[ruby-core:99159] [Ruby master Feature#16984] Remove write barrier examption for T_ICLASS

Issue #16984 has been updated by alanwu (Alan Wu).

Description updated

Edit: I noticed that T_ICLASS wasn't marking the shared method and constant table on master. My notes
about reducing the number of `gc_mark` references on the heap was incorrect.



----------------------------------------
Feature #16984: Remove write barrier examption for T_ICLASS
https://bugs.ruby-lang.org/issues/16984#change-86537

* Author: alanwu (Alan Wu)
* Status: Open
* Priority: Normal
----------------------------------------
Consider the following code:

```ruby
module M
  def foo; end
  def bar; end
end

class C
  include M
end
```

The object reference graph from running the code looks like this:

```
+---+              +-----+
| M |--------------| foo |-+
+---+              +-----+ |
  |                +-----+ |
  +----------------| bar | |
                   +-----+ |
+-----------+         |    |
| iclass(M) |---------+    |
+-----------+--------------+
```

Applying the proposed patch, the graph becomes

```
+---+         +--------------+   +-----+
| M |---------| method table |---| foo |
+---+         +--------------+   +-----+
+-----------+         |    |     +-----+
| iclass(M) |---------+    +-----| bar |
+-----------+                    +-----+

```

This change has a similar effect on the constant table. In addition to this, T_ICLASS no longer
holds a reference to a ivar table. Code that access the ivar table through iclasses
are changed to access it through the object from which the iclass was made. This change
impacts autoload and class variable lookup.

## Why?

The main goal of this change is to make iclasses and modules write barrier protected. At the moment, they are
"shady", which means the GC has to do extra work to handle them. In code bases that use modules a lot,
iclasses can easily take up a significant portion of the heap and impact GC time. Inserting write barriers was
tricky in the old setup, because of the way `M` and `iclass(M)` share the method table.

Having write barrier for iclasses mean they can age in the generational GC.
Once aged, the GC can sometimes skip subgraphs rooted at these objects, improving performance.

## Impact to GC time

I measured the impact to minor GC time with the following steps:
 - load an application
 - run `GC::Profile.enable`
 - allocate 50 million objects
 - run `GC::Profile.report`

Here is the impact to average minor GC time on various apps:

|Application             |     Before    |  After  | Speedup ratio |
|------------------------|---------------|---------|---------------|
|CRuby's test-all suite  |  2.438ms      | 2.289ms |   1.06        |
|`rails new` app         |  1.911ms      | 1.798ms |   1.06        |
|Private app A           |  5.182ms      | 5.168ms |   1.00        |
|Private app B           |  185.7ms      | 107.9ms |   1.72        |

Private app A's heap size is about 22 MiB compared to B's 250 MiB.
App B boots up about 15% faster with this change.

## Impact to class variable lookup

I included a benchmark in the patch to measure the impact to class variable lookup performance.
The difference seems negligible.

## Conclusion

This change seems to reduce minor GC time for real-world applications.


---

Code: https://github.com/ruby/ruby/pull/3238
Credits to @tenderlovemaking for coming up with the idea for this change.




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