[Feature #18239] Refactor RVARGC alloc functions

The allocation functions no longer assume that one RVALUE needs to be
allocated.

class.c

@@ -167,16 +167,16 @@ rb_class_detach_module_subclasses(VALUE klass)
 static VALUE
 class_alloc(VALUE flags, VALUE klass)
 {
-    size_t payload_size = 0;
+    size_t alloc_size = sizeof(struct RClass);
 
 #if USE_RVARGC
-    payload_size = sizeof(rb_classext_t);
+    alloc_size += sizeof(rb_classext_t);
 #endif
 
-    RVARGC_NEWOBJ_OF(obj, struct RClass, klass, (flags & T_MASK) | FL_PROMOTED1 /* start from age == 2 */ | (RGENGC_WB_PROTECTED_CLASS ? FL_WB_PROTECTED : 0), payload_size);
+    RVARGC_NEWOBJ_OF(obj, struct RClass, klass, (flags & T_MASK) | FL_PROMOTED1 /* start from age == 2 */ | (RGENGC_WB_PROTECTED_CLASS ? FL_WB_PROTECTED : 0), alloc_size);
 
 #if USE_RVARGC
-    obj->ptr = (rb_classext_t *)rb_gc_rvargc_object_data((VALUE)obj);
+    obj->ptr = (rb_classext_t *)((char *)obj + sizeof(struct RClass));
 #else
     obj->ptr = ZALLOC(rb_classext_t);
 #endif

gc.c

@@ -4577,7 +4577,9 @@ obj_memsize_of(VALUE obj, int use_all_types)
             if (RCLASS_CC_TBL(obj)) {
                 size += cc_table_memsize(RCLASS_CC_TBL(obj));
             }
+#if !USE_RVARGC
 	    size += sizeof(rb_classext_t);
+#endif
 	}
 	break;
       case T_ICLASS:
@@ -4666,7 +4668,7 @@ obj_memsize_of(VALUE obj, int use_all_types)
 	       BUILTIN_TYPE(obj), (void*)obj);
     }
 
-    return size + sizeof(RVALUE);
+    return size + GET_HEAP_PAGE(obj)->slot_size;
 }
 
 size_t

internal/gc.h

@@ -28,16 +28,18 @@ struct rb_objspace; /* in vm_core.h */
 # undef RB_OBJ_WRITE
 #endif
 
+#define RVALUE_SIZE (sizeof(struct RBasic) + sizeof(VALUE[RBIMPL_RVALUE_EMBED_LEN_MAX]))
+
 /* optimized version of NEWOBJ() */
 #define RB_NEWOBJ_OF(var, T, c, f) \
   T *(var) = (T *)(((f) & FL_WB_PROTECTED) ? \
-                   rb_wb_protected_newobj_of((c), (f) & ~FL_WB_PROTECTED, 0) : \
-                   rb_wb_unprotected_newobj_of((c), (f), 0))
+                   rb_wb_protected_newobj_of((c), (f) & ~FL_WB_PROTECTED, RVALUE_SIZE) : \
+                   rb_wb_unprotected_newobj_of((c), (f), RVALUE_SIZE))
 
 #define RB_EC_NEWOBJ_OF(ec, var, T, c, f) \
   T *(var) = (T *)(((f) & FL_WB_PROTECTED) ? \
-                   rb_ec_wb_protected_newobj_of((ec), (c), (f) & ~FL_WB_PROTECTED, 0) : \
-                   rb_wb_unprotected_newobj_of((c), (f), 0))
+                   rb_ec_wb_protected_newobj_of((ec), (c), (f) & ~FL_WB_PROTECTED, RVALUE_SIZE) : \
+                   rb_wb_unprotected_newobj_of((c), (f), RVALUE_SIZE))
 
 #define RB_RVARGC_NEWOBJ_OF(var, T, c, f, s) \
   T *(var) = (T *)(((f) & FL_WB_PROTECTED) ? \
@@ -100,7 +102,6 @@ static inline void *ruby_sized_xrealloc2_inlined(void *ptr, size_t new_count, si
 static inline void ruby_sized_xfree_inlined(void *ptr, size_t size);
 VALUE rb_class_allocate_instance(VALUE klass);
 void rb_gc_ractor_newobj_cache_clear(rb_ractor_newobj_cache_t *newobj_cache);
-void *rb_gc_rvargc_object_data(VALUE obj);
 
 RUBY_SYMBOL_EXPORT_BEGIN
 /* gc.c (export) */