shape.c: refactor frozen shape to no longer be final

This opens the door to store more informations in shapes, such
as the `object_id` or object address in case it has been observed
and the object has to be moved.

Author: byroot

object.c

@@ -358,7 +358,7 @@ rb_obj_copy_ivar(VALUE dest, VALUE obj)
 
     rb_shape_t *initial_shape = rb_shape_get_shape(dest);
 
-    if (initial_shape->heap_index != src_shape->heap_index) {
+    if (initial_shape->heap_index != src_shape->heap_index || !rb_shape_canonical_p(src_shape)) {
         RUBY_ASSERT(initial_shape->type == SHAPE_T_OBJECT);
 
         shape_to_set_on_dest = rb_shape_rebuild_shape(initial_shape, src_shape);

shape.c

@@ -50,6 +50,12 @@ static ID id_t_object;
 #define BLACK 0x0
 #define RED 0x1
 
+enum shape_flags {
+    SHAPE_FL_FROZEN = 1 << 0,
+
+    SHAPE_FL_NON_CANONICAL_MASK = SHAPE_FL_FROZEN,
+};
+
 static redblack_node_t *
 redblack_left(redblack_node_t *node)
 {
@@ -418,6 +424,7 @@ rb_shape_alloc(ID edge_name, rb_shape_t *parent, enum shape_type type)
 {
     rb_shape_t *shape = rb_shape_alloc_with_parent_id(edge_name, rb_shape_id(parent));
     shape->type = (uint8_t)type;
+    shape->flags = parent->flags;
     shape->heap_index = parent->heap_index;
     shape->capacity = parent->capacity;
     shape->edges = 0;
@@ -478,6 +485,7 @@ rb_shape_alloc_new_child(ID id, rb_shape_t *shape, enum shape_type shape_type)
         break;
       case SHAPE_FROZEN:
         new_shape->next_field_index = shape->next_field_index;
+        new_shape->flags |= SHAPE_FL_FROZEN;
         break;
       case SHAPE_OBJ_TOO_COMPLEX:
       case SHAPE_ROOT:
@@ -576,7 +584,7 @@ get_next_shape_internal(rb_shape_t *shape, ID id, enum shape_type shape_type, bo
 bool
 rb_shape_frozen_shape_p(rb_shape_t *shape)
 {
-    return SHAPE_FROZEN == (enum shape_type)shape->type;
+    return SHAPE_FL_FROZEN & shape->flags;
 }
 
 static rb_shape_t *
@@ -975,6 +983,9 @@ rb_shape_traverse_from_new_root(rb_shape_t *initial_shape, rb_shape_t *dest_shap
     return next_shape;
 }
 
+// Rebuild a similar shape with the same ivars but starting from
+// a different SHAPE_T_OBJECT, and don't cary over non-canonical transitions
+// such as SHAPE_FROZEN.
 rb_shape_t *
 rb_shape_rebuild_shape(rb_shape_t *initial_shape, rb_shape_t *dest_shape)
 {

shape.h

@@ -48,6 +48,7 @@ struct rb_shape {
     attr_index_t capacity; // Total capacity of the object with this shape
     uint8_t type;
     uint8_t heap_index;
+    uint8_t flags;
     shape_id_t parent_id;
     redblack_node_t *ancestor_index;
 };
@@ -171,6 +172,12 @@ rb_shape_t *rb_shape_get_next_no_warnings(rb_shape_t *shape, VALUE obj, ID id);
 
 rb_shape_t *rb_shape_rebuild_shape(rb_shape_t *initial_shape, rb_shape_t *dest_shape);
 
+static inline bool
+rb_shape_canonical_p(rb_shape_t *shape)
+{
+    return !shape->flags;
+}
+
 static inline uint32_t
 ROBJECT_FIELDS_CAPACITY(VALUE obj)
 {