diff -r 0893b9ee44ea Objects/setobject.c
--- a/Objects/setobject.c	Tue Jan 20 17:21:41 2015 -0800
+++ b/Objects/setobject.c	Wed Jan 21 15:16:06 2015 +0200
@@ -49,6 +49,32 @@ static PyObject _dummy_struct;
 #define PERTURB_SHIFT 5
 
 static setentry *
+set_find_free_slot(PySetObject *so, Py_hash_t hash)
+{
+    setentry *table = so->table;
+    setentry *entry;
+    size_t perturb = hash;
+    size_t mask = so->mask;
+    size_t i = (size_t)hash; /* Unsigned for defined overflow behavior. */
+    size_t j;
+
+    while (1) {
+        entry = &table[i & mask];
+        if (entry->key == NULL || entry->key == dummy)
+            return entry;
+
+        for (j = 1 ; j <= LINEAR_PROBES ; j++) {
+            entry = &table[(i + j) & mask];
+            if (entry->key == NULL || entry->key == dummy)
+                return entry;
+        }
+
+        perturb >>= PERTURB_SHIFT;
+        i = i * 5 + 1 + perturb;
+    }
+}
+
+static setentry *
 set_lookkey(PySetObject *so, PyObject *key, Py_hash_t hash)
 {
     setentry *table = so->table;
@@ -593,6 +619,7 @@ set_merge(PySetObject *so, PyObject *oth
     Py_hash_t hash;
     Py_ssize_t i;
     setentry *entry;
+    int clear;
 
     assert (PyAnySet_Check(so));
     assert (PyAnySet_Check(otherset));
@@ -601,6 +628,7 @@ set_merge(PySetObject *so, PyObject *oth
     if (other == so || other->used == 0)
         /* a.update(a) or a.update({}); nothing to do */
         return 0;
+    clear = so->used == 0;
     /* Do one big resize at the start, rather than
      * incrementally resizing as we insert new keys.  Expect
      * that there will be no (or few) overlapping keys.
@@ -615,10 +643,23 @@ set_merge(PySetObject *so, PyObject *oth
         hash = entry->hash;
         if (key != NULL &&
             key != dummy) {
-            Py_INCREF(key);
-            if (set_insert_key(so, key, hash) == -1) {
-                Py_DECREF(key);
-                return -1;
+            if (clear) {
+                assert(!(entry->key == NULL || entry->key == dummy));
+                entry = set_find_free_slot(so, hash);
+                assert(entry->key == NULL || entry->key == dummy);
+                if (entry->key == NULL)
+                    so->fill++;
+                Py_INCREF(key);
+                entry->key = key;
+                entry->hash = hash;
+                so->used++;
+            }
+            else {
+                Py_INCREF(key);
+                if (set_insert_key(so, key, hash) == -1) {
+                    Py_DECREF(key);
+                    return -1;
+                }
             }
         }
     }