[llvm-bugs] [Bug 44539] New: LIBGAV1 perf regression trunk vs. Clang 9

[via llvm-bugs]

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: unassignedbugs at nondot.org
          Reporter: andrea.dibiagio at gmail.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at 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.

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20200114/10e4c6e3/attachment.html>