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]

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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).

[thaliaarchi]

Looks like we overlooked that Chars::advance_by already came with basic tests. Is that still blocking this PR?

[scottmcm]

So Chars::advance_by does, but does CharIndices::advance_by? Since that's what's being changed here?

I think what you have here is correct, I'd just like to know that if, say, the front_offset update had an off-by-one error in it there's test(s) that'll catch it.

[thaliaarchi]

It's been a while since looking at this, so I missed this detail. Sorry for dropping it.