--- a/mercurial/revlog.py	Wed Jun 28 13:32:36 2017 +0200
+++ b/mercurial/revlog.py	Sun Jun 25 12:41:34 2017 -0700
@@ -570,6 +570,12 @@
         revs in ascending order and ``stopped`` is a bool indicating whether
         ``stoprev`` was hit.
         """
+        # Try C implementation.
+        try:
+            return self.index.deltachain(rev, stoprev, self._generaldelta)
+        except AttributeError:
+            pass
+
         chain = []
 
         # Alias to prevent attribute lookup in tight loop.