Differentiate T_OBJECT shapes from other objects

We would like to differentiate types of objects via their shape.  This
commit adds a special T_OBJECT shape when we allocate an instance of
T_OBJECT.  This allows us to avoid testing whether an object is an
instance of a T_OBJECT or not, we can just check the shape.

gc.c

@@ -2942,6 +2942,12 @@ rb_class_instance_allocate_internal(VALUE klass, VALUE flags, bool wb_protected)
 #endif
 
     VALUE obj = newobj_of(klass, flags, 0, 0, 0, wb_protected, size);
+    RUBY_ASSERT(rb_shape_get_shape(obj)->type == SHAPE_ROOT ||
+            rb_shape_get_shape(obj)->type == SHAPE_INITIAL_CAPACITY);
+
+    // Set the shape to the specific T_OBJECT shape which is always
+    // SIZE_POOL_COUNT away from the root shape.
+    ROBJECT_SET_SHAPE_ID(obj, ROBJECT_SHAPE_ID(obj) + SIZE_POOL_COUNT);
 
 #if RUBY_DEBUG
     VALUE *ptr = ROBJECT_IVPTR(obj);
@@ -10030,7 +10036,7 @@ gc_ref_update_object(rb_objspace_t *objspace, VALUE v)
         }
         ptr = ROBJECT(v)->as.ary;
         size_t size_pool_shape_id = size_pool_idx_for_size(embed_size);
-        rb_shape_t * initial_shape = rb_shape_get_shape_by_id((shape_id_t)size_pool_shape_id);
+        rb_shape_t * initial_shape = rb_shape_get_shape_by_id((shape_id_t)size_pool_shape_id + SIZE_POOL_COUNT);
         rb_shape_t * new_shape = rb_shape_rebuild_shape(initial_shape, rb_shape_get_shape(v));
         rb_shape_set_shape(v, new_shape);
     }

object.c

@@ -293,7 +293,7 @@ rb_obj_copy_ivar(VALUE dest, VALUE obj)
     rb_shape_t * initial_shape = rb_shape_get_shape(dest);
 
     if (initial_shape->size_pool_index != src_shape->size_pool_index) {
-        RUBY_ASSERT(initial_shape->parent_id == ROOT_SHAPE_ID || initial_shape->type == SHAPE_ROOT);
+        RUBY_ASSERT(initial_shape->type == SHAPE_T_OBJECT);
 
         shape_to_set_on_dest = rb_shape_rebuild_shape(initial_shape, src_shape);
     }

shape.c

@@ -8,6 +8,7 @@
 #include <stdbool.h>
 
 static ID id_frozen;
+static ID id_t_object;
 static ID size_pool_edge_names[SIZE_POOL_COUNT];
 
 /*
@@ -152,6 +153,7 @@ get_next_shape_internal(rb_shape_t * shape, ID id, enum shape_type shape_type)
                   case SHAPE_CAPACITY_CHANGE:
                   case SHAPE_IVAR_UNDEF:
                   case SHAPE_FROZEN:
+                  case SHAPE_T_OBJECT:
                     new_shape->next_iv_index = shape->next_iv_index;
                     break;
                   case SHAPE_INITIAL_CAPACITY:
@@ -264,6 +266,7 @@ rb_shape_get_iv_index(rb_shape_t * shape, ID id, attr_index_t *value)
               case SHAPE_IVAR_UNDEF:
               case SHAPE_ROOT:
               case SHAPE_INITIAL_CAPACITY:
+              case SHAPE_T_OBJECT:
                 return false;
               case SHAPE_FROZEN:
                 rb_bug("Ivar should not exist on transition\n");
@@ -333,14 +336,16 @@ rb_shape_rebuild_shape(rb_shape_t * initial_shape, rb_shape_t * dest_shape)
 {
     rb_shape_t * midway_shape;
 
-    if (dest_shape->type != SHAPE_ROOT) {
+    RUBY_ASSERT(initial_shape->type == SHAPE_T_OBJECT);
+
+    if (dest_shape->type != initial_shape->type) {
         midway_shape = rb_shape_rebuild_shape(initial_shape, rb_shape_get_parent(dest_shape));
     }
     else {
         midway_shape = initial_shape;
     }
 
-    switch (dest_shape->type) {
+    switch ((enum shape_type)dest_shape->type) {
       case SHAPE_IVAR:
         if (midway_shape->capacity <= midway_shape->next_iv_index) {
             // There isn't enough room to write this IV, so we need to increase the capacity
@@ -355,6 +360,8 @@ rb_shape_rebuild_shape(rb_shape_t * initial_shape, rb_shape_t * dest_shape)
       case SHAPE_ROOT:
       case SHAPE_FROZEN:
       case SHAPE_CAPACITY_CHANGE:
+      case SHAPE_INITIAL_CAPACITY:
+      case SHAPE_T_OBJECT:
         break;
     }
 
@@ -592,6 +599,7 @@ void
 Init_default_shapes(void)
 {
     id_frozen = rb_make_internal_id();
+    id_t_object = rb_make_internal_id();
 
