[llvm-dev] Fragmented DWARF
Alexey Lapshin via llvm-dev
llvm-dev at lists.llvm.org
Wed Nov 4 03:54:45 PST 2020
Hi James,
I did experiments with the clang code base and will do experiments with
our local codebase later.
Overall, both solutions("Fragmented DWARF" and "DWARFLinker without odr
types deduplication") look having similar size savings results for the
final binary. "DWARFLinker with odr types deduplication" has a bigger
size saving effect. "Fragmented DWARF" increases the size of original
object files up to 15%.
LLD with "fragmented DWARF" works significantly faster than with
"DWARFLinker".
Following are the results for "llvm-strings" and "clang" binaries:
1. llvm-strings:
source object files size: 381M.
fragmented source object files size: 451M(18% increase).
a. upstream version,
command line options: --gc-sections
binary size: 6,5M
compilation time: 0:00.13 sec
run-time memory: 111kb
b. "fragmented DWARF" version,
command line options: --gc-sections --mark-live-pc=0.45
binary size: 3,7M
compilation time: 0:00.10 sec
run-time memory: 122kb
c. DWARFLinker version,
command line options: --gc-sections --gc-debuginfo
binary size: 3,8M
compilation time: 0:00.33 sec
run-time memory: 141kb
d. DWARFLinker no-odr version,
command line options: --gc-sections --gc-debuginfo
--gc-debuginfo-no-odr
binary size: 4,3M
compilation time: 0:00.38 sec
run-time memory: 142kb
2. clang:
source object files size: 6,5G.
fragmented source object files size: 7,3G(13% increase).
a. upstream version,
command line options: --gc-sections
binary size: 1,5G
compilation time: 6 sec
run-time memory: 9.7G
b. "fragmented DWARF" version,
command line options: --gc-sections --mark-live-pc=0.43
binary size: 1,1G
compilation time: 9 sec
run-time memory: 11G
c. DWARFLinker version,
command line options: --gc-sections --gc-debuginfo
binary size: 836M
compilation time: 62 sec
run-time memory: 15G
d. DWARFLinker no-odr version,
command line options: --gc-sections --gc-debuginfo
--gc-debuginfo-no-odr
binary size: 1,3G
compilation time: 128 sec
run-time memory: 17G
Detailed size results:
1. llvm-strings
a)
FILE SIZE VM SIZE
-------------- --------------
41.1% 2.64Mi 0.0% 0 .debug_info
24.9% 1.60Mi 0.0% 0 .debug_str
12.6% 827Ki 0.0% 0 .debug_line
6.5% 428Ki 63.8% 428Ki .text
4.8% 317Ki 0.0% 0 .strtab
3.4% 223Ki 0.0% 0 .debug_ranges
2.0% 133Ki 19.8% 133Ki .eh_frame
1.7% 110Ki 0.0% 0 .symtab
1.2% 77.6Ki 0.0% 0 .debug_abbrev
b)
FILE SIZE VM SIZE
-------------- --------------
50.3% 1.85Mi 0.0% 0 .debug_info
43.6% 1.60Mi 0.0% 0 .debug_str
2.6% 98.2Ki 0.0% 0 .debug_line
2.1% 77.6Ki 0.0% 0 .debug_abbrev
0.5% 17.5Ki 54.9% 17.4Ki .text
0.3% 9.94Ki 0.0% 0 .strtab
0.2% 6.27Ki 0.0% 0 .symtab
0.1% 5.09Ki 15.9% 5.03Ki .eh_frame
0.1% 3.28Ki 0.0% 0 .debug_ranges
c)
FILE SIZE VM SIZE
-------------- --------------
33.0% 1.25Mi 0.0% 0 .debug_info
29.2% 1.11Mi 0.0% 0 .debug_str
11.0% 428Ki 63.8% 428Ki .text
8.2% 317Ki 0.0% 0 .strtab
7.8% 304Ki 0.0% 0 .debug_line
3.4% 133Ki 19.8% 133Ki .eh_frame
2.8% 110Ki 0.0% 0 .symtab
1.7% 65.9Ki 0.0% 0 .debug_ranges
1.0% 38.4Ki 5.7% 38.4Ki .rodata
d)
FILE SIZE VM SIZE
-------------- --------------
39.7% 1.68Mi 0.0% 0 .debug_info
26.3% 1.11Mi 0.0% 0 .debug_str
9.9% 428Ki 63.8% 428Ki .text
7.3% 317Ki 0.0% 0 .strtab
7.0% 304Ki 0.0% 0 .debug_line
3.1% 133Ki 19.8% 133Ki .eh_frame
2.6% 110Ki 0.0% 0 .symtab
1.5% 65.9Ki 0.0% 0 .debug_ranges
2. clang
a)
