized/deduplicated debuginfo
> output file.
>
Fair - this then looks a lot like the MachO debug info distribution/linking
model (with the advantage that the DWARF isn't in the .o files, so doesn't
have to be shipped to the machine doing the linking), so far as I know.
> Faster developer builds: Like previous, but omit step 2 -- running the
> debugger directly after step 1 can use the dwo files on-disk.
>
> I think we're not terribly far from that ideal, now, for ELF. Maybe only
> these three things need to be done? --
> 1. Teach lld how to emit a separated debuginfo output file directly,
> without requiring an objcopy step.
> 2. Integrate DWARFLinker into lld.
> 3. Create a new tool which takes the separated debuginfo and DWO/DWP
> files and uses DWARFLinker library to create a new (dwarf-linked)
> separated-debug file, that doesn't depend on DWO/DWP files.
>
> My hope is that the tool you're creating will be the implementation of #3,
> but I'm afraid the intent is for this tool to be an additional stage that
> non-split-dwarf users would need to run post-link, *instead of*
> integrating DWARFLinker into lld.
>
Yeah, that's the direction lld folks have pushed for - a post-processing,
rather than link-time. Mostly due to the current performance of DWARF-aware
linking being quite slow, so the idea that not many users would be willing
to take that link-time performance hit to use the feature. (whereas as a
post-processing step before archiving DWARF (like building a dwp from dwo
files) it might be more appealing/interesting - and maybe with sufficient
performance improvements, could then be rolled into lld as originally
proposed)
Curiously Alexey's needs include not wanting to use fission because a
single debuggable binary simplifies his users use-case/makes it easier to
distribute than two files. So he's probably not interested in the
strip-debug/only-keep-debug kind of debug info distribution model, at least
for his own users/use case. So far as I understand it.
I've got mixed feelings about that - and encourage you to
express/clarify/discuss your thoughts here, as I think the whole
conversation could use some more voices.
- Dave
>
> On Tue, Aug 25, 2020 at 10:29 AM 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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