     // Shapes by size pool
     for (int i = 0; i < SIZE_POOL_COUNT; i++) {
@@ -615,6 +623,16 @@ Init_default_shapes(void)
         RUBY_ASSERT(rb_shape_id(new_shape) == (shape_id_t)i);
     }
 
+    // Make shapes for T_OBJECT
+    for (int i = 0; i < SIZE_POOL_COUNT; i++) {
+        rb_shape_t * shape = rb_shape_get_shape_by_id(i);
+#if RUBY_DEBUG
+        rb_shape_t * t_object_shape =
+#endif
+            get_next_shape_internal(shape, id_t_object, SHAPE_T_OBJECT);
+        RUBY_ASSERT(rb_shape_id(t_object_shape) == (shape_id_t)(i + SIZE_POOL_COUNT));
+    }
+
     // Special const shape
 #if RUBY_DEBUG
     rb_shape_t * special_const_shape =
@@ -644,6 +662,7 @@ Init_shape(void)
     rb_define_method(rb_cShape, "capacity", rb_shape_capacity, 0);
     rb_define_const(rb_cShape, "SHAPE_ROOT", INT2NUM(SHAPE_ROOT));
     rb_define_const(rb_cShape, "SHAPE_IVAR", INT2NUM(SHAPE_IVAR));
+    rb_define_const(rb_cShape, "SHAPE_T_OBJECT", INT2NUM(SHAPE_T_OBJECT));
     rb_define_const(rb_cShape, "SHAPE_IVAR_UNDEF", INT2NUM(SHAPE_IVAR_UNDEF));
     rb_define_const(rb_cShape, "SHAPE_FROZEN", INT2NUM(SHAPE_FROZEN));
     rb_define_const(rb_cShape, "SHAPE_BITS", INT2NUM(SHAPE_BITS));

shape.h

@@ -42,7 +42,7 @@ typedef uint16_t shape_id_t;
 # define ROOT_SHAPE_ID 0x0
 // We use SIZE_POOL_COUNT number of shape IDs for transitions out of different size pools
 // The next available shapd ID will be the SPECIAL_CONST_SHAPE_ID
-# define SPECIAL_CONST_SHAPE_ID SIZE_POOL_COUNT
+# define SPECIAL_CONST_SHAPE_ID (SIZE_POOL_COUNT * 2)
 
 struct rb_shape {
     struct rb_id_table * edges; // id_table from ID (ivar) to next shape
@@ -63,6 +63,7 @@ enum shape_type {
     SHAPE_CAPACITY_CHANGE,
     SHAPE_IVAR_UNDEF,
     SHAPE_INITIAL_CAPACITY,
+    SHAPE_T_OBJECT,
 };
 
 #if SHAPE_IN_BASIC_FLAGS

test/ruby/test_shapes.rb

@@ -88,8 +88,8 @@ class TestShapes < Test::Unit::TestCase
 
   class TestObject; end
 
-  def test_new_obj_has_root_shape
-    assert_shape_equal(RubyVM::Shape.root_shape, RubyVM::Shape.of(TestObject.new))
+  def test_new_obj_has_t_object_shape
+    assert_shape_equal(RubyVM::Shape.root_shape, RubyVM::Shape.of(TestObject.new).parent)
   end
 
   def test_str_has_root_shape
@@ -114,14 +114,23 @@ class TestShapes < Test::Unit::TestCase
 
   def test_basic_shape_transition
     obj = Example.new
-    refute_equal(RubyVM::Shape.root_shape, RubyVM::Shape.of(obj))
-    assert_shape_equal(RubyVM::Shape.root_shape.edges[:@a], RubyVM::Shape.of(obj))
+    shape = RubyVM::Shape.of(obj)
+    refute_equal(RubyVM::Shape.root_shape, shape)
+    assert_equal :@a, shape.edge_name
+    assert_equal RubyVM::Shape::SHAPE_IVAR, shape.type
+
+    shape = shape.parent
+    assert_equal RubyVM::Shape::SHAPE_T_OBJECT, shape.type
+
+    shape = shape.parent
+    assert_equal(RubyVM::Shape.root_shape.id, shape.id)
     assert_equal(obj.instance_variable_get(:@a), 1)
   end
 
   def test_different_objects_make_same_transition
-    obj = Example.new
+    obj = []
     obj2 = ""
+    obj.instance_variable_set(:@a, 1)
     obj2.instance_variable_set(:@a, 1)
     assert_shape_equal(RubyVM::Shape.of(obj), RubyVM::Shape.of(obj2))
   end

variable.c

@@ -1606,6 +1606,7 @@ iterate_over_shapes_with_callback(rb_shape_t *shape, rb_ivar_foreach_callback_fu
       case SHAPE_CAPACITY_CHANGE:
       case SHAPE_FROZEN:
       case SHAPE_IVAR_UNDEF:
+      case SHAPE_T_OBJECT:
         iterate_over_shapes_with_callback(rb_shape_get_parent(shape), callback, itr_data);
         return;
     }