Move classpath to rb_classext_t

This commit moves the classpath (and tmp_classpath) from instance
variables to the rb_classext_t. This improves performance as we no
longer need to set an instance variable when assigning a classpath to
a class.

I benchmarked with the following script:

```ruby
name = :MyClass

puts(Benchmark.measure do
  10_000_000.times do |i|
    Object.const_set(name, Class.new)
    Object.send(:remove_const, name)
  end
end)
```

Before this patch:

```
  5.440119   0.025264   5.465383 (  5.467105)
```

After this patch:

```
  4.889646   0.028325   4.917971 (  4.942678)
```

gc.c

@@ -7259,6 +7259,8 @@ gc_mark_children(rb_objspace_t *objspace, VALUE obj)
             gc_mark(objspace, RCLASS_IVPTR(obj)[i]);
         }
         mark_const_tbl(objspace, RCLASS_CONST_TBL(obj));
+
+        gc_mark(objspace, RCLASS_EXT(obj)->classpath);
         break;
 
       case T_ICLASS:
@@ -10559,6 +10561,8 @@ gc_update_object_references(rb_objspace_t *objspace, VALUE obj)
 
         update_class_ext(objspace, RCLASS_EXT(obj));
         update_const_tbl(objspace, RCLASS_CONST_TBL(obj));
+
+        UPDATE_IF_MOVED(objspace, RCLASS_EXT(obj)->classpath);
         break;
 
       case T_ICLASS:

internal/class.h

@@ -59,6 +59,8 @@ struct rb_classext_struct {
 #endif
     uint32_t max_iv_count;
     unsigned char variation_count;
+    bool permanent_classpath;
+    VALUE classpath;
 };
 typedef struct rb_classext_struct rb_classext_t;
 
@@ -73,8 +75,10 @@ struct RClass {
 #endif
 };
 
-typedef struct rb_subclass_entry rb_subclass_entry_t;
+#if RCLASS_EXT_EMBEDDED
-typedef struct rb_classext_struct rb_classext_t;
+// Assert that classes can be embedded in size_pools[2] (which has 160B slot size)
+STATIC_ASSERT(sizeof_rb_classext_t, sizeof(struct RClass) + sizeof(rb_classext_t) <= 4 * RVALUE_SIZE);
+#endif
 
 #if RCLASS_EXT_EMBEDDED
 #  define RCLASS_EXT(c) ((rb_classext_t *)((char *)(c) + sizeof(struct RClass)))
@@ -184,4 +188,14 @@ RCLASS_SET_SUPER(VALUE klass, VALUE super)
     return super;
 }
 
+static inline void
+RCLASS_SET_CLASSPATH(VALUE klass, VALUE classpath, bool permanent)
+{
+    assert(BUILTIN_TYPE(klass) == T_CLASS || BUILTIN_TYPE(klass) == T_MODULE);
+    assert(classpath == 0 || BUILTIN_TYPE(classpath) == T_STRING);
+
+    RB_OBJ_WRITE(klass, &(RCLASS_EXT(klass)->classpath), classpath);
+    RCLASS_EXT(klass)->permanent_classpath = permanent;
+}
+
 #endif /* INTERNAL_CLASS_H */

spec/ruby/optional/capi/object_spec.rb

@@ -993,13 +993,19 @@ describe "CApiObject" do
       end
 
       it "calls the callback function for each cvar and ivar on a class" do
+        exp = [:@@cvar, :foo, :@@cvar2, :bar, :@ivar, :baz]
+        exp.unshift(:__classpath__, 'CApiObjectSpecs::CVars') if RUBY_VERSION < "3.3"
+
         ary = @o.rb_ivar_foreach(CApiObjectSpecs::CVars)
-        ary.should == [:__classpath__, 'CApiObjectSpecs::CVars', :@@cvar, :foo, :@@cvar2, :bar, :@ivar, :baz]
+        ary.should == exp
       end
 
