Optimize CharIndices::advance_by

[thaliaarchi]

Optimize CharIndices::advance_by by delegating to the optimized Chars::advance_by. Since this can also be used to count chars, add a benchmark for it.

(Interator::nth uses Iterator::advance_by.)

cc @orlp

[rustbot]

r? @scottmcm

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[thaliaarchi]

Benchmarking shows that .chars().advance_by(usize::MAX) compares favorably against .chars().count() for counting chars.

Also, whether the advance_by argument is a constant usize::MAX or black_boxed makes no difference in performance.

x bench library/coretests --stage 1 --test-args str::char_count

str::char_count::emoji_huge::case00_chars_count                26895.51ns/iter  +/- 69143.99
str::char_count::emoji_huge::case01_chars_advance_by           39908.10ns/iter   +/- 4735.18
str::char_count::emoji_huge::case02_filter_count_cont_bytes   143432.06ns/iter   +/- 6447.94
str::char_count::emoji_huge::case03_iter_chars_increment      294685.87ns/iter  +/- 19699.59
str::char_count::emoji_huge::case04_manual_char_len           294636.37ns/iter  +/- 20001.07
str::char_count::emoji_large::case00_chars_count                 374.25ns/iter     +/- 66.28
str::char_count::emoji_large::case01_chars_advance_by            586.73ns/iter    +/- 135.90
str::char_count::emoji_large::case02_filter_count_cont_bytes    2226.21ns/iter    +/- 104.03
str::char_count::emoji_large::case03_iter_chars_increment       4584.05ns/iter    +/- 270.57
str::char_count::emoji_large::case04_manual_char_len            4585.11ns/iter    +/- 258.49
str::char_count::emoji_medium::case00_chars_count                 60.87ns/iter     +/- 26.87
str::char_count::emoji_medium::case01_chars_advance_by            78.37ns/iter      +/- 6.77
str::char_count::emoji_medium::case02_filter_count_cont_bytes    288.41ns/iter     +/- 25.85
str::char_count::emoji_medium::case03_iter_chars_increment       583.22ns/iter     +/- 21.35
str::char_count::emoji_medium::case04_manual_char_len            589.36ns/iter     +/- 33.22
str::char_count::emoji_small::case00_chars_count                  21.08ns/iter      +/- 1.09
str::char_count::emoji_small::case01_chars_advance_by             12.49ns/iter      +/- 0.75
str::char_count::emoji_small::case02_filter_count_cont_bytes      29.74ns/iter      +/- 2.08
str::char_count::emoji_small::case03_iter_chars_increment         36.68ns/iter      +/- 2.66
str::char_count::emoji_small::case04_manual_char_len              37.47ns/iter      +/- 1.30
str::char_count::emoji_tiny::case00_chars_count                    7.77ns/iter      +/- 1.00
str::char_count::emoji_tiny::case01_chars_advance_by               5.81ns/iter      +/- 0.68
str::char_count::emoji_tiny::case02_filter_count_cont_bytes        6.24ns/iter      +/- 0.71
str::char_count::emoji_tiny::case03_iter_chars_increment           3.44ns/iter      +/- 0.16
str::char_count::emoji_tiny::case04_manual_char_len                3.87ns/iter      +/- 0.80
str::char_count::en_huge::case00_chars_count                   25270.06ns/iter   +/- 3517.67
str::char_count::en_huge::case01_chars_advance_by              36994.25ns/iter   +/- 8664.94
str::char_count::en_huge::case02_filter_count_cont_bytes      136098.38ns/iter   +/- 3237.48
str::char_count::en_huge::case03_iter_chars_increment         190540.45ns/iter +/- 375994.04
str::char_count::en_huge::case04_manual_char_len              286710.43ns/iter  +/- 33275.76
str::char_count::en_large::case00_chars_count                    355.26ns/iter     +/- 20.44
str::char_count::en_large::case01_chars_advance_by               571.21ns/iter     +/- 17.40
