[ruby-core:98669] [Ruby master Feature#13750] Improve String#casecmp? and Symbol#casecmp? performance with ASCII string

Issue #13750 has been updated by the_spectator (Akshay Birajdar).


@koic Made a new attempt with patch https://github.com/ruby/ruby/pull/2941

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Feature #13750: Improve String#casecmp? and Symbol#casecmp? performance with ASCII string
https://bugs.ruby-lang.org/issues/13750#change-86015

* Author: watson1978 (Shizuo Fujita)
* Status: Open
* Priority: Normal
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I think String#casecmp and String#casecmp? are similar methods.
But they have different performance on ASCII string.

Seems that String#casecmp handles ASCII string only,
however it is faster than String#casecmp?.

This patch will use the code of String#casecmp with ASCII string on String#casecmp?.
But, this patch will introduce a few penalties for UTF8 string in where detect ASCII/UTF8 string.

~~~
String#casecmp? ASCII -> 61.3 % up
String#casecmp? UTF8  ->  1.3 % down
Symbol#casecmp? ASCII -> 80.0 % up
Symbol#casecmp? UTF8  ->  4.0 % down
~~~

### Before
~~~
Calculating -------------------------------------
      String#casecmp      5.961M (3.8%) i/s -     29.838M in   5.017907s
String#casecmp? ASCII
                          3.530M (ア 8.6%) i/s -     17.554M in   5.034848s
String#casecmp? UTF8      1.252M (ア 7.4%) i/s -      6.213M in   5.012168s
      Symbol#casecmp      8.555M (ア 2.4%) i/s -     42.822M in   5.009280s
Symbol#casecmp? ASCII
                          4.235M (ア 9.7%) i/s -     20.824M in   5.001368s
Symbol#casecmp? UTF8      1.329M (ア 0.1%) i/s -      6.704M in   5.043725s
~~~

### After
~~~
Calculating -------------------------------------
      String#casecmp      5.984M (ア 6.4%) i/s -     29.829M in   5.020331s
String#casecmp? ASCII
                          5.658M (ア 1.5%) i/s -     28.308M in   5.004547s
String#casecmp? UTF8      1.215M (ア 4.3%) i/s -      6.132M in   5.060292s
      Symbol#casecmp      8.651M (ア 0.9%) i/s -     43.313M in   5.007215s
Symbol#casecmp? ASCII
                          7.462M (ア 0.5%) i/s -     37.489M in   5.023892s
Symbol#casecmp? UTF8      1.275M (ア 0.2%) i/s -      6.444M in   5.052743s
~~~


### Test code
~~~ruby
require 'benchmark/ips'

Benchmark.ips do |x|
  x.report "String#casecmp" do |loop|
    loop.times { "aBcDeF".casecmp("abcdefg") }
  end
  x.report "String#casecmp? ASCII" do |loop|
    loop.times { "aBcDeF".casecmp?("abcdefg") }
  end
  x.report "String#casecmp? UTF8" do |loop|
    loop.times { "\u{e4 f6 fc}".casecmp?("\u{c4 d6 dc}") }
  end

  x.report "Symbol#casecmp" do |loop|
    loop.times { :aBcDeF.casecmp(:abcdefg) }
  end
  x.report "Symbol#casecmp? ASCII" do |loop|
    loop.times { :aBcDeF.casecmp?(:abcdefg) }
  end
  x.report "Symbol#casecmp? UTF8" do |loop|
    loop.times { :"\u{e4 f6 fc}".casecmp?(:"\u{c4 d6 dc}") }
  end
end
~~~

### Patch
https://github.com/ruby/ruby/pull/1668



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