[llvm-dev] [Proposal][Debuginfo] dsymutil-like tool for ELF.
Alexey via llvm-dev
llvm-dev at lists.llvm.org
Thu Aug 27 12:36:03 PDT 2020
On 27.08.2020 03:57, Jonas Devlieghere wrote:
>
>
> On Wed, Aug 26, 2020 at 7:01 AM Alexey <avl.lapshin at gmail.com
> <mailto: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.
right. That is understood. I did not prepare detailed plan on this yet.
Generally, I think, For the point 1 we need to have multi-thread version
of DeclContext.
Then at the first stage all CUs would be analyzed(in parallel) and
canonical DIEs are determined.
At second stage all CUs would be emitted(in parallel) into own
container. Finally all CUs
containers would be sequentially glued into resulting output and offsets
referring
to de-duplicated types should be changed to offset of canonical DIE (in
the similar
manner like it is currently done in CompileUnit::fixupForwardReferences()).
It sounds pretty similar to your above description.
It would be good if such "per CU" processing allows not only speed up
execution time
but minimize memory requirements. So that it would not be necessary to
load all DIEs at a time.
But that is not clear how to do yet. Because of DW_FORM_ref_addr
attributes. Until we analyzed
all CUs(to understand whether we need to keep referenced dies) we could
not start cloning stage.
> 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 <mailto: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 <mailto:llvm-dev at lists.llvm.org>
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
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