gh-92810: Improve nested subclass check performance

Author: dolfinus

Lib/_py_abc.py

@@ -140,6 +140,11 @@ def __subclasscheck__(cls, subclass):
                 return True
         # Check if it's a subclass of a subclass (recursive)
         for scls in cls.__subclasses__():
+            if subclass is scls:
+                # Fast path
+                if not cls._abc_issubclasscheck_recursive:
+                    cls._abc_cache.add(subclass)
+                return True
             # If inside recursive issubclass check, avoid adding classes
             # to any cache because this may drastically increase memory usage.
             # Unfortunately, issubclass/__subclasscheck__ don't accept third
@@ -149,6 +154,7 @@ def __subclasscheck__(cls, subclass):
             if scls_is_abc:
                 scls._abc_issubclasscheck_recursive = True
             try:
+                # Perform recursive check
                 result = issubclass(subclass, scls)
             finally:
                 if scls_is_abc:

Modules/_abc.c

@@ -848,6 +848,17 @@ _abc__abc_subclasscheck_impl(PyObject *module, PyObject *self,
             goto end;
         }
 
+        if (scls == subclass) {
+            // Fast path
+            if (!is_issubclasscheck_recursive(impl)) {
+                if (_add_to_weak_set(impl, &impl->_abc_cache, subclass) < 0) {
+                    goto end;
+                }
+            }
+            result = Py_True;
+            goto end;
+        }
+
         _abc_data *scls_impl;
         int scls_is_abc = _get_optional_impl(state, scls, &scls_impl);
         if (scls_is_abc < 0) {
@@ -865,6 +876,7 @@ _abc__abc_subclasscheck_impl(PyObject *module, PyObject *self,
             set_issubclasscheck_recursive(scls_impl);
         }
 
+        // Perform recursive check
         int r = PyObject_IsSubclass(subclass, scls);
         Py_DECREF(scls);