       it "calls the callback function for each cvar and ivar on a module" do
+        exp = [:@@mvar, :foo, :@@mvar2, :bar, :@ivar, :baz]
+        exp.unshift(:__classpath__, 'CApiObjectSpecs::MVars') if RUBY_VERSION < "3.3"
+
         ary = @o.rb_ivar_foreach(CApiObjectSpecs::MVars)
-        ary.should == [:__classpath__, 'CApiObjectSpecs::MVars', :@@mvar, :foo, :@@mvar2, :bar, :@ivar, :baz]
+        ary.should == exp
       end
 
     end

variable.c

@@ -46,7 +46,7 @@ RUBY_EXTERN rb_serial_t ruby_vm_global_cvar_state;
 typedef void rb_gvar_compact_t(void *var);
 
 static struct rb_id_table *rb_global_tbl;
-static ID autoload, classpath, tmp_classpath;
+static ID autoload;
 
 // This hash table maps file paths to loadable features. We use this to track
 // autoload state until it's no longer needed.
@@ -75,10 +75,6 @@ Init_var_tables(void)
     rb_global_tbl = rb_id_table_create(0);
     generic_iv_tbl_ = st_init_numtable();
     autoload = rb_intern_const("__autoload__");
-    /* __classpath__: fully qualified class path */
-    classpath = rb_intern_const("__classpath__");
-    /* __tmp_classpath__: temporary class path which contains anonymous names */
-    tmp_classpath = rb_intern_const("__tmp_classpath__");
 
     autoload_mutex = rb_mutex_new();
     rb_obj_hide(autoload_mutex);
@@ -109,17 +105,16 @@ rb_namespace_p(VALUE obj)
  * Ruby level APIs that can change a permanent +classpath+.
  */
 static VALUE
-classname(VALUE klass, int *permanent)
+classname(VALUE klass, bool *permanent)
 {
-    *permanent = 0;
+    *permanent = false;
 
-    VALUE classpathv = rb_ivar_lookup(klass, classpath, Qnil);
+    VALUE classpath = RCLASS_EXT(klass)->classpath;
-    if (RTEST(classpathv)) {
+    if (classpath == 0) return Qnil;
-        *permanent = 1;
-        return classpathv;
-    }
 
-    return rb_ivar_lookup(klass, tmp_classpath, Qnil);;
+    *permanent = RCLASS_EXT(klass)->permanent_classpath;
+
+    return classpath;
 }
 
 /*
@@ -132,7 +127,7 @@ classname(VALUE klass, int *permanent)
 VALUE
 rb_mod_name(VALUE mod)
 {
-    int permanent;
+    bool permanent;
     return classname(mod, &permanent);
 }
 
@@ -158,7 +153,7 @@ make_temporary_path(VALUE obj, VALUE klass)
 typedef VALUE (*fallback_func)(VALUE obj, VALUE name);
 
 static VALUE
-rb_tmp_class_path(VALUE klass, int *permanent, fallback_func fallback)
+rb_tmp_class_path(VALUE klass, bool *permanent, fallback_func fallback)
 {
     VALUE path = classname(klass, permanent);
 
@@ -171,11 +166,11 @@ rb_tmp_class_path(VALUE klass, int *permanent, fallback_func fallback)
                 path = Qfalse;
             }
             else {
-                int perm;
+                bool perm;
                 path = rb_tmp_class_path(RBASIC(klass)->klass, &perm, fallback);
             }
         }
-        *permanent = 0;
+        *permanent = false;
         return fallback(klass, path);
     }
 }
@@ -183,7 +178,7 @@ rb_tmp_class_path(VALUE klass, int *permanent, fallback_func fallback)
 VALUE
 rb_class_path(VALUE klass)
 {
-    int permanent;
+    bool permanent;
     VALUE path = rb_tmp_class_path(klass, &permanent, make_temporary_path);
     if (!NIL_P(path)) path = rb_str_dup(path);
     return path;
@@ -204,7 +199,7 @@ no_fallback(VALUE obj, VALUE name)
 VALUE
 rb_search_class_path(VALUE klass)
 {
-    int permanent;
+    bool permanent;
     return rb_tmp_class_path(klass, &permanent, no_fallback);
 }
 
