Optimize dictionary iterators.

* Split into three separate types that share everything except the
  code for iternext.  Saves run time decision making and allows
  each iternext function to be specialized.

* Inlined PyDict_Next().  In addition to saving a function call, this
  allows a redundant test to be eliminated and further specialization
  of the code for the unique needs of each iterator type.

* Created a reusable result tuple for iteritems().  Saves the malloc
  time for tuples when the previous result was not kept by client code
  (this is the typical use case for iteritems).  If the client code
  does keep the reference, then a new tuple is created.

Results in a 20% to 30% speedup depending on the size and sparsity
of the dictionary.

Objects/dictobject.c

@@ -1726,52 +1726,27 @@ dict_tp_clear(PyObject *op)
 }
 
 
-static PyObject *dictiter_new(dictobject *, binaryfunc);
-
-static PyObject *
-select_key(PyObject *key, PyObject *value)
-{
-	Py_INCREF(key);
-	return key;
-}
-
-static PyObject *
-select_value(PyObject *key, PyObject *value)
-{
-	Py_INCREF(value);
-	return value;
-}
-
-static PyObject *
-select_item(PyObject *key, PyObject *value)
-{
-	PyObject *res = PyTuple_New(2);
-
-	if (res != NULL) {
-		Py_INCREF(key);
-		Py_INCREF(value);
-		PyTuple_SET_ITEM(res, 0, key);
-		PyTuple_SET_ITEM(res, 1, value);
-	}
-	return res;
-}
+extern PyTypeObject PyDictIterKey_Type; /* Forward */
+extern PyTypeObject PyDictIterValue_Type; /* Forward */
+extern PyTypeObject PyDictIterItem_Type; /* Forward */
+static PyObject *dictiter_new(dictobject *, PyTypeObject *);
 
 static PyObject *
 dict_iterkeys(dictobject *dict)
 {
-	return dictiter_new(dict, select_key);
+	return dictiter_new(dict, &PyDictIterKey_Type);
 }
 
 static PyObject *
 dict_itervalues(dictobject *dict)
 {
-	return dictiter_new(dict, select_value);
+	return dictiter_new(dict, &PyDictIterValue_Type);
 }
 
 static PyObject *
 dict_iteritems(dictobject *dict)
 {
-	return dictiter_new(dict, select_item);
+	return dictiter_new(dict, &PyDictIterItem_Type);
 }
 
 
@@ -1931,7 +1906,7 @@ dict_nohash(PyObject *self)
 static PyObject *
 dict_iter(dictobject *dict)
 {
-	return dictiter_new(dict, select_key);
+	return dictiter_new(dict, &PyDictIterKey_Type);
 }
 
 PyDoc_STRVAR(dictionary_doc,
@@ -2030,30 +2005,36 @@ PyDict_DelItemString(PyObject *v, const char *key)
 	return err;
 }
 
-/* Dictionary iterator type */
-
-extern PyTypeObject PyDictIter_Type; /* Forward */
+/* Dictionary iterator types */
 
 typedef struct {
 	PyObject_HEAD
 	dictobject *di_dict; /* Set to NULL when iterator is exhausted */
 	int di_used;
 	int di_pos;
-	binaryfunc di_select;
+	PyObject* di_result; /* reusable result tuple for iteritems */
 } dictiterobject;
 
 static PyObject *
-dictiter_new(dictobject *dict, binaryfunc select)
+dictiter_new(dictobject *dict, PyTypeObject *itertype)
 {
 	dictiterobject *di;
-	di = PyObject_New(dictiterobject, &PyDictIter_Type);
+	di = PyObject_New(dictiterobject, itertype);
 	if (di == NULL)
 		return NULL;
 	Py_INCREF(dict);
 	di->di_dict = dict;
 	di->di_used = dict->ma_used;
 	di->di_pos = 0;
-	di->di_select = select;
+	if (itertype == &PyDictIterItem_Type) {
+		di->di_result = PyTuple_Pack(2, Py_None, Py_None);
+		if (di->di_result == NULL) {
+			Py_DECREF(di);
+			return NULL;
+		}
+	}
+	else
+		di->di_result = NULL;
 	return (PyObject *)di;
 }
 
