diff -r 9d2c4d887c19 Objects/setobject.c
--- a/Objects/setobject.c	Tue May 12 14:28:08 2015 -0400
+++ b/Objects/setobject.c	Tue May 12 22:52:52 2015 +0300
@@ -557,7 +557,7 @@ set_merge(PySetObject *so, PyObject *oth
 
     other = (PySetObject*)otherset;
     if (other == so || other->used == 0)
-        /* a.update(a) or a.update({}); nothing to do */
+        /* a.update(a) or a.update(set()); nothing to do */
         return 0;
     /* Do one big resize at the start, rather than
      * incrementally resizing as we insert new keys.  Expect
@@ -567,16 +567,46 @@ set_merge(PySetObject *so, PyObject *oth
        if (set_table_resize(so, (so->used + other->used)*2) != 0)
            return -1;
     }
-    for (i = 0; i <= other->mask; i++) {
-        entry = &other->table[i];
-        key = entry->key;
-        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 (so->fill != 0) {
+        for (i = 0; i <= other->mask; i++) {
+            entry = &other->table[i];
+            key = entry->key;
+            hash = entry->hash;
+            if (key != NULL && key != dummy) {
+                Py_INCREF(key);
+                if (set_insert_key(so, key, hash) == -1) {
+                    Py_DECREF(key);
+                    return -1;
+                }
+            }
+        }
+    }
+    else if (other->fill == other->used && so->mask == other->mask) {
+        /* If no dummies, we can just copy entries at the same indices. */
+        for (i = 0; i <= other->mask; i++) {
+            entry = &other->table[i];
+            key = entry->key;
+            hash = entry->hash;
+            if (key != NULL && key != dummy) {
+                Py_INCREF(key);
+                entry = &so->table[i];
+                assert(entry->key == NULL);
+                entry->key = key;
+                entry->hash = hash;
+                so->used++;
+                so->fill++;
+            }
+        }
+    }
+    else {
+        /* If the table is clean, we can use fast set_insert_clean(). */
+        for (i = 0; i <= other->mask; i++) {
+            entry = &other->table[i];
+            key = entry->key;
+            hash = entry->hash;
+            if (key != NULL && key != dummy) {
+                Py_INCREF(key);
+                set_insert_clean(so, key, hash);
             }
         }
     }