diff -r 6f0f77333da5 Include/unicodeobject.h
--- a/Include/unicodeobject.h	Wed Nov 16 10:19:57 2016 +0200
+++ b/Include/unicodeobject.h	Wed Nov 16 19:27:48 2016 +0800
@@ -2037,7 +2037,11 @@
     );
 
 #ifndef Py_LIMITED_API
-PyAPI_FUNC(int) _PyUnicode_CompareWithId(
+/* Test whether a unicode is equal to ASCII identifier.  Return 1 if true,
+   0 otherwise.  Return 0 if any argument contains non-ASCII characters.
+   Any error occurs inside will be cleared before return. */
+
+PyAPI_FUNC(int) _PyUnicode_EqualToASCIIId(
     PyObject *left,             /* Left string */
     _Py_Identifier *right       /* Right identifier */
     );
diff -r 6f0f77333da5 Objects/typeobject.c
--- a/Objects/typeobject.c	Wed Nov 16 10:19:57 2016 +0200
+++ b/Objects/typeobject.c	Wed Nov 16 19:27:48 2016 +0800
@@ -863,7 +863,7 @@
         return NULL;
     }
 
-    if (mod != NULL && _PyUnicode_CompareWithId(mod, &PyId_builtins))
+    if (mod != NULL && !_PyUnicode_EqualToASCIIId(mod, &PyId_builtins))
         rtn = PyUnicode_FromFormat("<class '%U.%U'>", mod, name);
     else
         rtn = PyUnicode_FromFormat("<class '%s'>", type->tp_name);
@@ -2403,7 +2403,7 @@
             if (!valid_identifier(tmp))
                 goto error;
             assert(PyUnicode_Check(tmp));
-            if (_PyUnicode_CompareWithId(tmp, &PyId___dict__) == 0) {
+            if (_PyUnicode_EqualToASCIIId(tmp, &PyId___dict__)) {
                 if (!may_add_dict || add_dict) {
                     PyErr_SetString(PyExc_TypeError,
                         "__dict__ slot disallowed: "
@@ -2434,7 +2434,7 @@
         for (i = j = 0; i < nslots; i++) {
             tmp = PyTuple_GET_ITEM(slots, i);
             if ((add_dict &&
-                 _PyUnicode_CompareWithId(tmp, &PyId___dict__) == 0) ||
+                 _PyUnicode_EqualToASCIIId(tmp, &PyId___dict__)) ||
                 (add_weak &&
                  _PyUnicode_EqualToASCIIString(tmp, "__weakref__")))
                 continue;
@@ -3538,7 +3538,7 @@
         Py_XDECREF(mod);
         return NULL;
     }
-    if (mod != NULL && _PyUnicode_CompareWithId(mod, &PyId_builtins))
+    if (mod != NULL && !_PyUnicode_EqualToASCIIId(mod, &PyId_builtins))
         rtn = PyUnicode_FromFormat("<%U.%U object at %p>", mod, name, self);
     else
         rtn = PyUnicode_FromFormat("<%s object at %p>",
@@ -7238,7 +7238,7 @@
        (i.e. super, or a subclass), not the class of su->obj. */
     if (PyUnicode_Check(name) &&
         PyUnicode_GET_LENGTH(name) == 9 &&
-        _PyUnicode_CompareWithId(name, &PyId___class__) == 0)
+        _PyUnicode_EqualToASCIIId(name, &PyId___class__))
         goto skip;
 
     mro = starttype->tp_mro;
@@ -7450,7 +7450,7 @@
         for (i = 0; i < n; i++) {
             PyObject *name = PyTuple_GET_ITEM(co->co_freevars, i);
             assert(PyUnicode_Check(name));
-            if (!_PyUnicode_CompareWithId(name, &PyId___class__)) {
+            if (_PyUnicode_EqualToASCIIId(name, &PyId___class__)) {
                 Py_ssize_t index = co->co_nlocals +
                     PyTuple_GET_SIZE(co->co_cellvars) + i;
                 PyObject *cell = f->f_localsplus[index];
diff -r 6f0f77333da5 Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Wed Nov 16 10:19:57 2016 +0200
+++ b/Objects/unicodeobject.c	Wed Nov 16 19:27:48 2016 +0800
@@ -10993,15 +10993,6 @@
 }
 
