[ruby-core:114736] [Ruby master Bug#19875] Ruby 3.0 -> 3.1 Performance regression in String#count

Issue #19875 has been updated by byroot (Jean Boussier).


> May you please help me to find the root cause?

A start would be to decompile `rb_str_count` on two versions.

@alanwu guessed that it might be the change from signed to unsigned that is preventing some compilers from auto-vectorizing that loop.

Decompiling could confirm that, however it's unclear how it could be fixed.

----------------------------------------
Bug #19875: Ruby 3.0 -> 3.1 Performance regression in String#count
https://bugs.ruby-lang.org/issues/19875#change-104571

* Author: iz (Illia Zub)
* Status: Feedback
* Priority: Normal
* ruby -v: 3.2.2
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
`String#count` became slower since Ruby 3.1. Originally found by `@Freaky`: https://github.com/ruby/ruby/pull/4001#issuecomment-1714779781

Compared using the [`benchmark-driver` gem](https://github.com/benchmark-driver/benchmark-driver).

```
$ benchmark-driver tmp/string_count_benchmark_driver.yml --rbenv '3.1.1;3.1.4;2.7.2;3.2.2;3.0.6'                                                 
Calculating -------------------------------------
                          3.1.1       3.1.4       2.7.2       3.2.2       3.0.6
               count    465.804     463.741     865.783     462.711     857.395 i/s -     10.000k times in 21.468251s 21.563768s 11.550239s 21.611783s 11.663235s

Comparison:
                            count
               2.7.2:       865.8 i/s 
               3.0.6:       857.4 i/s - 1.01x  slower
               3.1.1:       465.8 i/s - 1.86x  slower
               3.1.4:       463.7 i/s - 1.87x  slower
               3.2.2:       462.7 i/s - 1.87x  slower
```

Benchmark:


```yml
$ cat ./tmp/string_count_benchmark_driver.yml 
loop_count: 10_000
prelude: |
  html = "\nruby\n" * 1024 * 1024
benchmark:
  count: html.count($/)
```

---

*Initially, I noticed the difference between `str.count($/)` and `str.lines.size` when working on the performance improvement: https://serpapi.com/blog/lines-count-failed-deployments/*



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