<div dir="ltr"><div>The script included in the patch can be used to convert an object containing normal DWARF into an object using fragmented DWARF. It does this by using llvm-dwarfdump to dump the various sections, parses the output to identify where it should split (using the offsets of the various entries), and then writes new section headers accordingly - you can see roughly what it's doing if you get a chance to watch the talk recording. The additional section headers are appended to the end of the ELF section header table, whilst the original DWARF is left in the same place it was before (making use of the fact that section headers don't have to appear in offset order). The script also parses and fragments the relocation sections targeting the DWARF sections so that they match up with the fragmented DWARF sections. This is clearly all suboptimal - in practice the compiler should be modified to do the fragmenting upfront, to save having to parse a tool's stdout, but that was just the simplest thing I could come up with to quickly write the script. Full details of the script usage are included in the patch description, if you want to play around with it.</div><div><br></div><div>If Alexey could point me at the latest version of his patch, I'd be happy to run that through either or both of the packages I used to see what happens. Equally, I'd be happy if Alexey is able to run my script to fragment and measure the performance of a couple of projects he's been working with. Based purely on the two packages I've tried this with, I can tell already that the results can vary wildly. My expectation is that Alexey's approach will be slower (at least in its current form, but probably more generally), but produce smaller output, but to what scale I have no idea.<br></div><div><br></div><div>I think linkers parse .eh_frame partly because they have no other choice. That being said, I think it's format is not too complex, so similarly the parser isn't too complex. You can see LLD's ELF implementation in ELF/EhFrame.cpp, how it is used in ELF/InputSection.cpp (see the bits to do with EhInputSection) and EhFrameSection in ELF/SyntheticSections.h (plus various usages of these two throughout the LLD code). I think the key to any structural changes in the DWARF format to make them more amenable to link-time parsing is being able to read a minimal amount without needing to parse the payload (e.g. a length field, some sort of type, and then using the relocations to associate it accordingly).</div><div><br></div><div>James<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, 12 Oct 2020 at 20:48, David Blaikie <<a href="mailto:dblaikie@gmail.com">dblaikie@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 dir="ltr">Awesome! Sorry I missed the lightning talk, but really interested to see this sort of thing (though it's not directly/immediately applicable to the use case I work with - Split DWARF, something similar could be used there with further work)<br><br>Though it looks like the patch has mostly linker changes - where/how do you generate the fragmented DWARF to begin with? Via the Python script? Run over assembly? I'd be surprised if it was achievable that way - curious to know more.<br><br>Got a rough sense/are you able to run apples-to-apples comparisons with Alexey's linker-based patches to compare linker time/memory overhead versus resulting output size gains?<br><br>(& yeah, I'm a bit curious about how the linkers do eh_frame rewriting, if the format is especially amenable to a lightweight parsing/rewriting and how we could make the DWARF more amenable to that too)</div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Oct 12, 2020 at 6:41 AM James Henderson <<a href="mailto:jh7370.2008@my.bristol.ac.uk" target="_blank">jh7370.2008@my.bristol.ac.uk</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 dir="ltr"><div>Hi all,</div><div><br></div><div>At the recent LLVM developers' meeting, I presented a lightning talk on an approach to reduce the amount of dead debug data left in an executable following operations such as --gc-sections and duplicate COMDAT removal. In that presentation, I presented some figures based on linking a game that had been built by our downstream clang port and fragmented using the described approach. Since recording the presentation, I ran the same experiment on a clang package (this time built with a GCC version). The comparable figures are below:</div><div><br></div><div>Link-time speed (s):</div><div><span