Issue #16557 has been updated by byroot (Jean Boussier).


From the developers meeting notes:

> Preliminary discussion:
>   mame: Looks fine, if CI failure is fixed
>   ko1: The patch creates a regexp object, and then find already-existing instance. It is slightly suboptimal
>   ko1: Regexp is immutable in onigmo? If so, the optimization can be applied not only to literals but also to all regexp instances.
> Discussion:
>   ko1: Is onigmo regex data structure immutable? If so, we can dedup all Regexp objects under lower layer (not only literals but also any regexp)
>   naruse: Yep. The key should be source string, encoding, and regexp options.
>   ko1: A simple reference count mechanism would be good enough
>   ko1: Regexp objects can be ignored
>   ko1: But there is no resource to developí─ At least I cannot make effort for that
>   ko1: And his patch has some problems (mame: I failed to log)
>   akr: How about drop-in replacement of regcomp/regexec/regfree? (regcomp lookup a cache and count up the reference count. regfree count down the reference count.)
> Conclusion:
>   ko1: I will propose the refernce count mechanism and tell the situation (I have no time to review and maintain the patch)


My opinion on this:

> ko1: The patch creates a regexp object, and then find already-existing instance. It is slightly suboptimal

Yes, my plan was to improve that a bit later, I just see no point tackling this before I fix the CI failures.

> is immutable in onigmo? If so, the optimization can be applied not only to literals but also to all regexp instances.

I don't have hard data, so I might be wrong, but:

  - I highly doubt there's much duplication among dynamic regexps.
  - If the registry is at this layer, it means it need to keep a copy of the `src` string. So it would increase each Regexp memory footprint.
  - So I'm really not convinced it would actually be a win.

>  if CI failure is fixed

I'm circling around the issue, I have no idea if I'll be able to figure it out myself, but I've added some debug info on the PR, it might ring someone's bell.

----------------------------------------
Feature #16557: Deduplicate Regexp literals
https://bugs.ruby-lang.org/issues/16557#change-84431

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
Pull Request: https://github.com/ruby/ruby/pull/2859

### Context

Real world application contain many duplicated Regexp literals.

From a rails/console in Redmine:

```
>> ObjectSpace.each_object(Regexp).count
=> 6828
>> ObjectSpace.each_object(Regexp).uniq.count
=> 4162
>> ObjectSpace.each_object(Regexp).to_a.map { |r| ObjectSpace.memsize_of(r) }.sum
=> 4611957 # 4.4 MB total
>> ObjectSpace.each_object(Regexp).to_a.map { |r| ObjectSpace.memsize_of(r) }.sum - ObjectSpace.each_object(Regexp).to_a.uniq.map { |r| ObjectSpace.memsize_of(r) }.sum
=> 1490601 # 1.42 MB could be saved
```

Here's the to 10 most duplicated regexps in Redmine:

```
147: /"/
107: /\s+/
103: //
89: /\n/
83: /'/
76: /\s+/m
37: /\d+/
35: /\[/
33: /./
33: /\\./
```

Any empty Rails application will have a similar amount of regexps.

### The feature

Since https://bugs.ruby-lang.org/issues/16377 made literal regexps frozen, it is possible to deduplicate literal regexps without changing any semantic and save a decent amount of resident memory.


### The patch

I tried implementing this feature in a way very similar to the `frozen_strings` table, it's functional but I'm having trouble with a segfault on Linux: https://github.com/ruby/ruby/pull/2859



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