[ruby-core:99713] [Ruby master Bug#17030] Enumerable#grep{_v} should be optimized for Regexp

[daniel@dan42.com]

Issue #17030 has been updated by Dan0042 (Daniel DeLorme).


> Couldn't static analysis of the code determine in most cases if match data need be generated or not?

scivola20 had a good idea, but this is even better. We can automatically get the best performance without having to manually optimize each case.

But static analysis has other limits besides `const_get(:Regexp).last_match`

```ruby
def foo(v)
  /x/ =~ 'x' # needs_last_match? depends on whether 'v' is regexp
  case method
  when v
    $1
  end
end
```

So a simpler approach would be to check if the match operation's scope (in this case the method body) contains any of the regexp-related pseudo-globals.

----------------------------------------
Bug #17030: Enumerable#grep{_v} should be optimized for Regexp
https://bugs.ruby-lang.org/issues/17030#change-87203

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
Currently:

```ruby
array.select { |e| e.match?(REGEXP) }

# about 3x faster and 6x more memory efficient than
array.grep(REGEXP)
```

This is because `grep` calls `Regexp#===` which creates useless `MatchData`



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