[all-commits] [llvm/llvm-project] f8afc5: [libc++] Speed up classic locale (#72112)

[Dmitry Vyukov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: f8afc53d641ce9d4ad8565aae9e7b5911b572a02
      https://github.com/llvm/llvm-project/commit/f8afc53d641ce9d4ad8565aae9e7b5911b572a02
  Author: Dmitry Vyukov <dvyukov at google.com>
  Date:   2023-11-27 (Mon, 27 Nov 2023)

  Changed paths:
    M libcxx/benchmarks/stringstream.bench.cpp
    M libcxx/include/CMakeLists.txt
    M libcxx/include/__locale
    A libcxx/include/__utility/no_destroy.h
    M libcxx/include/module.modulemap.in
    M libcxx/src/locale.cpp

  Log Message:
  -----------
  [libc++] Speed up classic locale (#72112)

Locale objects use atomic reference counting, which may be very
expensive in parallel applications. The classic locale is used by
default by all streams and can be very contended. But it's never
destroyed, so the reference counting is also completely pointless on the
classic locale. Currently ~70% of time in the parallel stringstream
benchmarks is spent in locale ctor/dtor. And the execution radically
slows down with more threads.

Avoid reference counting on the classic locale. With this change
parallel benchmarks start to scale with threads.

Co-authored-by: Louis Dionne <ldionne.2 at gmail.com>

```
                              │   baseline   │    optimized                            │
                              │    sec/op    │    sec/op      vs base                  │
Istream_numbers/0/threads:1      4.672µ ± 0%   4.419µ ± 0%     -5.42% (p=0.000 n=30+39)
Istream_numbers/0/threads:72   539.817µ ± 0%   9.842µ ± 1%    -98.18% (p=0.000 n=30+40)
Istream_numbers/1/threads:1      4.890µ ± 0%   4.750µ ± 0%     -2.85% (p=0.000 n=30+40)
Istream_numbers/1/threads:72     66.44µ ± 1%   10.14µ ± 1%    -84.74% (p=0.000 n=30+40)
Istream_numbers/2/threads:1      4.888µ ± 0%   4.746µ ± 0%     -2.92% (p=0.000 n=30+40)
Istream_numbers/2/threads:72     494.8µ ± 0%   410.2µ ± 1%    -17.11% (p=0.000 n=30+40)
Istream_numbers/3/threads:1      4.697µ ± 0%   4.695µ ± 5%          ~ (p=0.391 n=30+37)
Istream_numbers/3/threads:72     421.5µ ± 7%   421.9µ ± 9%          ~ (p=0.665 n=30)
Ostream_number/0/threads:1       183.0n ± 0%   141.0n ± 2%    -22.95% (p=0.000 n=30)
Ostream_number/0/threads:72    24196.5n ± 1%   343.5n ± 3%    -98.58% (p=0.000 n=30)
Ostream_number/1/threads:1       250.0n ± 0%   196.0n ± 2%    -21.60% (p=0.000 n=30)
Ostream_number/1/threads:72    16260.5n ± 0%   407.0n ± 2%    -97.50% (p=0.000 n=30)
Ostream_number/2/threads:1       254.0n ± 0%   196.0n ± 1%    -22.83% (p=0.000 n=30)
Ostream_number/2/threads:72      28.49µ ± 1%   18.89µ ± 5%    -33.72% (p=0.000 n=30)
Ostream_number/3/threads:1       185.0n ± 0%   185.0n ± 0%      0.00% (p=0.017 n=30)
Ostream_number/3/threads:72      19.38µ ± 4%   19.33µ ± 5%          ~ (p=0.425 n=30)
```