diff --git a/contrib/perf.py b/contrib/perf.py
--- a/contrib/perf.py
+++ b/contrib/perf.py
@@ -1576,7 +1576,11 @@
     header = struct.unpack(b'>I', data[0:4])[0]
     version = header & 0xFFFF
     if version == 1:
-        revlogio = revlog.revlogio()
+        # Renamed to revlogv1io in Mercurial 4.9.
+        try:
+            revlogio = revlog.revlogv1io()
+        except AttributeError:
+            revlogio = revlog.revlogio()
         inline = header & (1 << 16)
     else:
         raise error.Abort((b'unsupported revlog version: %d') % version)
diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -240,7 +240,7 @@
             return (0, 0, 0, -1, -1, -1, -1, nullid)
         return list.__getitem__(self, i)
 
-class revlogoldio(object):
+class revlogv0io(object):
     def __init__(self):
         self.size = indexformatv0.size
 
@@ -291,7 +291,7 @@
 # signed integer)
 _maxentrysize = 0x7fffffff
 
-class revlogio(object):
+class revlogv1io(object):
     def __init__(self):
         self.size = indexformatng.size
 
@@ -505,9 +505,9 @@
 
         self._storedeltachains = True
 
-        self._io = revlogio()
+        self._io = revlogv1io()
         if self.version == REVLOGV0:
-            self._io = revlogoldio()
+            self._io = revlogv0io()
         try:
             d = self._io.parseindex(indexdata, self._inline)
         except (ValueError, IndexError):