This commit implements the Object Shapes technique in CRuby.

Object Shapes is used for accessing instance variables and representing the
"frozenness" of objects.  Object instances have a "shape" and the shape
represents some attributes of the object (currently which instance variables are
set and the "frozenness").  Shapes form a tree data structure, and when a new
instance variable is set on an object, that object "transitions" to a new shape
in the shape tree.  Each shape has an ID that is used for caching. The shape
structure is independent of class, so objects of different types can have the
same shape.

For example:

```ruby
class Foo
  def initialize
    # Starts with shape id 0
    @a = 1 # transitions to shape id 1
    @b = 1 # transitions to shape id 2
  end
end

class Bar
  def initialize
    # Starts with shape id 0
    @a = 1 # transitions to shape id 1
    @b = 1 # transitions to shape id 2
  end
end

foo = Foo.new # `foo` has shape id 2
bar = Bar.new # `bar` has shape id 2
```

Both `foo` and `bar` instances have the same shape because they both set
instance variables of the same name in the same order.

This technique can help to improve inline cache hits as well as generate more
efficient machine code in JIT compilers.

This commit also adds some methods for debugging shapes on objects.  See
`RubyVM::Shape` for more details.

For more context on Object Shapes, see [Feature: #18776]

Co-Authored-By: Aaron Patterson <tenderlove@ruby-lang.org>
Co-Authored-By: Eileen M. Uchitelle <eileencodes@gmail.com>
Co-Authored-By: John Hawthorn <john@hawthorn.email>

1 files changed, 10 insertions, 1 deletions

diff --git a/vm_core.h b/vm_core.h
index e11838a59a..4dd873c1f8 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -99,6 +99,7 @@ extern int ruby_assert_critical_section_entered;
 #include "ruby/st.h"
 #include "ruby_atomic.h"
 #include "vm_opts.h"
+#include "shape.h"
 
 #include "ruby/thread_native.h"
 
@@ -272,7 +273,9 @@ struct iseq_inline_constant_cache {
 };
 
 struct iseq_inline_iv_cache_entry {
-    struct rb_iv_index_tbl_entry *entry;
+    shape_id_t source_shape_id;
+    shape_id_t dest_shape_id;
+    attr_index_t attr_index;
 };
 
 struct iseq_inline_cvar_cache_entry {
@@ -687,6 +690,12 @@ typedef struct rb_vm_struct {
     VALUE mark_object_ary;
     const VALUE special_exceptions[ruby_special_error_count];
 
+    /* object shapes */
+    rb_shape_t *shape_list;
+    rb_shape_t *root_shape;
+    rb_shape_t *frozen_root_shape;
+    shape_id_t next_shape_id;
+
     /* load */
     VALUE top_self;
     VALUE load_path;