[ruby-core:117169] [Ruby master Bug#20338] calling new has regressed allocations over past few ruby releases

["fablestales (Fable Tales) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20338 has been reported by fablestales (Fable Tales).

----------------------------------------
Bug #20338: calling new has regressed allocations over past few ruby releases
https://bugs.ruby-lang.org/issues/20338

* Author: fablestales (Fable Tales)
* Status: Open
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
Given this script:

```
class Foo
  def initialize(**kwargs)
  end
end

class Bar
  def initialize(y:)
  end
end

b = GC.stat[:total_allocated_objects]
1000.times {
  Foo.new(y:1)
}
p GC.stat[:total_allocated_objects]-b


b = GC.stat[:total_allocated_objects]
1000.times {
  Bar.new(y:1)
}
p GC.stat[:total_allocated_objects]-b
```

On Ruby 3.2 it prints
```
 (fable-3_3_dead_with_patches) $ ruby --version
ruby 3.2.3 (2024-01-18 revision 52bb2ac0a6) [arm64-darwin23]
 (fable-3_3_dead_with_patches) $ ruby hi.rb
3006
2005
```

On Ruby 3.3 it prints:
```
 ((HEAD detached at v3_3_0)) $ ./ruby --version
ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [arm64-darwin23]
 ((HEAD detached at v3_3_0)) $ ./ruby hi.rb
3008
3005
```

On master:
```
 (master) $ git rev-parse HEAD
83618f2cfa004accdd1514de7dcbba291aa7e831
 (master) $ ./ruby --version
ruby 3.4.0dev (2024-03-14T12:33:30Z master 83618f2cfa) [arm64-darwin23]
 (master) $ ./ruby hi.rb
3006
4005
```

so that is: since Ruby 3.2 `.new`ing an object with static keyword args has grown from 2 allocations per object to 4 allocations per object.

I have a pull request which introduces a new instruction to the VM `opt_new` which basically inlines calling allocate and initialize directly, avoiding a lot of what is causing these extra objects to be allocated. My PR is here: https://github.com/ruby/ruby/pull/10254. With my patch the script prints 2006 and 1005 respectively. This is the smallest number of objects that _can_ be allocated for such calls.




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