https://bugs.llvm.org/show_bug.cgi?id=44539
Bug ID: 44539
Summary: LIBGAV1 perf regression trunk vs. Clang 9
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: andrea.dibia...@gmail.com
CC: craig.top...@gmail.com, llvm-bugs@lists.llvm.org,
llvm-...@redking.me.uk, spatel+l...@rotateright.com
This is related to bug 44411.
There is a significant perf regression in benchmark LIBGAV1.
---
-O3 -march=znver1
Numbers are FPS (frames per second); more is better.
```
single thread -- 2000 frames
========
| GCC 7.4 | CLANG 9.x | CLANG Master
chimera_8b_1080p.ivf | 22.77 | 21.86 | 18.71
chimera_10b_1080p.ivf | 11.31 | 12.68 | 11.67
summer_nature_1080p.ivf | 21.10 | 21.02 | 18.29
summer_nature_4K.ivf | 4.74 | 4.57 | 3.94
multi threaded (8) -- no frame limit
========
| GCC 7.4 | CLANG 9.x | CLANG master
chimera_8b_1080p.ivf | 43.51 | 42.76 | 34.80
chimera_10b_1080p.ivf | 16.18 | 18.98 | 17.12
summer_nature_1080p.ivf | 64.22 | 63.66 | 53.89
summer_nature_4K.ivf | 17.57 | 17.17 | 14.70
multi threaded (16) -- no frame limit
========
| GCC 7.4 | CLANG 9.x | CLANG master
chimera_8b_1080p.ivf | 43.40 | 43.05 | 38.73
chimera_10b_1080p.ivf | 16.54 | 19.68 | 18.67
summer_nature_1080p.ivf | 62.72 | 62.20 | 54.96
summer_nature_4K.ivf | 19.31 | 19.11 | 17.13
```
The single threaded execution is ~14% slower on master vs clang 9.x.
Later I will post a full description of the underlying issue that caused this
perf regression.
tl;dr: performance degradation in libgav is caused by poor decisions made by
pass "x86 cmov converter". In particular, a bunch of CMOVs from a hot loop are
now sub-optimally expanded into if-then blocks. Those CMOVs weren't expanded by
the Clang 9 compiler (that was the correct decision).
If we disable that pass then we fully get back the performance loss. For
example, decoding "chimera_8b_1080p.ivf" with a single thread gives us an
average of 22.14 fps.
As I wrote, I plan to post all my findings in a follow-up comment.
NOTE: this is unlikely to be AMD specific. For example, I can reproduce the
poor CMOV expansions if I generate code for Skylake.
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