str::char_count::en_large::case02_filter_count_cont_bytes       2123.16ns/iter    +/- 503.87
str::char_count::en_large::case03_iter_chars_increment          2921.19ns/iter    +/- 356.64
str::char_count::en_large::case04_manual_char_len               4431.56ns/iter    +/- 536.13
str::char_count::en_medium::case00_chars_count                    55.92ns/iter      +/- 8.05
str::char_count::en_medium::case01_chars_advance_by               73.93ns/iter      +/- 4.66
str::char_count::en_medium::case02_filter_count_cont_bytes       282.48ns/iter    +/- 139.73
str::char_count::en_medium::case03_iter_chars_increment          377.55ns/iter     +/- 72.04
str::char_count::en_medium::case04_manual_char_len               556.65ns/iter     +/- 66.79
str::char_count::en_small::case00_chars_count                     18.46ns/iter      +/- 0.93
str::char_count::en_small::case01_chars_advance_by                12.49ns/iter      +/- 6.43
str::char_count::en_small::case02_filter_count_cont_bytes         18.91ns/iter      +/- 0.52
str::char_count::en_small::case03_iter_chars_increment            19.19ns/iter      +/- 1.00
str::char_count::en_small::case04_manual_char_len                 41.81ns/iter     +/- 20.10
str::char_count::en_tiny::case00_chars_count                       7.78ns/iter      +/- 3.34
str::char_count::en_tiny::case01_chars_advance_by                 13.17ns/iter      +/- 0.98
str::char_count::en_tiny::case02_filter_count_cont_bytes           6.23ns/iter      +/- 0.24
str::char_count::en_tiny::case03_iter_chars_increment              5.34ns/iter      +/- 0.79
str::char_count::en_tiny::case04_manual_char_len                   8.90ns/iter      +/- 0.50
str::char_count::ru_huge::case00_chars_count                   23521.49ns/iter   +/- 4525.93
str::char_count::ru_huge::case01_chars_advance_by              34957.96ns/iter  +/- 16442.47
str::char_count::ru_huge::case02_filter_count_cont_bytes      128222.60ns/iter   +/- 9503.80
str::char_count::ru_huge::case03_iter_chars_increment         176097.38ns/iter  +/- 26276.12
str::char_count::ru_huge::case04_manual_char_len              231660.15ns/iter  +/- 28604.21
str::char_count::ru_large::case00_chars_count                    337.69ns/iter     +/- 32.97
str::char_count::ru_large::case01_chars_advance_by               565.93ns/iter     +/- 45.96
str::char_count::ru_large::case02_filter_count_cont_bytes       1989.00ns/iter     +/- 53.45
str::char_count::ru_large::case03_iter_chars_increment          2709.66ns/iter    +/- 241.61
str::char_count::ru_large::case04_manual_char_len               3510.50ns/iter    +/- 617.17
str::char_count::ru_medium::case00_chars_count                    56.29ns/iter      +/- 3.25
str::char_count::ru_medium::case01_chars_advance_by              117.97ns/iter      +/- 3.20
str::char_count::ru_medium::case02_filter_count_cont_bytes       262.44ns/iter     +/- 69.23
str::char_count::ru_medium::case03_iter_chars_increment          341.12ns/iter    +/- 157.75
str::char_count::ru_medium::case04_manual_char_len               395.95ns/iter    +/- 730.95
str::char_count::ru_small::case00_chars_count                     16.23ns/iter      +/- 0.95
str::char_count::ru_small::case01_chars_advance_by                 7.02ns/iter      +/- 0.23
str::char_count::ru_small::case02_filter_count_cont_bytes         16.08ns/iter      +/- 1.56
str::char_count::ru_small::case03_iter_chars_increment            17.23ns/iter      +/- 1.29
str::char_count::ru_small::case04_manual_char_len                 17.67ns/iter      +/- 1.08
str::char_count::ru_tiny::case00_chars_count                       8.52ns/iter      +/- 0.40