 int
-_PyUnicode_CompareWithId(PyObject *left, _Py_Identifier *right)
-{
-    PyObject *right_str = _PyUnicode_FromId(right);   /* borrowed */
-    if (right_str == NULL)
-        return -1;
-    return PyUnicode_Compare(left, right_str);
-}
-
-int
 PyUnicode_CompareWithASCIIString(PyObject* uni, const char* str)
 {
     Py_ssize_t i;
@@ -11083,6 +11074,48 @@
            memcmp(PyUnicode_1BYTE_DATA(unicode), str, len) == 0;
 }
 
+int
+_PyUnicode_EqualToASCIIId(PyObject *left, _Py_Identifier *right)
+{
+    PyObject *right_uni;
+    Py_hash_t hash;
+    size_t len;
+
+    assert(_PyUnicode_CHECK(left));
+    assert(right->string);
+
+    if (PyUnicode_READY(left) == -1) {
+        /* memory error or bad data */
+        PyErr_Clear();
+        return non_ready_unicode_equal_to_ascii_string(left, right->string);
+    }
+
+    if (!PyUnicode_IS_ASCII(left))
+        return 0;
+    len = (size_t)PyUnicode_GET_LENGTH(left);
+
+    right_uni = _PyUnicode_FromId(right);       /* borrowed */
+    if (right_uni == NULL) {
+        /* memory error or bad data */
+        PyErr_Clear();
+        return strlen(right->string) == len &&
+            memcmp(PyUnicode_1BYTE_DATA(left), right->string, len) == 0;
+    }
+
+    if (left == right_uni) {
+        return 1;
+    }
+    else if (PyUnicode_CHECK_INTERNED(left)) {
+        return 0;
+    }
+
+    assert(_PyUnicode_HASH(right_uni) != 1);
+    hash = _PyUnicode_HASH(left);
+    if ((hash != -1) && hash != _PyUnicode_HASH(right_uni))
+        return 0;
+
+    return unicode_compare_eq(left, right_uni);
+}
 
 #define TEST_COND(cond)                         \
     ((cond) ? Py_True : Py_False)
diff -r 6f0f77333da5 Python/errors.c
--- a/Python/errors.c	Wed Nov 16 10:19:57 2016 +0200
+++ b/Python/errors.c	Wed Nov 16 19:27:48 2016 +0800
@@ -984,7 +984,7 @@
             goto done;
     }
     else {
-        if (_PyUnicode_CompareWithId(moduleName, &PyId_builtins) != 0) {
+        if (!_PyUnicode_EqualToASCIIId(moduleName, &PyId_builtins)) {
             if (PyFile_WriteObject(moduleName, f, Py_PRINT_RAW) < 0)
                 goto done;
             if (PyFile_WriteString(".", f) < 0)
diff -r 6f0f77333da5 Python/pythonrun.c
--- a/Python/pythonrun.c	Wed Nov 16 10:19:57 2016 +0200
+++ b/Python/pythonrun.c	Wed Nov 16 19:27:48 2016 +0800
@@ -751,7 +751,7 @@
             err = PyFile_WriteString("<unknown>", f);
         }
         else {
-            if (_PyUnicode_CompareWithId(moduleName, &PyId_builtins) != 0)
+            if (!_PyUnicode_EqualToASCIIId(moduleName, &PyId_builtins))
             {
                 err = PyFile_WriteObject(moduleName, f, Py_PRINT_RAW);
                 err += PyFile_WriteString(".", f);