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, 12 insertions, 38 deletions

diff --git a/gc.c b/gc.c
index 0ba28ba67b..ac5ed1e7cb 100644
--- a/gc.c
+++ b/gc.c
@@ -2895,8 +2895,7 @@ rb_class_instance_allocate_internal(VALUE klass, VALUE flags, bool wb_protected)
     GC_ASSERT((flags & RUBY_T_MASK) == T_OBJECT);
     GC_ASSERT(flags & ROBJECT_EMBED);
 
-    st_table *index_tbl = RCLASS_IV_INDEX_TBL(klass);
-    uint32_t index_tbl_num_entries = index_tbl == NULL ? 0 : (uint32_t)index_tbl->num_entries;
+    uint32_t index_tbl_num_entries = RCLASS_EXT(klass)->max_iv_count;
 
     size_t size;
     bool embed = true;
@@ -2931,7 +2930,7 @@ rb_class_instance_allocate_internal(VALUE klass, VALUE flags, bool wb_protected)
 #endif
     }
     else {
-        rb_init_iv_list(obj);
+        rb_ensure_iv_list_size(obj, 0, index_tbl_num_entries);
     }
 
     return obj;
@@ -3206,20 +3205,6 @@ rb_free_const_table(struct rb_id_table *tbl)
     rb_id_table_free(tbl);
 }
 
-static int
-free_iv_index_tbl_free_i(st_data_t key, st_data_t value, st_data_t data)
-{
-    xfree((void *)value);
-    return ST_CONTINUE;
-}
-
-static void
-iv_index_tbl_free(struct st_table *tbl)
-{
-    st_foreach(tbl, free_iv_index_tbl_free_i, 0);
-    st_free_table(tbl);
-}
-
 // alive: if false, target pointers can be freed already.
 //        To check it, we need objspace parameter.
 static void
@@ -3435,6 +3420,16 @@ obj_free(rb_objspace_t *objspace, VALUE obj)
             RB_DEBUG_COUNTER_INC(obj_obj_transient);
         }
         else {
+            rb_shape_t *shape = rb_shape_get_shape_by_id(ROBJECT_SHAPE_ID(obj));
+            if (shape) {
+                VALUE klass = RBASIC_CLASS(obj);
+
+                // Increment max_iv_count if applicable, used to determine size pool allocation
+                uint32_t num_of_ivs = shape->iv_count;
+                if (RCLASS_EXT(klass)->max_iv_count < num_of_ivs) {
+                    RCLASS_EXT(klass)->max_iv_count = num_of_ivs;
+                }
+            }
             xfree(RANY(obj)->as.object.as.heap.ivptr);
             RB_DEBUG_COUNTER_INC(obj_obj_ptr);
         }
@@ -3449,9 +3444,6 @@ obj_free(rb_objspace_t *objspace, VALUE obj)
         if (RCLASS_CONST_TBL(obj)) {
             rb_free_const_table(RCLASS_CONST_TBL(obj));
         }
-        if (RCLASS_IV_INDEX_TBL(obj)) {
-            iv_index_tbl_free(RCLASS_IV_INDEX_TBL(obj));
-        }
         if (RCLASS_CVC_TBL(obj)) {
             rb_id_table_foreach_values(RCLASS_CVC_TBL(obj), cvar_table_free_i, NULL);
             rb_id_table_free(RCLASS_CVC_TBL(obj));
@@ -4873,10 +4865,6 @@ obj_memsize_of(VALUE obj, int use_all_types)
             if (RCLASS_CVC_TBL(obj)) {
                 size += rb_id_table_memsize(RCLASS_CVC_TBL(obj));
             }
-            if (RCLASS_IV_INDEX_TBL(obj)) {
-                // TODO: more correct value
-                size += st_memsize(RCLASS_IV_INDEX_TBL(obj));
-            }
             if (RCLASS_EXT(obj)->iv_tbl) {
                 size += st_memsize(RCLASS_EXT(obj)->iv_tbl);
             }
@@ -10407,15 +10395,6 @@ update_subclass_entries(rb_objspace_t *objspace, rb_subclass_entry_t *entry)
     }
 }
 
-static int
-update_iv_index_tbl_i(st_data_t key, st_data_t value, st_data_t arg)
-{
-    rb_objspace_t *objspace = (rb_objspace_t *)arg;
-    struct rb_iv_index_tbl_entry *ent = (struct rb_iv_index_tbl_entry *)value;
-    UPDATE_IF_MOVED(objspace, ent->class_value);
-    return ST_CONTINUE;
-}
-
 static void
 update_class_ext(rb_objspace_t *objspace, rb_classext_t *ext)
 {
@@ -10423,11 +10402,6 @@ update_class_ext(rb_objspace_t *objspace, rb_classext_t *ext)
     UPDATE_IF_MOVED(objspace, ext->includer);
     UPDATE_IF_MOVED(objspace, ext->refined_class);
     update_subclass_entries(objspace, ext->subclasses);
-
-    // ext->iv_index_tbl
-    if (ext->iv_index_tbl) {
-        st_foreach(ext->iv_index_tbl, update_iv_index_tbl_i, (st_data_t)objspace);
-    }
 }
 
 static void