avoid useless metaclass searching overhead when creating Python class without base classes

Cython/Compiler/ExprNodes.py

@@ -6966,10 +6966,14 @@ class Py3ClassNode(ExprNode):
             mkw = self.mkw.py_result()
         else:
             mkw = 'NULL'
+        if self.metaclass:
+            metaclass = self.metaclass.result()
+        else:
+            metaclass = "((PyObject*)&__Pyx_DefaultClassType)"
         code.putln(
             '%s = __Pyx_Py3ClassCreate(%s, %s, %s, %s, %s, %d, %d); %s' % (
                 self.result(),
-                self.metaclass.result(),
+                metaclass,
                 cname,
                 self.bases.py_result(),
                 self.dict.py_result(),
@@ -7163,10 +7167,14 @@ class PyClassNamespaceNode(ExprNode, ModuleNameMixin):
             mkw = self.mkw.py_result()
         else:
             mkw = '(PyObject *) NULL'
+        if self.metaclass:
+            metaclass = self.metaclass.result()
+        else:
+            metaclass = "(PyObject *) NULL"
         code.putln(
             "%s = __Pyx_Py3MetaclassPrepare(%s, %s, %s, %s, %s, %s, %s); %s" % (
                 self.result(),
-                self.metaclass.result(),
+                metaclass,
                 self.bases.result(),
                 cname,
                 qualname,

Cython/Compiler/Nodes.py

@@ -4044,8 +4044,13 @@ class PyClassDefNode(ClassDefNode):
                     mkdict = self.mkw
                 else:
                     mkdict = None
-                self.metaclass = ExprNodes.PyClassMetaclassNode(
-                    pos, mkw=mkdict, bases=self.bases)
+                if (not mkdict and
+                        self.bases.is_sequence_constructor and
+                        not self.bases.args):
+                    pass  # no base classes => no inherited metaclass
+                else:
+                    self.metaclass = ExprNodes.PyClassMetaclassNode(
+                        pos, mkw=mkdict, bases=self.bases)
                 needs_metaclass_calculation = False
             else:
                 needs_metaclass_calculation = True

Cython/Utility/ModuleSetupCode.c

@@ -109,10 +109,12 @@
   #define __Pyx_BUILTIN_MODULE_NAME "__builtin__"
   #define __Pyx_PyCode_New(a, k, l, s, f, code, c, n, v, fv, cell, fn, name, fline, lnos) \
           PyCode_New(a, l, s, f, code, c, n, v, fv, cell, fn, name, fline, lnos)
+  #define __Pyx_DefaultClassType PyClass_Type
 #else
   #define __Pyx_BUILTIN_MODULE_NAME "builtins"
   #define __Pyx_PyCode_New(a, k, l, s, f, code, c, n, v, fv, cell, fn, name, fline, lnos) \
           PyCode_New(a, k, l, s, f, code, c, n, v, fv, cell, fn, name, fline, lnos)
+  #define __Pyx_DefaultClassType PyType_Type
 #endif
 
 #if PY_VERSION_HEX < 0x02060000

Cython/Utility/ObjectHandling.c

@@ -830,25 +830,26 @@ static PyObject *__Pyx_Py3ClassCreate(PyObject *metaclass, PyObject *name, PyObj
 
 static PyObject *__Pyx_Py3MetaclassPrepare(PyObject *metaclass, PyObject *bases, PyObject *name,
                                            PyObject *qualname, PyObject *mkw, PyObject *modname, PyObject *doc) {
-    PyObject *prep;
-    PyObject *pargs;
     PyObject *ns;
-
-    prep = __Pyx_PyObject_GetAttrStr(metaclass, PYIDENT("__prepare__"));
-    if (!prep) {
-        if (unlikely(!PyErr_ExceptionMatches(PyExc_AttributeError)))
-            return NULL;
-        PyErr_Clear();
-        ns = PyDict_New();
-    } else {
-        pargs = PyTuple_Pack(2, name, bases);
-        if (unlikely(!pargs)) {
+    if (metaclass) {
+        PyObject *prep = __Pyx_PyObject_GetAttrStr(metaclass, PYIDENT("__prepare__"));
+        if (prep) {
+            PyObject *pargs = PyTuple_Pack(2, name, bases);
+            if (unlikely(!pargs)) {
+                Py_DECREF(prep);
+                return NULL;
+            }
+            ns = PyObject_Call(prep, pargs, mkw);
             Py_DECREF(prep);
-            return NULL;
+            Py_DECREF(pargs);
+        } else {
+            if (unlikely(!PyErr_ExceptionMatches(PyExc_AttributeError)))
+                return NULL;
+            PyErr_Clear();
+            ns = PyDict_New();
         }
-        ns = PyObject_Call(prep, pargs, mkw);
-        Py_DECREF(prep);
-        Py_DECREF(pargs);
+    } else {
+        ns = PyDict_New();
     }
 
     if (unlikely(!ns))