@@ -226,21 +221,18 @@ build_const_path(VALUE head, ID tail)
 void
 rb_set_class_path_string(VALUE klass, VALUE under, VALUE name)
 {
-    VALUE str;
+    bool permanent = true;
-    ID pathid = classpath;
 
+    VALUE str;
     if (under == rb_cObject) {
         str = rb_str_new_frozen(name);
     }
     else {
-        int permanent;
         str = rb_tmp_class_path(under, &permanent, make_temporary_path);
         str = build_const_pathname(str, name);
-        if (!permanent) {
-            pathid = tmp_classpath;
-        }
     }
-    rb_ivar_set(klass, pathid, str);
+
+    RCLASS_SET_CLASSPATH(klass, str, permanent);
 }
 
 void
@@ -311,7 +303,7 @@ rb_class_name(VALUE klass)
 const char *
 rb_class2name(VALUE klass)
 {
-    int permanent;
+    bool permanent;
     VALUE path = rb_tmp_class_path(rb_class_real(klass), &permanent, make_temporary_path);
     if (NIL_P(path)) return NULL;
     return RSTRING_PTR(path);
@@ -3217,17 +3209,21 @@ set_namespace_path_i(ID id, VALUE v, void *payload)
 {
     rb_const_entry_t *ce = (rb_const_entry_t *)v;
     VALUE value = ce->value;
-    int has_permanent_classpath;
     VALUE parental_path = *((VALUE *) payload);
     if (!rb_is_const_id(id) || !rb_namespace_p(value)) {
         return ID_TABLE_CONTINUE;
     }
+
+    bool has_permanent_classpath;
     classname(value, &has_permanent_classpath);
     if (has_permanent_classpath) {
         return ID_TABLE_CONTINUE;
     }
     set_namespace_path(value, build_const_path(parental_path, id));
-    rb_attr_delete(value, tmp_classpath);
+
+    if (!RCLASS_EXT(value)->permanent_classpath) {
+        RCLASS_SET_CLASSPATH(value, 0, false);
+    }
 
     return ID_TABLE_CONTINUE;
 }
@@ -3244,7 +3240,8 @@ set_namespace_path(VALUE named_namespace, VALUE namespace_path)
 
     RB_VM_LOCK_ENTER();
     {
-        rb_class_ivar_set(named_namespace, classpath, namespace_path);
+        RCLASS_SET_CLASSPATH(named_namespace, namespace_path, true);
+
         if (const_table) {
             rb_id_table_foreach(const_table, set_namespace_path_i, &namespace_path);
         }
@@ -3304,24 +3301,24 @@ const_set(VALUE klass, ID id, VALUE val)
      * and avoid order-dependency on const_tbl
      */
     if (rb_cObject && rb_namespace_p(val)) {
-        int val_path_permanent;
+        bool val_path_permanent;
         VALUE val_path = classname(val, &val_path_permanent);
         if (NIL_P(val_path) || !val_path_permanent) {
             if (klass == rb_cObject) {
                 set_namespace_path(val, rb_id2str(id));
             }
             else {
-                int parental_path_permanent;
+                bool parental_path_permanent;
                 VALUE parental_path = classname(klass, &parental_path_permanent);
                 if (NIL_P(parental_path)) {
-                    int throwaway;
+                    bool throwaway;
                     parental_path = rb_tmp_class_path(klass, &throwaway, make_temporary_path);
                 }
                 if (parental_path_permanent && !val_path_permanent) {
                     set_namespace_path(val, build_const_path(parental_path, id));
                 }
                 else if (!parental_path_permanent && NIL_P(val_path)) {
-                    ivar_set(val, tmp_classpath, build_const_path(parental_path, id));
+                    RCLASS_SET_CLASSPATH(val, build_const_path(parental_path, id), false);
                 }
             }
         }