Optimize also PyFrozenDict_New(frozendict)

Author: vstinner

Lib/test/test_capi/test_dict.py

@@ -619,6 +619,16 @@ def test_frozendict_new(self):
         self.assertEqual(dct, frozendict(x=1, y=2))
         self.assertIs(type(dct), frozendict)
 
+        # PyFrozenDict_New(frozendict) returns the same object unmodified
+        fd = frozendict(a=1, b=2, c=3)
+        fd2 = frozendict_new(fd)
+        self.assertIs(fd2, fd)
+
+        fd = FrozenDictSubclass(a=1, b=2, c=3)
+        fd2 = frozendict_new(fd)
+        self.assertIsNot(fd2, fd)
+        self.assertEqual(fd2, fd)
+
         # PyFrozenDict_New(NULL) creates an empty dictionary
         dct = frozendict_new(NULL)
         self.assertEqual(dct, frozendict())

Objects/dictobject.c

@@ -8203,6 +8203,11 @@ PyObject*
 PyFrozenDict_New(PyObject *iterable)
 {
     if (iterable != NULL) {
+        if (PyFrozenDict_CheckExact(iterable)) {
+            // PyFrozenDict_New(frozendict) returns the same object unmodified
+            return Py_NewRef(iterable);
+        }
+
         PyObject *args = PyTuple_Pack(1, iterable);
         if (args == NULL) {
             return NULL;