diff -r 3e6792af95f0 Objects/longobject.c
--- a/Objects/longobject.c	Fri Jun 17 00:01:30 2016 +0200
+++ b/Objects/longobject.c	Fri Jun 17 11:37:36 2016 +0300
@@ -667,6 +667,12 @@
         sign = -1;
         i = -i;
     }
+
+    /* No need to iterate over digits whose bits would be shifted
+       out of x anyway.
+       Round upward this way instead of calling ceil, to allow the
+       expression to be calculated at compile time. */
+    i = Py_MIN(i, (sizeof(x) * 8 - 1) / PyLong_SHIFT + 1);
     while (--i >= 0) {
         x = (x << PyLong_SHIFT) | v->ob_digit[i];
     }
@@ -1301,6 +1307,12 @@
         sign = -1;
         i = -i;
     }
+
+    /* No need to iterate over digits whose bits would be shifted
+       out of x anyway.
+       Round upward this way instead of calling ceil, to allow the
+       expression to be calculated at compile time. */
+    i = Py_MIN(i, (sizeof(x) * 8 - 1) / PyLong_SHIFT + 1);
     while (--i >= 0) {
         x = (x << PyLong_SHIFT) | v->ob_digit[i];
     }