shape.c: Implement a lock-free version of get_next_shape_internal

Whenever we run into an inline cache miss when we try to set
an ivar, we may need to take the global lock, just to be able to
lookup inside `shape->edges`.

To solve that, when we're in multi-ractor mode, we can treat
the `shape->edges` as immutable. When we need to add a new
edge, we first copy the table, and then replace it with
CAS.

This increases memory allocations, however we expect that
creating new transitions becomes increasingly rare over time.

```ruby
class A
  def initialize(bool)
    @a = 1
    if bool
      @b = 2
    else
      @c = 3
    end
  end

  def test
    @d = 4
  end
end

def bench(iterations)
  i = iterations
  while i > 0
    A.new(true).test
    A.new(false).test
    i -= 1
  end
end

if ARGV.first == "ractor"
  ractors = 8.times.map do
    Ractor.new do
      bench(20_000_000 / 8)
    end
  end
  ractors.each(&:take)
else
  bench(20_000_000)
end
```

The above benchmark takes 27 seconds in Ractor mode on Ruby 3.4,
and only 1.7s with this branch.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/shape.h b/shape.h
index e14f1576a0..b9809c4010 100644
--- a/shape.h
+++ b/shape.h
@@ -42,7 +42,7 @@ extern ID ruby_internal_object_id;
 typedef struct redblack_node redblack_node_t;
 
 struct rb_shape {
-    struct rb_id_table *edges; // id_table from ID (ivar) to next shape
+    VALUE edges; // id_table from ID (ivar) to next shape
     ID edge_name; // ID (ivar) for transition from parent to rb_shape
     attr_index_t next_field_index; // Fields are either ivars or internal properties like `object_id`
     attr_index_t capacity; // Total capacity of the object with this shape
@@ -75,7 +75,7 @@ typedef struct {
     /* object shapes */
     rb_shape_t *shape_list;
     rb_shape_t *root_shape;
-    shape_id_t next_shape_id;
+    rb_atomic_t next_shape_id;
 
     redblack_node_t *shape_cache;
     unsigned int cache_size;