<div dir="ltr">A quick note: The feature as currently proposed sounds like it's an exact match for 'dwz'? Is there any benefit to this over the existing dwz project? Is it different in some ways I'm not aware of? (I haven't actually used dwz, so I might have some mistaken ideas about how it should work)<br><br>If it's going to solve the same general problem, but be in the llvm project instead, then maybe it should be called llvm-dwz.<br><br>Though I understand the desire for this to grow other functionality, like DWARF-aware dwp-ing. Might be better for this to busybox and provide that functionality under llvm-dwp instead, or more likely I Suspect, that the existing llvm-dwp will be rewritten (probably by me) to use more of lld's infrastructure to be more efficient (it's current object reading/writing logic is using LLVM's libObject and MCStreamer, which is a bit inefficient for a very content-unaware linking process) and then maybe that could be taught to use DwarfLinker as a library to optionally do DWARF-aware linking depending on the users time/space tradeoff desires. Still benefiting from any improvements to the underlying DwarfLinker library (at which point that would be shared between llvm-dsymutil, llvm-dwz, and llvm-dwp).</div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Aug 25, 2020 at 7:29 AM Alexey <<a href="mailto:avl.lapshin@gmail.com">avl.lapshin@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi,<br>
<br>
We propose llvm-dwarfutil - a dsymutil-like tool for ELF.<br>
Any thoughts on this?<br>
Thanks in advance, Alexey.<br>
<br>
======================================================================<br>
<br>
llvm-dwarfutil(Apndx A) - is a tool that is used for processing debug <br>
info(DWARF)<br>
located in built binary files to improve debug info quality,<br>
reduce debug info size and accelerate debug info processing.<br>
Supported object files formats: ELF, MachO(Apndx B), COFF(Apndx C), <br>
WASM(Apndx C).<br>
<br>
======================================================================<br>
<br>
Specifically, the tool would do:<br>
<br>
- Remove obsolete debug info which refers to code deleted by the linker<br>
doing the garbage collection (gc-sections).<br>
<br>
- Deduplicate debug type definitions for reducing resulting size of <br>
binary.<br>
<br>
- Build accelerator/index tables.<br>
= .debug_aranges, .debug_names, .gdb_index, .debug_pubnames, <br>
.debug_pubtypes.<br>
<br>
- Strip unneeded tables.<br>
= .debug_aranges, .debug_names, .gdb_index, .debug_pubnames, <br>
.debug_pubtypes.<br>
<br>
- Compress or decompress debug info as requested.<br>
<br>
Possible feature:<br>
<br>
- Join split dwarf .dwo files in a single file containing all debug info<br>
(convert split DWARF into monolithic DWARF).<br>
<br>
======================================================================<br>
<br>
User interface:<br>
<br>
OVERVIEW: A tool for optimizing debug info located in the built binary.<br>
<br>
USAGE: llvm-dwarfutil [options] input output<br>
<br>
OPTIONS: (Apndx E)<br>
<br>
======================================================================<br>
<br>
Implementation notes:<br>
<br>
1. Removing obsolete debug info would be done using DWARFLinker llvm <br>
library.<br>
<br>
2. Data types deduplication would be done using DWARFLinker llvm library.<br>
<br>
3. Accelerator/index tables would be generated using DWARFLinker llvm <br>
library.<br>
<br>
4. Interface of DWARFLinker library would be changed in such way that it<br>
would be possible to switch on/off various stages:<br>
<br>
class DWARFLinker {<br>
setDoRemoveObsoleteInfo ( bool DoRemoveObsoleteInfo = false);<br>
<br>
setDoAppleNames ( bool DoAppleNames = false );<br>
setDoAppleNamespaces ( bool DoAppleNamespaces = false );<br>
setDoAppleTypes ( bool DoAppleTypes = false );<br>
setDoObjC ( bool DoObjC = false );<br>
setDoDebugPubNames ( bool DoDebugPubNames = false );<br>
setDoDebugPubTypes ( bool DoDebugPubTypes = false );<br>
<br>
setDoDebugNames (bool DoDebugNames = false);<br>
setDoGDBIndex (bool DoGDBIndex = false);<br>
}<br>
<br>
5. Copying source file contents, stripping tables, <br>
compressing/decompressing tables<br>
would be done by ObjCopy llvm library(extracted from llvm-objcopy):<br>
<br>
Error executeObjcopyOnBinary(const CopyConfig &Config,<br>
object::COFFObjectFile &In, Buffer &Out);<br>
Error executeObjcopyOnBinary(const CopyConfig &Config,<br>
object::ELFObjectFileBase &In, Buffer &Out);<br>
Error executeObjcopyOnBinary(const CopyConfig &Config,<br>
object::MachOObjectFile &In, Buffer &Out);<br>
Error executeObjcopyOnBinary(const CopyConfig &Config,<br>
object::WasmObjectFile &In, Buffer &Out);<br>
<br>
6. Address ranges and single addresses pointing to removed code should <br>
be marked<br>
with tombstone value in the input file:<br>
<br>
-2 for .debug_ranges and .debug_loc.<br>
-1 for other .debug* tables.<br>
<br>
7. Prototype implementation - <a href="https://reviews.llvm.org/D86539" rel="noreferrer" target="_blank">https://reviews.llvm.org/D86539</a>.<br>