style="font-family:monospace">+--------------------+-------+---------------+------+------+------+------+------+</span><br></div><div><font face="monospace">| Package variant | No GC | GC 1 (normal) | GC 2 | GC 3 | GC 4 | GC 5 | GC 6 |</font></div><div><font face="monospace">+--------------------+-------+---------------+------+------+------+------+------+<br></font></div><div><font face="monospace">| Game (plain) | 4.5 | 4.9 | 4.2 | 3.6 | 3.4 | 3.3 | 3.2 |<br></font></div><div><font face="monospace">| Game (fragmented) | 11.1 | 11.8 | 9.7 | 8.6 | 7.9 | 7.7 | 7.5 |<br></font></div><div><font face="monospace">| Clang (plain) | 13.9 | 17.9 | 17.0 | 16.7 | 16.3 | 16.2 | 16.1 |<br></font></div><div><font face="monospace">| Clang (fragmented) | 18.6 | 22.8 | 21.6 | 21.1 | 20.8 | 20.5 | 20.2 |</font></div><div><font face="monospace">+--------------------+-------+---------------+------+------+------+------+------+</font></div><div><font face="monospace"><br></font></div><div><font face="monospace"><font face="arial,sans-serif">Output size - Game package (MB):</font></font></div><div><font face="monospace"><font face="arial,sans-serif"><span style="font-family:monospace">+---------------------+-------+------+------+------+------+------+------+</span><br></font></font></div><div><span style="font-family:monospace">| Category | No GC | GC 1 | GC 2 | GC 3 | GC 4 | GC 5 | GC 6 |<br></span></div><div><span style="font-family:monospace">+---------------------+-------+------+------+------+------+------+------+<br></span></div><div><span style="font-family:monospace">| Plain (total) | 1149 | 1121 | 1017 | 965 | 938 | 930 | 928 |<br></span></div><div><span style="font-family:monospace">| Plain (DWARF*) | 845 | 845 | 845 | 845 | 845 | 845 | 845 |<br></span></div><div><span style="font-family:monospace">| Plain (other) | 304 | 276 | 172 | 120 | 93 | 85 | 82 |<br></span></div><div><span style="font-family:monospace">| Fragmented (total) | 1044 | 940 | 556 | 373 | 287 | 263 | 255 |<br></span></div><div><span style="font-family:monospace">| Fragmented (DWARF*) | 740 | 664 | 384 | 253 | 194 | 178 | 173 |<br></span></div><div><span style="font-family:monospace">| Fragmented (other) | 304 | 276 | 172 | 120 | 93 | 85 | 82 |<br></span></div><div><font face="monospace">+---------------------+-------+------+------+------+------+------+------+
</font><div><font face="monospace"><br></font></div><div><font face="monospace"><font face="arial,sans-serif">Output size - Clang (MB):</font></font></div><div><font face="monospace"><font face="arial,sans-serif"><span style="font-family:monospace">+---------------------+-------+------+------+------+------+------+------+</span><br></font></font></div><div><span style="font-family:monospace">| Category | No GC | GC 1 | GC 2 | GC 3 | GC 4 | GC 5 | GC 6 |<br></span></div><div><span style="font-family:monospace">+---------------------+-------+------+------+------+------+------+------+<br></span></div><div><span style="font-family:monospace">| Plain (total) | 2596 | 2546 | 2406 | 2332 | 2293 | 2273 | 2251 |<br></span></div><div><span style="font-family:monospace">| Plain (DWARF*) | 1979 | 1979 | 1979 | 1979 | 1979 | 1979 | 1979 |<br></span></div><div><span style="font-family:monospace">| Plain (other) | 616 | 567 | 426 | 353 | 314 | 294 | 272 |<br></span></div><div><span style="font-family:monospace">| Fragmented (total) | 2397 | 2346 | 2164 | 2069 | 2017 | 1990 | 1963 |<br></span></div><div><span style="font-family:monospace">| Fragmented (DWARF*) | 1780 | 1780 | 1738 | 1716 | 1703 | 1696 | 1691 |<br></span></div><div><span style="font-family:monospace">| Fragmented (other) | 616 | 567 | 426 | 353 | 314 | 294 | 272 |<br></span></div><div><font face="monospace">+---------------------+-------+------+------+------+------+------+------+</font></div>
</div><div><font face="monospace"><br></font></div><div><font face="monospace">*DWARF size == total size of .debug_info + .debug_line + .debug_ranges + .debug_aranges + .debug_loc<br></font></div><div><font face="monospace"><br></font></div><div><font face="monospace"><font face="arial,sans-serif">Additionally, I have posted <a href="https://reviews.llvm.org/D89229" target="_blank">https://reviews.llvm.org/D89229</a> which provides the python script and linker patches used to reproduce the above results on my machine. The GC 1/2/3/4/5/6 correspond to the linker option added in that patch --mark-live-pc with values 1/0.8/0.6/0.4/0.2/0 respectively.<br></font></font></div><div><font face="monospace"><font face="arial,sans-serif"><br></font></font></div><div><font face="monospace"><font face="arial,sans-serif">During the conference, the question was asked what the memory usage and input size impact was. I've summarised these below:</font></font></div><div><font face="monospace"><font face="arial,sans-serif"><br></font></font></div><div><font face="monospace"><font face="arial,sans-serif">Input file size total (GB):</font></font></div><div><font face="monospace"><font face="arial,sans-serif">
