diff -r 2a4fb01fa1a3 Lib/test/test_dict_split.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/test/test_dict_split.py	Sat Sep 17 00:14:58 2016 +0900
@@ -0,0 +1,88 @@
+"""Tests for split table form of dict"""
+
+import struct
+import sys
+from test import support
+import unittest
+
+
+def combined_dict_size(keysize):
+    """size of combined dict"""
+    size = support.calcobjsize('n2P')  # PyDictObject
+    size += struct.calcsize('2nPn')    # PyDictKeysObject
+    size += struct.calcsize('n2P') * keysize # dk_entries
+    return size
+
+
+def split_dict_size(keysize):
+    """size of split, but not shared dict"""
+    size = combined_dict_size(keysize)
+    size += struct.calcsize('P') * keysize  # ma_values
+    return size
+
+
+def shared_dict_size(keysize):
+    """size of shared dict"""
+    size = support.calcobjsize('n2P')  # PyDictObject
+    size += struct.calcsize('P') * keysize  # ma_values
+    return size
+
+
+@support.cpython_only
+class SplitTableTest(unittest.TestCase):
+
+    check_sizeof = support.check_sizeof
+
+    def make_shared_key_dict(self, n):
+        class C:
+            pass
+
+        dicts = []
+        for i in range(n):
+            a = C()
+            a.x, a.y, a.z = 1, 2, 3
+            dicts.append(a.__dict__)
+
+        return dicts
+
+    def test_pop_pending(self):
+        """pop a pending key in a splitted table should not crash"""
+        a, b = self.make_shared_key_dict(2)
+
+        a['a'] = 4
+        with self.assertRaises(KeyError):
+            b.pop('a')
+
+    def test_popitem(self):
+        """split table must be combined when d.popitem()"""
+        a, b = self.make_shared_key_dict(2)
+
+        orig_size = sys.getsizeof(a)
+
+        key = a.popitem()[0]  # split table is combined
+        with self.assertRaises(KeyError):
+            del a[key]
+
+        self.assertGreater(sys.getsizeof(a), orig_size)
+
+    def test_setitem_after_pop(self):
+        class C:
+            pass
+        a = C()
+
+        a.a = 1
+        self.check_sizeof({}, combined_dict_size(8))
+        self.check_sizeof(a.__dict__, shared_dict_size(4))
+
+        # dict.popitem() convert it to combined table
+        # C doesn't stop using shared key when bypass delitem.
+        a.__dict__.popitem()
+        self.check_sizeof(a.__dict__, combined_dict_size(8))
+
+        # But C should not convert a.__dict__ to split table again.
+        a.a = 1
+        self.check_sizeof(a.__dict__, combined_dict_size(8))
+
+
+if __name__ == "__main__":
+    unittest.main()
diff -r 2a4fb01fa1a3 Objects/dictobject.c
--- a/Objects/dictobject.c	Fri Sep 16 17:31:06 2016 +0300
+++ b/Objects/dictobject.c	Sat Sep 17 00:14:58 2016 +0900
@@ -985,8 +985,10 @@
             return NULL;
         }
         else if (mp->ma_keys->dk_lookup == lookdict_unicode) {
-            /* Remove dummy keys */
-            if (dictresize(mp, DK_SIZE(mp->ma_keys)))
+            /* Remove dummy keys
+             * -1 is required since dictresize() uses key size > minused
+             */
+            if (dictresize(mp, DK_SIZE(mp->ma_keys) - 1))
                 return NULL;
         }
         assert(mp->ma_keys->dk_lookup == lookdict_unicode_nodummy);
@@ -2473,7 +2475,8 @@
     }
     /* Convert split table to combined table */
     if (mp->ma_keys->dk_lookup == lookdict_split) {
-        if (dictresize(mp, DK_SIZE(mp->ma_keys))) {
+        /* -1 is required since dictresize() uses key size > minused */
+        if (dictresize(mp, DK_SIZE(mp->ma_keys) - 1)) {
             Py_DECREF(res);
             return NULL;
         }
@@ -3848,9 +3851,11 @@
                 CACHED_KEYS(tp) = NULL;
                 DK_DECREF(cached);
             }
-        } else {
+        }
+        else {
+            int shared = cached == ((PyDictObject *)dict)->ma_keys;
             res = PyDict_SetItem(dict, key, value);
-            if (cached != ((PyDictObject *)dict)->ma_keys) {
+            if (shared && cached != ((PyDictObject *)dict)->ma_keys) {
                 /* Either update tp->ht_cached_keys or delete it */
                 if (cached->dk_refcnt == 1) {
                     CACHED_KEYS(tp) = make_keys_shared(dict);