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>

2 files changed, 4 insertions, 18 deletions

diff --git a/include/ruby/internal/core/robject.h b/include/ruby/internal/core/robject.h
index 7823061d8f..bec0b45fd4 100644
--- a/include/ruby/internal/core/robject.h
+++ b/include/ruby/internal/core/robject.h
@@ -46,7 +46,6 @@
 #define ROBJECT_EMBED         ROBJECT_EMBED
 #define ROBJECT_NUMIV         ROBJECT_NUMIV
 #define ROBJECT_IVPTR         ROBJECT_IVPTR
-#define ROBJECT_IV_INDEX_TBL  ROBJECT_IV_INDEX_TBL
 /** @endcond */
 
 /**
@@ -132,7 +131,7 @@ struct RObject {
              *
              * This is a shortcut for `RCLASS_IV_INDEX_TBL(rb_obj_class(obj))`.
              */
-            struct st_table *iv_index_tbl;
+            struct rb_id_table *iv_index_tbl;
         } heap;
 
 #if USE_RVARGC
diff --git a/include/ruby/internal/fl_type.h b/include/ruby/internal/fl_type.h
index c51bd2e9d9..7383426b23 100644
--- a/include/ruby/internal/fl_type.h
+++ b/include/ruby/internal/fl_type.h
@@ -941,21 +941,8 @@ RB_OBJ_FREEZE_RAW(VALUE obj)
     RB_FL_SET_RAW(obj, RUBY_FL_FREEZE);
 }
 
-/**
- * Prevents further modifications to the given object.  ::rb_eFrozenError shall
- * be raised if modification is attempted.
- *
- * @param[out]  x  Object in question.
- */
-static inline void
-rb_obj_freeze_inline(VALUE x)
-{
-    if (RB_FL_ABLE(x)) {
-        RB_OBJ_FREEZE_RAW(x);
-        if (RBASIC_CLASS(x) && !(RBASIC(x)->flags & RUBY_FL_SINGLETON)) {
-            rb_freeze_singleton_class(x);
-        }
-    }
-}
+RUBY_SYMBOL_EXPORT_BEGIN
+void rb_obj_freeze_inline(VALUE obj);
+RUBY_SYMBOL_EXPORT_END
 
 #endif /* RBIMPL_FL_TYPE_H */