<br>
======================================================================<br>
<br>
Roadmap:<br>
<br>
1. Refactor llvm-objcopy to extract it`s implementation into separate <br>
library<br>
ObjCopy(in LLVM tree).<br>
<br>
2. Create a command line utility using existed DWARFLinker and ObjCopy<br>
implementation. First version is supposed to work with only ELF <br>
input object files.<br>
It would take input ELF file with unoptimized debug info and create <br>
output<br>
ELF file with optimized debug info. That version would be done out <br>
of the llvm tree.<br>
<br>
3. Make a tool to be able to work in multi-thread mode.<br>
<br>
4. Consider it to be included into LLVM tree.<br>
<br>
5. Support DWARF5 tables.<br>
<br>
======================================================================<br>
<br>
Appendix A. Should this tool be implemented as a new tool or as an extension<br>
to dsymutil/llvm-objcopy?<br>
<br>
There already exists a tool which removes obsolete debug info on <br>
darwin - dsymutil.<br>
Why create another tool instead of extending the already existed <br>
dsymutil/llvm-objcopy?<br>
<br>
The main functionality of dsymutil is located in a separate library <br>
- DWARFLinker.<br>
Thus, dsymutil utility is a command-line interface for DWARFLinker. <br>
dsymutil has<br>
another type of input/output data: it takes several object files and <br>
address map<br>
as input and creates a .dSYM bundle with linked debug info as <br>
output. llvm-dwarfutil<br>
would take a built executable as input and create an optimized <br>
executable as output.<br>
Additionally, there would be many command-line options specific for <br>
only one utility.<br>
This means that these utilities(implementing command line interface) <br>
would significantly<br>
differ. It makes sense not to put another command-line utility <br>
inside existing dsymutil,<br>
but make it as a separate utility. That is the reason why <br>
llvm-dwarfutil suggested to be<br>
implemented not as sub-part of dsymutil but as a separate tool.<br>
<br>
Please share your preference: whether llvm-dwarfutil should be<br>
separate utility, or a variant of dsymutil compiled for ELF?<br>
<br>
======================================================================<br>
<br>
Appendix B. The machO object file format is already supported by dsymutil.<br>
Depending on the decision whether llvm-dwarfutil would be done as a <br>
subproject<br>
of dsymutil or as a separate utility - machO would be supported or not.<br>
<br>
======================================================================<br>
<br>
Appendix C. Support for the COFF and WASM object file formats presented as<br>
possible future improvement. It would be quite easy to add them <br>
assuming<br>
that llvm-objcopy already supports these formats. It also would require<br>
supporting DWARF6-suggested tombstone values(-1/-2).<br>
<br>
======================================================================<br>
<br>
Appendix D. Documentation.<br>
<br>
- proposal for DWARF6 which suggested -1/-2 values for marking bad <br>
addresses<br>
<a href="http://www.dwarfstd.org/ShowIssue.php?issue=200609.1" rel="noreferrer" target="_blank">http://www.dwarfstd.org/ShowIssue.php?issue=200609.1</a><br>
- dsymutil tool <a href="https://llvm.org/docs/CommandGuide/dsymutil.html" rel="noreferrer" target="_blank">https://llvm.org/docs/CommandGuide/dsymutil.html</a>.<br>
- proposal "Remove obsolete debug info in lld."<br>
<a href="http://lists.llvm.org/pipermail/llvm-dev/2020-May/141468.html" rel="noreferrer" target="_blank">http://lists.llvm.org/pipermail/llvm-dev/2020-May/141468.html</a><br>
<br>
======================================================================<br>
<br>
Appendix E. Possible command line options:<br>
<br>
DwarfUtil Options:<br>
<br>
--build-aranges - generate .debug_aranges table.<br>
--build-debug-names - generate .debug_names table.<br>
--build-debug-pubnames - generate .debug_pubnames table.<br>
--build-debug-pubtypes - generate .debug_pubtypes table.<br>
--build-gdb-index - generate .gdb_index table.<br>
--compress - Compress debug tables.<br>
--decompress - Decompress debug tables.<br>
--deduplicate-types - Do ODR deduplication for debug types.<br>
--garbage-collect - Do garbage collecting for debug info.<br>
--num-threads=<n> - Specify the maximum number (n) of <br>
simultaneous threads<br>
to use when optimizing input file.<br>
Defaults to the number of cores on the <br>
current machine.<br>
--strip-all - Strip all debug tables.<br>
--strip=<name1,name2> - Strip specified debug info tables.<br>
--strip-unoptimized-debug - Strip all unoptimized debug tables.<br>
--tombstone=<value> - Tombstone value used as a marker of <br>
invalid address.<br>
=bfd - BFD default value<br>
=dwarf6 - Dwarf v6.<br>
--verbose - Enable verbose logging and encoding details.<br>
<br>
Generic Options:<br>
<br>
--help - Display available options (--help-hidden <br>
for more)<br>
--version - Display the version of this program<br>
<br>
</blockquote></div>