<span style="font-family:monospace">+--------------------+------------+
</span></font></font></div><div><span style="font-family:monospace">
| Package variant | Total Size | <br></span></div><div><span style="font-family:monospace">
+--------------------+------------+<br></span></div><div><span style="font-family:monospace">
| Game (plain) | 2.9 | <br></span></div><div><span style="font-family:monospace">
| Game (fragmented) | 4.2 |<br></span></div><div><span style="font-family:monospace">
| Clang (plain) | 10.9 |<br></span></div><div><span style="font-family:monospace">
| Clang (fragmented) | 12.3 |<br></span></div><div><span style="font-family:monospace">
+--------------------+------------+</span></div><div><span style="font-family:monospace"><br></span></div><div><span style="font-family:monospace"><span style="font-family:arial,sans-serif">Peak Working Set Memory usage (GB):</span><br></span></div><div><span style="font-family:monospace">
</span><div><font face="monospace"><font face="arial,sans-serif"><span style="font-family:monospace">+--------------------+-------+------+
</span></font></font></div><div><span style="font-family:monospace">
| Package variant | No GC | GC 1 |<br></span></div><div><span style="font-family:monospace">
+--------------------+-------+------+<br></span></div><div><span style="font-family:monospace">
| Game (plain) | 4.3 | 4.7 |<br></span></div><div><span style="font-family:monospace">
| Game (fragmented) | 8.9 | 8.6 |<br></span></div><div><span style="font-family:monospace">
| Clang (plain) | 15.7 | 15.6 |<br></span></div><div><span style="font-family:monospace">
| Clang (fragmented) | 19.4 | 19.2 |<br></span></div><div><span style="font-family:monospace">
+--------------------+-------+------+</span></div><div><span style="font-family:monospace"><br></span></div><div><span style="font-family:monospace"><font face="arial,sans-serif">I'm keen to hear what people's feedback is, and also interested to see what results others might see by running this experiment on other input packages. Also, if anybody has any alternative ideas that meet the goals listed below, I'd love to hear them!<br></font></span></div><div><span style="font-family:monospace"><font face="arial,sans-serif"><br></font></span></div><div><span style="font-family:monospace"><font face="arial,sans-serif">To reiterate some key goals of fragmented DWARF, similar to what I said in the presentation:</font></span></div><div><span style="font-family:monospace"><font face="arial,sans-serif">1) Devise a scheme that gives significant size savings without being too costly. It's clear from just the two packages I've tried this on that there is a fairly hefty link time performance cost, although the exact cost depends on the nature of the input package. On the other hand, depending on the nature of the input package, there can also be some big gains.<br></font></span></div><div><span style="font-family:monospace"><font face="arial,sans-serif">2) Devise a scheme that doesn't require any linker knowledge of DWARF. The current approach doesn't quite achieve this properly due to the slight misuse of SHF_LINK_ORDER, but I expect that a pivot to using non-COMDAT group sections should solve this problem.</font></span></div><div><span style="font-family:monospace"><font face="arial,sans-serif">3) Provide some kind of halfway house between simply writing tombstone values into dead DWARF and fully parsing the DWARF to reoptimise its/discard the dead bits.<br></font></span></div><div><span style="font-family:monospace"><font face="arial,sans-serif"><br></font></span></div><div><span style="font-family:monospace"><font face="arial,sans-serif">I'm hopeful that changes could be made to the linker to improve the link-time cost. There seems to be a significant amount of the link time spent creating the input sections. An alternative would be to devise a scheme that would avoid the literal splitting into section headers, in favour of some sort of list of split-points that the linker uses to split things up (a bit like it already does for .eh_frame or mergeable sections).</font><br></span></div>
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