From: eregontp@...
Date: 2020-09-30T18:18:26+00:00
Subject: [ruby-core:100242] [Ruby master Feature#17206] Introduce new Regexp option to avoid global MatchData allocations

Issue #17206 has been updated by Eregon (Benoit Daloze).


IMHO hardcoding such knowledge in the pattern feels wrong (vs in the matching method like `Regexp#match?` which is fine).
It seems to me that it could cause confusing bugs, e.g. when using `/f` in the `case` above if a `when` clause starts to use one of the `$~`-derived variables.
Then it would unexpectedly always be `nil`, causing a potentially very subtle bug.

I have a hard time to believe that allocating the MatchData is so expensive.
If that's the case, then there must be a lot of optimization potential for faster allocation of MatchData in CRuby.
What I think rather is this is due to having to set $~ in the caller, and maybe to compute group offsets.

I think it would be worth investigating more in details where does the performance overhead from `$~` & friends come from in CRuby.

----------------------------------------
Feature #17206: Introduce new Regexp option to avoid global MatchData allocations
https://bugs.ruby-lang.org/issues/17206#change-87829

* Author: fatkodima (Dima Fatko)
* Status: Open
* Priority: Normal
----------------------------------------
Originates from https://bugs.ruby-lang.org/issues/17030

When this option is specified, ruby will not create global `MatchData` objects, when not explicitly needed by the method.

If the new option is named `f`, we can write as `/o/f`, and `grep(/o/f)` is faster than `grep(/o/)`.

This speeds up not only `grep`, but also `all?`, `any?`, `case` and so on.

Many people have written code like this:
```ruby
IO.foreach("foo.txt") do |line|
  case line
  when /^#/
    # do nothing 
  when /^(\d+)/
    # using $1
  when /xxx/
    # using $&
  when /yyy/
    # not using $&
  else
    # ...
  end
end
```

This is slow, because of the above mentioned problem.
Replacing `/^#/` with `/^#/f`, and `/yyy/` with `/yyy/f` will make it faster.

Some benchmarks - https://bugs.ruby-lang.org/issues/17030#note-9 which show `2.5x` to `5x` speedup.

PR: https://github.com/ruby/ruby/pull/3455



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