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>

3 files changed, 12 insertions, 26 deletions

diff --git a/internal/class.h b/internal/class.h
index ae680564a6..4a3e09ddc7 100644
--- a/internal/class.h
+++ b/internal/class.h
@@ -14,6 +14,7 @@
 #include "ruby/internal/stdbool.h"     /* for bool */
 #include "ruby/intern.h"        /* for rb_alloc_func_t */
 #include "ruby/ruby.h"          /* for struct RBasic */
+#include "shape.h"
 
 #ifdef RCLASS_SUPER
 # undef RCLASS_SUPER
@@ -27,8 +28,8 @@ struct rb_subclass_entry {
 
 struct rb_iv_index_tbl_entry {
     uint32_t index;
-    rb_serial_t class_serial;
-    VALUE class_value;
+    shape_id_t source_shape_id;
+    shape_id_t dest_shape_id;
 };
 
 struct rb_cvar_class_tbl_entry {
@@ -38,7 +39,6 @@ struct rb_cvar_class_tbl_entry {
 };
 
 struct rb_classext_struct {
-    struct st_table *iv_index_tbl; // ID -> struct rb_iv_index_tbl_entry
     struct st_table *iv_tbl;
 #if SIZEOF_SERIAL_T == SIZEOF_VALUE /* otherwise m_tbl is in struct RClass */
     struct rb_id_table *m_tbl;
@@ -64,6 +64,10 @@ struct rb_classext_struct {
     const VALUE refined_class;
     rb_alloc_func_t allocator;
     const VALUE includer;
+    uint32_t max_iv_count;
+#if !SHAPE_IN_BASIC_FLAGS
+    shape_id_t shape_id;
+#endif
 };
 
 struct RClass {
@@ -102,7 +106,6 @@ typedef struct rb_classext_struct rb_classext_t;
 #define RCLASS_CALLABLE_M_TBL(c) (RCLASS_EXT(c)->callable_m_tbl)
 #define RCLASS_CC_TBL(c) (RCLASS_EXT(c)->cc_tbl)
 #define RCLASS_CVC_TBL(c) (RCLASS_EXT(c)->cvc_tbl)
-#define RCLASS_IV_INDEX_TBL(c) (RCLASS_EXT(c)->iv_index_tbl)
 #define RCLASS_ORIGIN(c) (RCLASS_EXT(c)->origin_)
 #define RCLASS_REFINED_CLASS(c) (RCLASS_EXT(c)->refined_class)
 #if SIZEOF_SERIAL_T == SIZEOF_VALUE
diff --git a/internal/object.h b/internal/object.h
index 88f3a44bc6..7b54e13dd2 100644
--- a/internal/object.h
+++ b/internal/object.h
@@ -9,11 +9,6 @@
  * @brief      Internal header for Object.
  */
 #include "ruby/ruby.h"          /* for VALUE */
-#include "internal/class.h"     /* for RCLASS_IV_INDEX_TBL */
-
-#ifdef ROBJECT_IV_INDEX_TBL
-# undef ROBJECT_IV_INDEX_TBL
-#endif
 
 /* object.c */
 VALUE rb_class_search_ancestor(VALUE klass, VALUE super);
@@ -26,7 +21,6 @@ int rb_bool_expected(VALUE, const char *, int raise);
 static inline void RBASIC_CLEAR_CLASS(VALUE obj);
 static inline void RBASIC_SET_CLASS_RAW(VALUE obj, VALUE klass);
 static inline void RBASIC_SET_CLASS(VALUE obj, VALUE klass);
-static inline struct st_table *ROBJECT_IV_INDEX_TBL_inline(VALUE obj);
 
 RUBY_SYMBOL_EXPORT_BEGIN
 /* object.c (export) */
@@ -64,20 +58,4 @@ RBASIC_SET_CLASS(VALUE obj, VALUE klass)
     RBASIC_SET_CLASS_RAW(obj, klass);
     RB_OBJ_WRITTEN(obj, oldv, klass);
 }
-
-RBIMPL_ATTR_PURE()
-static inline struct st_table *
-ROBJECT_IV_INDEX_TBL_inline(VALUE obj)
-{
-    if (RB_FL_ANY_RAW(obj, ROBJECT_EMBED)) {
-        VALUE klass = rb_obj_class(obj);
-        return RCLASS_IV_INDEX_TBL(klass);
-    }
-    else {
-        const struct RObject *const ptr = ROBJECT(obj);
-        return ptr->as.heap.iv_index_tbl;
-    }
-}
-#define ROBJECT_IV_INDEX_TBL ROBJECT_IV_INDEX_TBL_inline
-
 #endif /* INTERNAL_OBJECT_H */
diff --git a/internal/variable.h b/internal/variable.h
index 1a19e8964b..47037a3392 100644
--- a/internal/variable.h
+++ b/internal/variable.h
@@ -37,6 +37,9 @@ static inline void ROBJ_TRANSIENT_SET(VALUE obj);
 static inline void ROBJ_TRANSIENT_UNSET(VALUE obj);
 uint32_t rb_obj_ensure_iv_index_mapping(VALUE obj, ID id);
 
+struct gen_ivtbl;
+int rb_gen_ivtbl_get(VALUE obj, ID id, struct gen_ivtbl **ivtbl);
+
 RUBY_SYMBOL_EXPORT_BEGIN
 /* variable.c (export) */
 void rb_mark_generic_ivar(VALUE);
@@ -52,6 +55,8 @@ VALUE rb_gvar_set(ID, VALUE);
 VALUE rb_gvar_defined(ID);
 void rb_const_warn_if_deprecated(const rb_const_entry_t *, VALUE, ID);
 void rb_init_iv_list(VALUE obj);
+void rb_ensure_iv_list_size(VALUE obj, uint32_t len, uint32_t newsize);
+struct gen_ivtbl * rb_ensure_generic_iv_list_size(VALUE obj, uint32_t newsize);
 MJIT_SYMBOL_EXPORT_END
 
 static inline bool