Get rid of frozen shapes.

Instead `shape_id_t` higher bits contain flags, and the first one
tells whether the shape is frozen.

This has multiple benefits:
  - Can check if a shape is frozen with a single bit check instead of
    dereferencing a pointer.
  - Guarantees it is always possible to transition to frozen.
  - This allow reclaiming `FL_FREEZE` (not done yet).

The downside is you have to be careful to preserve these flags
when transitioning.

Author: byroot

ext/objspace/objspace_dump.c

@@ -817,9 +817,6 @@ shape_id_i(shape_id_t shape_id, void *data)
         dump_append(dc, ",\"edge_name\":");
         dump_append_id(dc, shape->edge_name);
 
-        break;
-      case SHAPE_FROZEN:
-        dump_append(dc, ", \"shape_type\":\"FROZEN\"");
         break;
       case SHAPE_T_OBJECT:
         dump_append(dc, ", \"shape_type\":\"T_OBJECT\"");

internal/class.h

@@ -297,7 +297,6 @@ static inline void RCLASS_WRITE_CLASSPATH(VALUE klass, VALUE classpath, bool per
 #define RCLASS_PRIME_CLASSEXT_WRITABLE FL_USER2
 #define RCLASS_IS_INITIALIZED FL_USER3
 // 3 is RMODULE_IS_REFINEMENT for RMODULE
-// 4-19: SHAPE_FLAG_MASK
 
 /* class.c */
 rb_classext_t * rb_class_duplicate_classext(rb_classext_t *orig, VALUE obj, const rb_namespace_t *ns);

object.c

@@ -496,12 +496,7 @@ rb_obj_clone_setup(VALUE obj, VALUE clone, VALUE kwfreeze)
 
         if (RB_OBJ_FROZEN(obj)) {
             shape_id_t next_shape_id = rb_shape_transition_frozen(clone);
-            if (!rb_shape_obj_too_complex_p(clone) && rb_shape_too_complex_p(next_shape_id)) {
-                rb_evict_ivars_to_hash(clone);
-            }
-            else {
-                rb_obj_set_shape_id(clone, next_shape_id);
-            }
+            rb_obj_set_shape_id(clone, next_shape_id);
         }
         break;
       case Qtrue: {
@@ -518,14 +513,7 @@ rb_obj_clone_setup(VALUE obj, VALUE clone, VALUE kwfreeze)
         rb_funcallv_kw(clone, id_init_clone, 2, argv, RB_PASS_KEYWORDS);
         RBASIC(clone)->flags |= FL_FREEZE;
         shape_id_t next_shape_id = rb_shape_transition_frozen(clone);
-        // If we're out of shapes, but we want to freeze, then we need to
-        // evacuate this clone to a hash
-        if (!rb_shape_obj_too_complex_p(clone) && rb_shape_too_complex_p(next_shape_id)) {
-            rb_evict_ivars_to_hash(clone);
-        }
-        else {
-            rb_obj_set_shape_id(clone, next_shape_id);
-        }
+        rb_obj_set_shape_id(clone, next_shape_id);
         break;
       }
       case Qfalse: {