* Bring in INIT_NONZERO_SET_SLOTS macro from dictionary code.

* Bring in free list from dictionary code.
* Improve several comments.
* Differencing can leave many dummy entries.  If more than
  1/6 are dummies, then resize them away.
* Factor-out common code with new macro, PyAnySet_CheckExact.

Include/setobject.h

@@ -59,12 +59,16 @@ struct _setobject {
 PyAPI_DATA(PyTypeObject) PySet_Type;
 PyAPI_DATA(PyTypeObject) PyFrozenSet_Type;
 
-/* Invariants for frozensets only:
+/* Invariants for frozensets:
  *     data is immutable.
  *     hash is the hash of the frozenset or -1 if not computed yet.
+ * Invariants for sets:
+ *     hash is -1
  */
 
 #define PyFrozenSet_CheckExact(ob) ((ob)->ob_type == &PyFrozenSet_Type)
+#define PyAnySet_CheckExact(ob) \
+	((ob)->ob_type == &PySet_Type || (ob)->ob_type == &PyFrozenSet_Type)
 #define PyAnySet_Check(ob) \
 	((ob)->ob_type == &PySet_Type || (ob)->ob_type == &PyFrozenSet_Type || \
 	  PyType_IsSubtype((ob)->ob_type, &PySet_Type) || \

Objects/setobject.c

@@ -15,13 +15,22 @@
 /* Object used as dummy key to fill deleted entries */
 static PyObject *dummy = NULL; /* Initialized by first call to make_new_set() */
 
+#define INIT_NONZERO_SET_SLOTS(so) do {				\
+	(so)->table = (so)->smalltable;				\
+	(so)->mask = PySet_MINSIZE - 1;				\
+	(so)->hash = -1;					\
+    } while(0)
+
 #define EMPTY_TO_MINSIZE(so) do {				\
 	memset((so)->smalltable, 0, sizeof((so)->smalltable));	\
 	(so)->used = (so)->fill = 0;				\
-	(so)->table = (so)->smalltable;				\
-	(so)->mask = PySet_MINSIZE - 1;				\
+	INIT_NONZERO_SET_SLOTS(so);				\
     } while(0)
 
+/* Reuse scheme to save calls to malloc, free, and memset */
+#define MAXFREESETS 80
+static PySetObject *free_sets[MAXFREESETS];
+static int num_free_sets = 0;
 
 /*
 The basic lookup function used by all operations.
@@ -30,13 +39,15 @@ Open addressing is preferred over chaining since the link overhead for
 chaining would be substantial (100% with typical malloc overhead).
 
 The initial probe index is computed as hash mod the table size. Subsequent
-probe indices are computed as explained earlier.
+probe indices are computed as explained in Objects/dictobject.c.
 
 All arithmetic on hash should ignore overflow.
 
-This function must never return NULL; failures are indicated by returning
-a setentry* for which the value field is NULL.  Exceptions are never
-reported by this function, and outstanding exceptions are maintained.
+The lookup function always succeeds and nevers return NULL.  This simplifies
+and speeds client functions which do won't have to test for and handle
+errors.  To meet that requirement, any errors generated by a user defined 
+__cmp__() function are simply cleared and ignored. 
+Previously outstanding exceptions are maintained.
 */
 
 static setentry *
@@ -187,7 +198,7 @@ set_lookkey_string(PySetObject *so, PyObject *key, register long hash)
 				freeslot = entry;
 		}
 	} else {
-		/* Simplified loop that can assume are no dummy entries */
+		/* Simplified loop when there are no dummy entries. */
 		if (entry->hash == hash && _PyString_Eq(entry->key, key))
 			return entry;
 		for (perturb = hash; ; perturb >>= PERTURB_SHIFT) {
@@ -347,7 +358,7 @@ set_add_internal(register PySetObject *so, PyObject *key)
 	set_insert_key(so, key, hash);
 	if (!(so->used > n_used && so->fill*3 >= (so->mask+1)*2))
 		return 0;
-	return set_table_resize(so, so->used*(so->used>50000 ? 2 : 4));
+	return set_table_resize(so, so->used>50000 ? so->used*2 : so->used*4);
 }
 
 #define DISCARD_NOTFOUND 0
@@ -439,7 +450,6 @@ set_clear_internal(PySetObject *so)
 
 	if (table_is_malloced)
 		PyMem_DEL(table);
-	so->hash = -1;
 	return 0;
 }
 
@@ -710,16 +720,24 @@ make_new_set(PyTypeObject *type, PyObject *iterable)
 	}
 
 	/* create PySetObject structure */
-	so = (PySetObject *)type->tp_alloc(type, 0);
-	if (so == NULL)
-		return NULL;
+	if (num_free_sets && 
+	    (type == &PySet_Type  ||  type == &PyFrozenSet_Type)) {
+		so = free_sets[--num_free_sets];
+		assert (so != NULL && PyAnySet_CheckExact(so));
+		so->ob_type = type;
+		_Py_NewReference((PyObject *)so);
+		EMPTY_TO_MINSIZE(so);
+		PyObject_GC_Track(so);
+	} else {
+		so = (PySetObject *)type->tp_alloc(type, 0);
+		if (so == NULL)
+			return NULL;
+		/* tp_alloc has already zeroed the structure */
+		assert(so->table == NULL && so->fill == 0 && so->used == 0);
+		INIT_NONZERO_SET_SLOTS(so);
+	}
 
-	/* tp_alloc has already zeroed the structure */
-	assert(so->table == NULL && so->fill == 0 && so->used == 0);
-	so->table = so->smalltable;
-	so->mask = PySet_MINSIZE - 1;
 	so->lookup = set_lookkey_string;
-	so->hash = -1;
 	so->weakreflist = NULL;
 
 	if (iterable != NULL) {
@@ -767,6 +785,13 @@ frozenset_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 void
 PySet_Fini(void)
 {
+	PySetObject *so;
+
+	while (num_free_sets) {
+		num_free_sets--;
+		so = free_sets[num_free_sets];
+		PyObject_GC_Del(so);
+	}
 	Py_XDECREF(dummy);
 	Py_XDECREF(emptyfrozenset);
 }
@@ -797,7 +822,10 @@ set_dealloc(PySetObject *so)
 	if (so->table != so->smalltable)
 		PyMem_DEL(so->table);
 
-	so->ob_type->tp_free(so);
+	if (num_free_sets < MAXFREESETS && PyAnySet_CheckExact(so))
+		free_sets[num_free_sets++] = so;
+	else 
+		so->ob_type->tp_free(so);
 	Py_TRASHCAN_SAFE_END(so)
 }
 
@@ -1079,6 +1107,11 @@ set_difference_update(PySetObject *so, PyObject *other)
 	Py_DECREF(it);
 	if (PyErr_Occurred())
 		return NULL;
+	/* If more than 1/6 are dummies, then resize them away. */
+	if ((so->fill - so->used) * 6 < so->mask)
+		Py_RETURN_NONE;
+	if (set_table_resize(so, so->used>50000 ? so->used*2 : so->used*4) == -1)
+		return NULL;
 	Py_RETURN_NONE;
 }