[all-commits] [llvm/llvm-project] 2f5151: [libc++][format] Improves escaping performance.

[Mark de Wever via All-commits]

  Branch: refs/heads/users/mordante/improves_format_escaping_performance
  Home:   https://github.com/llvm/llvm-project
  Commit: 2f515101e0b79dc7021ca9dd0aa81f08d704c62e
      https://github.com/llvm/llvm-project/commit/2f515101e0b79dc7021ca9dd0aa81f08d704c62e
  Author: Mark de Wever <koraq at xs4all.nl>
  Date:   2024-04-25 (Thu, 25 Apr 2024)

  Changed paths:
    M libcxx/include/__format/escaped_output_table.h
    M libcxx/include/format
    A libcxx/test/libcxx/utilities/format/format.string/format.string.std/escaped_output.pass.cpp
    M libcxx/utils/generate_escaped_output_table.py

  Log Message:
  -----------
  [libc++][format] Improves escaping performance.

The previous patch implemented
- P2713R1 Escaping improvements in std::format
- LWG3965 Incorrect example in [format.string.escaped] p3 for formatting of combining characters

These changes were correct, but has a size and performance penalty. This
patch improves the size and performance of the previous patch. The
performance is still worse than before since the lookups may require two
property lookups instead of one before implementing the paper. The changes
give a tighter coupling between the Unicode data and the algorithm.
An additional tests are added to notify about changes in future Unicode
updates.

Before
-----------------------------------------------------------------------
Benchmark                             Time             CPU   Iterations
-----------------------------------------------------------------------
BM_ascii_escaped<char>           110704 ns       110696 ns         6206
BM_unicode_escaped<char>         101371 ns       101374 ns         6862
BM_cyrillic_escaped<char>         63329 ns        63327 ns        11013
BM_japanese_escaped<char>         41223 ns        41225 ns        16938
BM_emoji_escaped<char>           111022 ns       111021 ns         6304
BM_ascii_escaped<wchar_t>        112441 ns       112443 ns         6231
BM_unicode_escaped<wchar_t>      102776 ns       102779 ns         6813
BM_cyrillic_escaped<wchar_t>      58977 ns        58975 ns        11868
BM_japanese_escaped<wchar_t>      36885 ns        36886 ns        18975
BM_emoji_escaped<wchar_t>        115885 ns       115881 ns         6051

The first change is to manually encode the entire last area and make a
manual exception for the 240 excluded entries. This reduced the table
from 1077 to 729 entries and gave the following benchmark results.
-----------------------------------------------------------------------
Benchmark                             Time             CPU   Iterations
-----------------------------------------------------------------------
BM_ascii_escaped<char>           104777 ns       104776 ns         6550
BM_unicode_escaped<char>          96980 ns        96982 ns         7238
BM_cyrillic_escaped<char>         60254 ns        60251 ns        11670
BM_japanese_escaped<char>         44452 ns        44452 ns        15734
BM_emoji_escaped<char>           104557 ns       104551 ns         6685
BM_ascii_escaped<wchar_t>        107456 ns       107454 ns         6505
BM_unicode_escaped<wchar_t>       96219 ns        96216 ns         7301
BM_cyrillic_escaped<wchar_t>      56921 ns        56904 ns        12288
BM_japanese_escaped<wchar_t>      39530 ns        39529 ns        17492
BM_emoji_escaped<wchar_t>        108494 ns       108496 ns         6408

An entry in the table can only contain 2048 code points. For larger ranges
there are multiple entries split in chunks with a maximum size of 2048
entries. To encode the entire Unicode code point range 21 bits are
required. The manual part starts at 0x323B0 this means all entries in the
table fit in 18 bits. This allows to allocate 3 additional bits for the
range. This allows entries to have 16384 elements. This range always
avoids splitting the range in multiple chunks.

This reduces the number of table elements from 729 to 711 and gives the
following benchmark results.
-----------------------------------------------------------------------
Benchmark                             Time             CPU   Iterations
-----------------------------------------------------------------------
BM_ascii_escaped<char>           104289 ns       104289 ns         6619
BM_unicode_escaped<char>          96682 ns        96681 ns         7215
BM_cyrillic_escaped<char>         59673 ns        59673 ns        11732
BM_japanese_escaped<char>         41983 ns        41982 ns        16646
BM_emoji_escaped<char>           104119 ns       104120 ns         6683
BM_ascii_escaped<wchar_t>        104503 ns       104505 ns         6693
BM_unicode_escaped<wchar_t>       93426 ns        93423 ns         7489
BM_cyrillic_escaped<wchar_t>      54858 ns        54859 ns        12742
BM_japanese_escaped<wchar_t>      36385 ns        36384 ns        19259
BM_emoji_escaped<wchar_t>        105608 ns       105610 ns         6592



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