@@ -2061,34 +2042,52 @@ static void
 dictiter_dealloc(dictiterobject *di)
 {
 	Py_XDECREF(di->di_dict);
+	Py_XDECREF(di->di_result);
 	PyObject_Del(di);
 }
 
-static PyObject *dictiter_iternext(dictiterobject *di)
+static PyObject *dictiter_iternextkey(dictiterobject *di)
 {
-	PyObject *key, *value;
+	PyObject *key;
+	register int i, mask;
+	register dictentry *ep;
+	dictobject *d = di->di_dict;
 
-	if (di->di_dict == NULL)
+	if (d == NULL)
 		return NULL;
+	assert (PyDict_Check(d));
 
-	if (di->di_used != di->di_dict->ma_used) {
+	if (di->di_used != d->ma_used) {
 		PyErr_SetString(PyExc_RuntimeError,
 				"dictionary changed size during iteration");
 		di->di_used = -1; /* Make this state sticky */
 		return NULL;
 	}
-	if (PyDict_Next((PyObject *)(di->di_dict), &di->di_pos, &key, &value))
-		return (*di->di_select)(key, value);
 
-	Py_DECREF(di->di_dict);
+	i = di->di_pos;
+	if (i < 0)
+		goto fail;
+	ep = d->ma_table;
+	mask = d->ma_mask;
+	while (i <= mask && ep[i].me_value == NULL)
+		i++;
+	di->di_pos = i+1;
+	if (i > mask)
+		goto fail;
+	key = ep[i].me_key;
+	Py_INCREF(key);
+	return key;
+
+fail:
+	Py_DECREF(d);
 	di->di_dict = NULL;
 	return NULL;
 }
 