str::char_count::ru_tiny::case01_chars_advance_by                  8.51ns/iter      +/- 1.06
str::char_count::ru_tiny::case02_filter_count_cont_bytes           7.39ns/iter      +/- 0.40
str::char_count::ru_tiny::case03_iter_chars_increment              6.33ns/iter      +/- 0.82
str::char_count::ru_tiny::case04_manual_char_len                   6.79ns/iter      +/- 0.59
str::char_count::zh_huge::case00_chars_count                   21915.29ns/iter   +/- 1553.56
str::char_count::zh_huge::case01_chars_advance_by              32533.42ns/iter   +/- 2794.91
str::char_count::zh_huge::case02_filter_count_cont_bytes      119467.06ns/iter   +/- 2970.87
str::char_count::zh_huge::case03_iter_chars_increment         319110.20ns/iter  +/- 15128.61
str::char_count::zh_huge::case04_manual_char_len              316619.37ns/iter  +/- 14316.98
str::char_count::zh_large::case00_chars_count                    316.41ns/iter     +/- 45.45
str::char_count::zh_large::case01_chars_advance_by               492.66ns/iter     +/- 20.85
str::char_count::zh_large::case02_filter_count_cont_bytes       1850.94ns/iter     +/- 70.57
str::char_count::zh_large::case03_iter_chars_increment          4927.22ns/iter    +/- 145.58
str::char_count::zh_large::case04_manual_char_len               4944.76ns/iter    +/- 480.41
str::char_count::zh_medium::case00_chars_count                    55.57ns/iter      +/- 5.57
str::char_count::zh_medium::case01_chars_advance_by               67.27ns/iter      +/- 1.85
str::char_count::zh_medium::case02_filter_count_cont_bytes       242.38ns/iter     +/- 20.20
str::char_count::zh_medium::case03_iter_chars_increment          599.00ns/iter     +/- 57.02
str::char_count::zh_medium::case04_manual_char_len               593.71ns/iter     +/- 19.71
str::char_count::zh_small::case00_chars_count                     19.19ns/iter      +/- 0.66
str::char_count::zh_small::case01_chars_advance_by                 9.03ns/iter      +/- 3.14
str::char_count::zh_small::case02_filter_count_cont_bytes         17.37ns/iter      +/- 1.16
str::char_count::zh_small::case03_iter_chars_increment            23.59ns/iter      +/- 0.93
str::char_count::zh_small::case04_manual_char_len                 24.84ns/iter      +/- 1.11
str::char_count::zh_tiny::case00_chars_count                       8.12ns/iter      +/- 0.32
str::char_count::zh_tiny::case01_chars_advance_by                  7.63ns/iter      +/- 0.48
str::char_count::zh_tiny::case02_filter_count_cont_bytes           6.65ns/iter      +/- 0.44
str::char_count::zh_tiny::case03_iter_chars_increment              5.00ns/iter      +/- 1.01
str::char_count::zh_tiny::case04_manual_char_len                   5.26ns/iter      +/- 0.96

[scottmcm]

Benchmarking shows that .chars().advance_by(usize::MAX) compares favorably against .chars().count() for counting chars.

Not this PR, but if that's better than the super::count::count_chars(self.as_str()) that Chars::count is using, might as well update whatever count_chars is doing...

[thaliaarchi]

It seems Chars::count is still a little faster than Chars::advance_by in most cases.

[scottmcm]

The change looks good to me, but just to be sure can you add tests either for advance_by or nth in library/alloc/tests/str.rs?

I didn't spot any, and since it doesn't seem to be exercised by a doc-comment either, it'd be good to have a couple of basic things.

[thaliaarchi]

I've added a test for using Chars::advance_by for counting chars. I have yet to write tests for it in a general sense (mostly because it's code I didn't write and I haven't yet taken the time to understand it completely to write good tests which would exercise it fully).