FILE SIZE VM SIZE
-------------- --------------
58.3% 878Mi 0.0% 0 .debug_info
11.8% 177Mi 0.0% 0 .debug_str
7.7% 115Mi 62.2% 115Mi .text
7.7% 115Mi 0.0% 0 .debug_line
6.0% 90.7Mi 0.0% 0 .strtab
2.4% 35.4Mi 0.0% 0 .debug_ranges
1.5% 23.3Mi 12.5% 23.3Mi .eh_frame
1.5% 23.0Mi 12.4% 23.0Mi .rodata
1.2% 17.9Mi 0.0% 0 .symtab
b)
FILE SIZE VM SIZE
-------------- --------------
71.5% 772Mi 0.0% 0 .debug_info
16.5% 177Mi 0.0% 0 .debug_str
3.7% 40.2Mi 59.2% 40.2Mi .text
2.4% 25.8Mi 0.0% 0 .debug_line
2.1% 23.0Mi 0.0% 0 .strtab
1.0% 10.6Mi 15.6% 10.6Mi .dynstr
0.7% 7.18Mi 10.6% 7.18Mi .eh_frame
0.5% 5.60Mi 0.0% 0 .symtab
0.4% 4.28Mi 0.0% 0 .debug_ranges
0.4% 4.04Mi 0.0% 0 .debug_abbrev
c)
FILE SIZE VM SIZE
-------------- --------------
35.1% 293Mi 0.0% 0 .debug_info
21.2% 177Mi 0.0% 0 .debug_str
13.9% 115Mi 62.2% 115Mi .text
10.9% 90.7Mi 0.0% 0 .strtab
6.9% 57.4Mi 0.0% 0 .debug_line
2.8% 23.3Mi 12.5% 23.3Mi .eh_frame
2.8% 23.0Mi 12.4% 23.0Mi .rodata
2.1% 17.9Mi 0.0% 0 .symtab
1.5% 12.4Mi 0.0% 0 .debug_ranges
1.3% 10.6Mi 5.7% 10.6Mi .dynstr
d)
FILE SIZE VM SIZE
-------------- --------------
58.3% 758Mi 0.0% 0 .debug_info
13.6% 177Mi 0.0% 0 .debug_str
8.9% 115Mi 62.2% 115Mi .text
7.0% 90.7Mi 0.0% 0 .strtab
4.4% 57.4Mi 0.0% 0 .debug_line
1.8% 23.3Mi 12.5% 23.3Mi .eh_frame
1.8% 23.0Mi 12.4% 23.0Mi .rodata
1.4% 17.9Mi 0.0% 0 .symtab
1.0% 12.4Mi 0.0% 0 .debug_ranges
0.8% 10.6Mi 5.7% 10.6Mi .dynstr
Thank you, Alexey.
On 19.10.2020 11:50, James Henderson wrote:
> Great, thanks Alexey! I'll try to take a look at this in the near
> future, and will report my results back here. I imagine our clang
> results will differ, purely because we probably used different
> toolchains to build the input in the first place.
>
> On Thu, 15 Oct 2020 at 10:08, Alexey Lapshin <avl.lapshin at gmail.com
> <mailto:avl.lapshin at gmail.com>> wrote:
>
>
> On 13.10.2020 10:20, James Henderson wrote:
>> 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.
>>
>> 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.
>
> James, I updated the patch - https://reviews.llvm.org/D74169.
>
> To make it working it is necessary to build example with
> -ffunction-sections and specify following options to the linker :
>
> --gc-sections --gc-debuginfo --gc-debuginfo-no-odr
>
> For clang binary I got following results:
>
> 1. --gc-sections = binary size 1,5G, Debug Info size(*)1.2G
>
> 2. --gc-sections --gc-debuginfo = binary size 840M, 8x performance
> decrease, Debug Info size 542M
>
> 3. --gc-sections --gc-debuginfo --gc-debuginfo-no-odr = binary
> size 1,3G, 16x performance decrease, Debug Info size 1G
>
> (*) .debug_info+.debug_str+.debug_line+.debug_ranges+.debug_loc
>
>
> I added option --gc-debuginfo-no-odr, so that size reduction could
> be compared correctly. Without that option D74169 does types
> deduplication and then it is not correct to compare resulting size
> with "Fragmented DWARF" solution which does not do types
> deduplication.
>
> Also, I look at your D89229 <https://reviews.llvm.org/D89229> and
> would share results some time later.
>
> Thank you, Alexey.
>
>>
>> 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).
>>
>> James
>>
>> On Mon, 12 Oct 2020 at 20:48, David Blaikie <dblaikie at gmail.com
>> <mailto:dblaikie at gmail.com>> wrote:
>>
>> 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)
>>
>> 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.
