Don't add singleton classes to subclasses list

Singleton classes don't need to be iterated in the subclasses list, we
only need to know that a subclass exists so that we can use the "method
entry" path for invalidation.

Author: jhawthorn

class.c

@@ -743,7 +743,15 @@ class_associate_super(VALUE klass, VALUE super, bool init)
         // Include/prepend inserts ICLASSes into the super chain, but T_CLASS
         // subclass lists should track only the immutable T_CLASS→T_CLASS link.
         if (RB_TYPE_P(klass, T_CLASS) && RB_TYPE_P(super, T_CLASS)) {
-            class_switch_superclass(super, klass);
+            if (RCLASS_SINGLETON_P(klass)) {
+                // Instead of adding singleton classes to the subclass list,
+                // just set a flag so that method cache invalidation takes the
+                // tree path.
+                FL_SET_RAW(super, RCLASS_HAS_SUBCLASSES);
+            }
+            else {
+                class_switch_superclass(super, klass);
+            }
         }
     }
     if (init) {
@@ -1328,9 +1336,14 @@ static inline VALUE
 make_singleton_class(VALUE obj)
 {
     VALUE orig_class = METACLASS_OF(obj);
-    VALUE klass = class_boot_boxable(orig_class, FL_TEST_RAW(orig_class, RCLASS_BOXABLE));
-
+    VALUE klass = class_alloc0(T_CLASS, rb_cClass, FL_TEST_RAW(orig_class, RCLASS_BOXABLE));
     FL_SET(klass, FL_SINGLETON);
+    class_initialize_method_table(klass);
+    class_associate_super(klass, orig_class, true);
+    if (orig_class && !UNDEF_P(orig_class)) {
+        rb_class_set_initialized(klass);
+    }
+
     RBASIC_SET_CLASS(obj, klass);
     rb_singleton_class_attached(klass, obj);
     rb_yjit_invalidate_no_singleton_class(orig_class);

internal/class.h

@@ -256,6 +256,7 @@ static inline void RCLASS_WRITE_CLASSPATH(VALUE klass, VALUE classpath, bool per
 // 3 is RMODULE_IS_REFINEMENT for RMODULE
 #define RCLASS_BOXABLE FL_USER4
 #define RCLASS_ALLOCATOR_DEFINED FL_USER5
+#define RCLASS_HAS_SUBCLASSES FL_USER6
 
 static inline st_table *
 RCLASS_CLASSEXT_TBL(VALUE klass)

vm_method.c

@@ -440,6 +440,7 @@ clear_method_cache_by_id_in_class(VALUE klass, ID mid)
         rb_vm_barrier();
 
         if (LIKELY(RCLASS_SUBCLASSES_FIRST(klass) == NULL) &&
+            !FL_TEST_RAW(klass, RCLASS_HAS_SUBCLASSES) &&
             // Non-refinement ICLASSes (from module inclusion) previously had
             // subclasses reparented onto them, so they need the tree path for
             // broader cme-based invalidation even though they now have no subclasses.