<div dir="ltr">Fair enough - thanks for clarifying the differences! (I'd still lean a bit towards this being dwz-esque, as you say "an extension of classic dwz" using a bit more domain knowledge (of terminators and C++ odr - though I'm not sure dsymutil does rely on the ODR, does it? It relies on it to know that two names represent the same type, I suppose, but doesn't assume they're already identical, instead it merges their members))<br><br>But I don't have super strong feelings about the naming.</div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Sep 1, 2020 at 6:36 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">
<div>
<p><br>
</p>
<div>On 01.09.2020 06:27, David Blaikie
wrote:<br>
</div>
<blockquote type="cite">
<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>
</div>
</blockquote>
It looks like dwz and llvm-dwarfutil are not exactly matched in
functionality. <br>
<br>
dwz is a program that attempts to optimize DWARF debugging
information <br>
contained in ELF shared libraries and ELF executables for *size*.<br>
<br>
llvm-dwarfutil is a tool that is used for processing debug<br>
info(DWARF) located in built binary files to improve debug info
*quality*,<br>
reduce debug info *size* and accelerate debug info *processing*.<br>
<br>
Things which are supposed to be done by llvm-dwarfutil and which are
not <br>
done by dwz: removing obsolete debug info, building indexes,
stripping <br>
unneeded debug sections, compress/decompress debug sections.<br>
<br>
Common thing is that both of these tools do debug info size
reduction. <br>
But they do this using different approaches:<br>
<br>
1. dwz reduces the size of debug info by creating partial
compilation units <br>
for duplicated parts. So that these partial compilation units
could be imported <br>
in every duplicated place. AFAIU, That optimization gives the
most size saving effect.<br>
<br>
another size saving optimization is ODR types deduplication.<br>
<br>
2. llvm-dwarfutil reduces the size of debug info by ODR types
deduplication <br>
which gives the most size saving effect in llvm-dwarfutil case. <br>
<br>
another size saving optimization is removing obsolete debug info.<br>
(which actually is not only about size but about correctness
also)<br>
<br>
So, it looks like these tools are not equal. If we would consider
that <br>
llvm-dwz is an extension of classic dwz then we could probably<br>
name it as llvm-dwz.<br>
<br>
<blockquote type="cite">
<div dir="ltr"><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" target="_blank">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>
</blockquote>
</div>
</blockquote></div>