Slides for the OptView2 talk given at the Munich C++ meetup. Full talk is available at https://www.twitch.tv/videos/1268889367

1. OptView2
View and improve
compiler optimizations
Ofek Shilon
Istra Research
Jan 2022
@OfekShilon
1

2. Things I’d like the compiler to tell me:
• I couldn’t inline that function
…because I had only its declaration.
• I’m re-evaluating these expressions on every loop iteration
…because I couldn’t prove they stay fixed.
• I’m reloading this variable from memory many times
…because I don’t know whether unrelated code lines change it.
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3. Some raw optimization data is exposed
• Clang/gcc: -Rpass
code.cc:4:25: remark: foo inlined into bar [-Rpass=inline]
• Intel: -qopt-report=[0..5]
• Microsoft (very partial):
/Qpar-report, /Qvec-report
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4. 4

5. Presentation Matters.
• opt-viewer, 2016 work by Adam Nemet (Apple) and others
• Focus on Clang, over Linux
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6. Opt-Viewer Usage
• Build with an extra clang switch:
-fsave-optimization-record
*.opt.yaml files are generated, by
default in the obj folder.
• Generate htmls:
$ opt-viewer.py
--output-dir <htmls folder>
--source-dir <repo>
<yamls folder>
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7. opt-viewer.py Sample Output
Inlining
context
Hotness
(PGO)
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8. Opt-Viewer Usage
• Speed up yaml generation ~x50 by installing libyaml:
$ sudo apt install libyaml-dev
$ pip --no-cache-dir install --verbose --force-reinstall -I pyyaml
• The script would detect missing libyaml and suggest installing it:
“For faster parsing, you may want to install libyaml for PyYAL”
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9. 9

10. Great work, still not much traction
https://www.youtube.com/watch?v=qq0q1hfzidg
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11. Great work, still not much traction
11

12. Why?
• Heavy
• High I/O
• High memory
• >1G htmls
• Designed (and presented) for compiler authors
• Mostly non actionable to developers
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13. Introducing optview2
• https://github.com/OfekShilon/optview2
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14. Target Developers, Not Compiler Authors
• Denoise:
• Collect only optimization failures
• Ignore system headers
• Display a single entry per type/source loc (in index)
• …
• ~1.5M lines ==> 22K lines
• Don’t mention ‘passes’
• Make the index sortable, resizable & pageable
• Abridged function names
• ...
14

15. Example - OpenCV
• file://wsl.localhost/Ubuntu-20.04/home/ofek/html/opencv-
clean/modules/photo/index.html
15

16. “Definition is unavailable”
16

17. !
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18. “Clobbered by store”
• https://godbolt.org/z/K9a79MhcE
• Aliasing
• Forces reloading redistBatch on every loop iteration
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19. Aliasing – the silent killer
• Distribution of 22K remarks in a C++ project:
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20. 20

21. “Clobbered by call”
• https://godbolt.org/z/bb9fTKj9d
• https://github.com/llvm/llvm-project/issues/53102
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22. 22

23. Aliasing: Counter-measures
• LTO ?
• __restrict__
• Non standard, tells the compiler an argument doesn’t alias with anything else
• __attribute__((pure)), __attribute__((const))
• Non standard, tell the compiler a function doesn’t write/read global state.
• Potentially resolve ‘clobbered by call’ opt failures,
• Strict-Aliasing
• The only C++ standard compliant way.
23

24. Strict Aliasing
• “Strict aliasing is an assumption made by the compiler, that
objects of different types will never refer to the same
memory location (i.e. alias each other.)”
Mike Acton https://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html
• Allowed optimization. on by default for -O2 +
• char* is an exception – always allowed to alias with any type.
• Can be disabled with –fno-strict-aliasing (pessimization!)
24

25. Strict Aliasing: Forcing type difference
• typedef / using ?
• Inherit int?
• “I considered …to allow derivation from built-in classes … However, I
restrained myself. … the C conversion rules are so chaotic that pretending that
int, short, etc., are well-behaved ordinary classes is not going to work. They
are either C compatible, or they obey the relatively well-behaved C++ rules
for classes, but not both.”
Bjarne Stroustrup, The Design and Evolution of C++, §15.11.3.
• Strong-Typedefs
• Wrappers
25

26. Strict Aliasing demos
• https://godbolt.org/z/qGdj4qoG1
• https://godbolt.org/z/zWfYdsorE
• https://godbolt.org/z/oxq1K3n8W
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27. Demo 3: loop hoisting
27

28. • Check foreach!!
• Try to form a toy example
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29. Demo 3: loop hoisting
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30. 30

31. GCC work
• https://gcc.gnu.org/legacy-ml/gcc-patches/2018-
05/msg01675.html
• https://github.com/davidmalcolm/gcc-opt-viewer
• YAML -> JSON.gz
• Actually a better choice
https://stackoverflow.com/questions/27743711/can-i-speedup-yaml
• Active only during 2018
• Currently seems unusable
https://github.com/davidmalcolm/gcc-opt-viewer/issues/3
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32. Conclusions applicable to other compilers?
• In many cases – yes.
• Shows where to look
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33. OptView2 Future work
• Improve filtering,
• Reduce run time & memory,
• Run on windows,
• Consume binary optimization remarks,
• Revive (possibly integrate) gcc-opt-viewer,
• Support code annotations,
• Report LLVM bugs.
33

34. Lots of help needed 
• https://github.com/Of
ekShilon/optview2
• ofekshilon@gmail.com
34

35. Bkp
35

36. llvm-opt-report
$ clang -O3 -o /tmp/v.o -c /tmp/v.c -fsave-optimization-record
$ llvm-opt-report /tmp/v.yaml > /tmp/v.lst
$ cat /tmp/v.lst
• https://reviews.llvm.org/D25262
• https://github.com/llvm/llvm-
project/tree/main/llvm/tools/llvm-opt-
report
36

37. Impact
https://github.com/opencv/opencv/wiki/HowToUsePerfTests
$ cd ~/src/opencv/modules/ts/misc/
$ python3 ./run.py ~/src/opencv-clean/build/ -w ~/logs-opencv-clean
$ python3 ./run.py ~/src/opencv-opt/build/ -w ~/logs-opencv-opt
$ python3 ./summary.py ~/logs-opencv-clean/core* ~/logs-opencv-opt/core*
-u mks -m median
• Seems substantial.
• Will publish more detailed steps in the optview2 github page (and
possibly fork opencv)
Detour
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