-PyTypeObject PyDictIter_Type = {
+PyTypeObject PyDictIterKey_Type = {
 	PyObject_HEAD_INIT(&PyType_Type)
 	0,					/* ob_size */
-	"dictionary-iterator",			/* tp_name */
+	"dictionary-keyiterator",			/* tp_name */
 	sizeof(dictiterobject),			/* tp_basicsize */
 	0,					/* tp_itemsize */
 	/* methods */
@@ -2114,12 +2113,158 @@ PyTypeObject PyDictIter_Type = {
 	0,					/* tp_richcompare */
 	0,					/* tp_weaklistoffset */
 	PyObject_SelfIter,			/* tp_iter */
-	(iternextfunc)dictiter_iternext,	/* tp_iternext */
-	0,					/* tp_methods */
-	0,					/* tp_members */
-	0,					/* tp_getset */
-	0,					/* tp_base */
-	0,					/* tp_dict */
-	0,					/* tp_descr_get */
-	0,					/* tp_descr_set */
+	(iternextfunc)dictiter_iternextkey,	/* tp_iternext */
+};
+
+static PyObject *dictiter_iternextvalue(dictiterobject *di)
+{
+	PyObject *value;
+	register int i, mask;
+	register dictentry *ep;
+	dictobject *d = di->di_dict;
+
+	if (d == NULL)
+		return NULL;
+	assert (PyDict_Check(d));
+
+	if (di->di_used != d->ma_used) {
+		PyErr_SetString(PyExc_RuntimeError,
+				"dictionary changed size during iteration");
+		di->di_used = -1; /* Make this state sticky */
+		return NULL;
+	}
+
+	i = di->di_pos;
+	if (i < 0)
+		goto fail;
+	ep = d->ma_table;
+	mask = d->ma_mask;
+	while (i <= mask && (value=ep[i].me_value) == NULL)
+		i++;
+	di->di_pos = i+1;
+	if (i > mask)
+		goto fail;
+	Py_INCREF(value);
+	return value;
+
+fail:
+	Py_DECREF(d);
+	di->di_dict = NULL;
+	return NULL;
+}
+
+PyTypeObject PyDictIterValue_Type = {
+	PyObject_HEAD_INIT(&PyType_Type)
+	0,					/* ob_size */
+	"dictionary-valueiterator",		/* tp_name */
+	sizeof(dictiterobject),			/* tp_basicsize */
+	0,					/* tp_itemsize */
+	/* methods */
+	(destructor)dictiter_dealloc, 		/* tp_dealloc */
+	0,					/* tp_print */
+	0,					/* tp_getattr */
+	0,					/* tp_setattr */
+	0,					/* tp_compare */
+	0,					/* tp_repr */
+	0,					/* tp_as_number */
+	0,					/* tp_as_sequence */
+	0,					/* tp_as_mapping */
+	0,					/* tp_hash */
+	0,					/* tp_call */
+	0,					/* tp_str */
+	PyObject_GenericGetAttr,		/* tp_getattro */
+	0,					/* tp_setattro */
+	0,					/* tp_as_buffer */
+	Py_TPFLAGS_DEFAULT,			/* tp_flags */
+ 	0,					/* tp_doc */
+ 	0,					/* tp_traverse */
+ 	0,					/* tp_clear */
+	0,					/* tp_richcompare */
+	0,					/* tp_weaklistoffset */
+	PyObject_SelfIter,			/* tp_iter */
+	(iternextfunc)dictiter_iternextvalue,	/* tp_iternext */
+};
+
+static PyObject *dictiter_iternextitem(dictiterobject *di)
+{
+	PyObject *key, *value, *result = di->di_result;
+	register int i, mask;
+	register dictentry *ep;
+	dictobject *d = di->di_dict;
+
+	if (d == NULL)
+		return NULL;
+	assert (PyDict_Check(d));
+
+	if (di->di_used != d->ma_used) {
+		PyErr_SetString(PyExc_RuntimeError,
+				"dictionary changed size during iteration");
+		di->di_used = -1; /* Make this state sticky */
+		return NULL;
+	}
+
+	i = di->di_pos;
+	if (i < 0)
+		goto fail;
+	ep = d->ma_table;
+	mask = d->ma_mask;
+	while (i <= mask && ep[i].me_value == NULL)
+		i++;
+	di->di_pos = i+1;
+	if (i > mask)
+		goto fail;
+
+	if (result->ob_refcnt == 1) {
+		Py_INCREF(result);
+		Py_DECREF(PyTuple_GET_ITEM(result, 0));
+		Py_DECREF(PyTuple_GET_ITEM(result, 1));
+	} else {
+		result = PyTuple_New(2);
+		if (result == NULL) 
+			return NULL;
+	}
+	key = ep[i].me_key;
+	value = ep[i].me_value;
+	Py_INCREF(key);
+	Py_INCREF(value);
+	PyTuple_SET_ITEM(result, 0, key);
+	PyTuple_SET_ITEM(result, 1, value);
+	return result;
+
+fail:
+	Py_DECREF(d);
+	di->di_dict = NULL;
+	return NULL;
+}
+
+PyTypeObject PyDictIterItem_Type = {
+	PyObject_HEAD_INIT(&PyType_Type)
+	0,					/* ob_size */
+	"dictionary-itemiterator",		/* tp_name */
+	sizeof(dictiterobject),			/* tp_basicsize */
+	0,					/* tp_itemsize */
+	/* methods */
+	(destructor)dictiter_dealloc, 		/* tp_dealloc */
+	0,					/* tp_print */
+	0,					/* tp_getattr */
+	0,					/* tp_setattr */
+	0,					/* tp_compare */
+	0,					/* tp_repr */
+	0,					/* tp_as_number */
+	0,					/* tp_as_sequence */
+	0,					/* tp_as_mapping */
+	0,					/* tp_hash */
+	0,					/* tp_call */
+	0,					/* tp_str */
+	PyObject_GenericGetAttr,		/* tp_getattro */
+	0,					/* tp_setattro */
+	0,					/* tp_as_buffer */
+	Py_TPFLAGS_DEFAULT,			/* tp_flags */
+ 	0,					/* tp_doc */
+ 	0,					/* tp_traverse */
+ 	0,					/* tp_clear */
+	0,					/* tp_richcompare */
+	0,					/* tp_weaklistoffset */
+	PyObject_SelfIter,			/* tp_iter */
+	(iternextfunc)dictiter_iternextitem,	/* tp_iternext */
 };