Reclaim one VALUE from rb_classext_t

The `includer` field is only used for `T_ICLASS`, so by moving
it into the existing union we can save one `VALUE` per class
and module.

Author: byroot

gc.c

@@ -3718,12 +3718,14 @@ update_superclasses(rb_objspace_t *objspace, rb_classext_t *ext)
 }
 
 static void
-update_classext_values(rb_objspace_t *objspace, rb_classext_t *ext)
+update_classext_values(rb_objspace_t *objspace, rb_classext_t *ext, bool is_iclass)
 {
     UPDATE_IF_MOVED(objspace, RCLASSEXT_ORIGIN(ext));
     UPDATE_IF_MOVED(objspace, RCLASSEXT_REFINED_CLASS(ext));
-    UPDATE_IF_MOVED(objspace, RCLASSEXT_INCLUDER(ext));
     UPDATE_IF_MOVED(objspace, RCLASSEXT_CLASSPATH(ext));
+    if (is_iclass) {
+        UPDATE_IF_MOVED(objspace, RCLASSEXT_INCLUDER(ext));
+    }
 }
 
 static void
@@ -3757,7 +3759,7 @@ update_classext(rb_classext_t *ext, bool is_prime, VALUE namespace, void *arg)
     update_superclasses(objspace, ext);
     update_subclasses(objspace, ext);
 
-    update_classext_values(objspace, ext);
+    update_classext_values(objspace, ext, false);
 }
 
 static void
@@ -3774,7 +3776,7 @@ update_iclass_classext(rb_classext_t *ext, bool is_prime, VALUE namespace, void
     update_cc_tbl(objspace, RCLASSEXT_CC_TBL(ext));
     update_subclasses(objspace, ext);
 
-    update_classext_values(objspace, ext);
+    update_classext_values(objspace, ext, true);
 }
 
 extern rb_symbols_t ruby_global_symbols;

internal/class.h

@@ -116,8 +116,10 @@ struct rb_classext_struct {
         struct {
             VALUE attached_object;
         } singleton_class;
+        struct {
+            const VALUE includer;
+        } iclass;
     } as;
-    const VALUE includer;
     attr_index_t max_iv_count;
     unsigned char variation_count;
     bool permanent_classpath : 1;
@@ -184,7 +186,7 @@ static inline rb_classext_t * RCLASS_EXT_WRITABLE(VALUE obj);
 #define RCLASSEXT_ORIGIN(ext) (ext->origin_)
 #define RCLASSEXT_REFINED_CLASS(ext) (ext->refined_class)
 // class.allocator/singleton_class.attached_object are not accessed directly via RCLASSEXT_*
-#define RCLASSEXT_INCLUDER(ext) (ext->includer)
+#define RCLASSEXT_INCLUDER(ext) (ext->as.iclass.includer)
 #define RCLASSEXT_MAX_IV_COUNT(ext) (ext->max_iv_count)
 #define RCLASSEXT_VARIATION_COUNT(ext) (ext->variation_count)
 #define RCLASSEXT_PERMANENT_CLASSPATH(ext) (ext->permanent_classpath)
@@ -237,7 +239,7 @@ static inline void RCLASSEXT_SET_INCLUDER(rb_classext_t *ext, VALUE klass, VALUE
 // namespaces don't make changes on these refined_class/attached_object/includer
 #define RCLASS_REFINED_CLASS(c) (RCLASS_EXT_PRIME(c)->refined_class)
 #define RCLASS_ATTACHED_OBJECT(c) (RCLASS_EXT_PRIME(c)->as.singleton_class.attached_object)
-#define RCLASS_INCLUDER(c) (RCLASS_EXT_PRIME(c)->includer)
+#define RCLASS_INCLUDER(c) (RCLASS_EXT_PRIME(c)->as.iclass.includer)
 
 // Writable classext entries (instead of RCLASS_SET_*) because member data will be operated directly
 #define RCLASS_WRITABLE_M_TBL(c) (RCLASS_EXT_WRITABLE(c)->m_tbl)
@@ -459,6 +461,7 @@ RCLASSEXT_SET_ORIGIN(rb_classext_t *ext, VALUE klass, VALUE origin)
 static inline void
 RCLASSEXT_SET_INCLUDER(rb_classext_t *ext, VALUE klass, VALUE includer)
 {
+    RUBY_ASSERT(RB_TYPE_P(klass, T_ICLASS));
     RB_OBJ_WRITE(klass, &(RCLASSEXT_INCLUDER(ext)), includer);
 }
 
@@ -651,7 +654,7 @@ RCLASS_SET_REFINED_CLASS(VALUE klass, VALUE refined)
 static inline rb_alloc_func_t
 RCLASS_ALLOCATOR(VALUE klass)
 {
-    if (RCLASS_SINGLETON_P(klass)) {
+    if (RCLASS_SINGLETON_P(klass) || RB_TYPE_P(klass, T_ICLASS)) {
         return 0;
     }
     return RCLASS_EXT_PRIME(klass)->as.class.allocator;
@@ -702,6 +705,7 @@ RICLASS_OWNS_M_TBL_P(VALUE iclass)
 static inline void
 RCLASS_SET_INCLUDER(VALUE iclass, VALUE klass)
 {
+    RUBY_ASSERT(RB_TYPE_P(iclass, T_ICLASS));
     RB_OBJ_WRITE(iclass, &RCLASS_INCLUDER(iclass), klass);
 }