>>
>> 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?
>>
>> (& 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)
>>
>> On Mon, Oct 12, 2020 at 6:41 AM James Henderson
>> <jh7370.2008 at my.bristol.ac.uk
>> <mailto:jh7370.2008 at my.bristol.ac.uk>> wrote:
>>
>> Hi all,
>>
>> 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:
>>
>> Link-time speed (s):
>> +--------------------+-------+---------------+------+------+------+------+------+
>> | Package variant | No GC | GC 1 (normal) | GC 2 | GC 3 |
>> GC 4 | GC 5 | GC 6 |
>> +--------------------+-------+---------------+------+------+------+------+------+
>> | Game (plain) | 4.5 | 4.9 | 4.2 | 3.6 |
>> 3.4 | 3.3 | 3.2 |
>> | Game (fragmented) | 11.1 | 11.8 | 9.7 | 8.6
>> | 7.9 | 7.7 | 7.5 |
>> | Clang (plain) | 13.9 | 17.9 | 17.0 | 16.7 |
>> 16.3 | 16.2 | 16.1 |
>> | Clang (fragmented) | 18.6 | 22.8 | 21.6 |
>> 21.1 | 20.8 | 20.5 | 20.2 |
>> +--------------------+-------+---------------+------+------+------+------+------+
>>
>> Output size - Game package (MB):
>> +---------------------+-------+------+------+------+------+------+------+
>> | Category | No GC | GC 1 | GC 2 | GC 3 | GC 4
>> | GC 5 | GC 6 |
>> +---------------------+-------+------+------+------+------+------+------+
>> | Plain (total) | 1149 | 1121 | 1017 | 965 | 938
>> | 930 | 928 |
>> | Plain (DWARF*) | 845 | 845 | 845 | 845 | 845
>> | 845 | 845 |
>> | Plain (other) | 304 | 276 | 172 | 120 | 93
>> | 85 | 82 |
>> | Fragmented (total) | 1044 | 940 | 556 | 373 | 287
>> | 263 | 255 |
>> | Fragmented (DWARF*) | 740 | 664 | 384 | 253 | 194
>> | 178 | 173 |
>> | Fragmented (other) | 304 | 276 | 172 | 120 | 93
>> | 85 | 82 |
>> +---------------------+-------+------+------+------+------+------+------+
>>
>>
>> Output size - Clang (MB):
>> +---------------------+-------+------+------+------+------+------+------+
>> | Category | No GC | GC 1 | GC 2 | GC 3 | GC 4
>> | GC 5 | GC 6 |
>> +---------------------+-------+------+------+------+------+------+------+
>> | Plain (total) | 2596 | 2546 | 2406 | 2332 | 2293
>> | 2273 | 2251 |
>> | Plain (DWARF*) | 1979 | 1979 | 1979 | 1979 | 1979
>> | 1979 | 1979 |
>> | Plain (other) | 616 | 567 | 426 | 353 | 314
>> | 294 | 272 |
>> | Fragmented (total) | 2397 | 2346 | 2164 | 2069 | 2017
>> | 1990 | 1963 |
>> | Fragmented (DWARF*) | 1780 | 1780 | 1738 | 1716 | 1703
>> | 1696 | 1691 |
>> | Fragmented (other) | 616 | 567 | 426 | 353 | 314
>> | 294 | 272 |
>> +---------------------+-------+------+------+------+------+------+------+
>>
>> *DWARF size == total size of .debug_info + .debug_line +
>> .debug_ranges + .debug_aranges + .debug_loc
>>
>> Additionally, I have posted
>> https://reviews.llvm.org/D89229 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.
>>
>> During the conference, the question was asked what the
>> memory usage and input size impact was. I've summarised
>> these below:
>>
>> Input file size total (GB):
>> +--------------------+------------+
>> | Package variant | Total Size |
>> +--------------------+------------+
>> | Game (plain) | 2.9 |
>> | Game (fragmented) | 4.2 |
>> | Clang (plain) | 10.9 |
>> | Clang (fragmented) | 12.3 |
>> +--------------------+------------+
>>
>> Peak Working Set Memory usage (GB):
>> +--------------------+-------+------+
>> | Package variant | No GC | GC 1 |
>> +--------------------+-------+------+
>> | Game (plain) | 4.3 | 4.7 |
>> | Game (fragmented) | 8.9 | 8.6 |
>> | Clang (plain) | 15.7 | 15.6 |
>> | Clang (fragmented) | 19.4 | 19.2 |
>> +--------------------+-------+------+
>>
>> 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!
>>
>> To reiterate some key goals of fragmented DWARF, similar
>> to what I said in the presentation:
>> 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.
>> 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.
>> 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.
>>
>> 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).
>>
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