diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index 83cd3dca8..d2211d7bf 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -416,12 +416,13 @@ reset:
   return ret;
 }
 
-void
+int
 hb_buffer_t::sync_so_far ()
 {
   bool had_output = have_output;
   unsigned out_i = out_len;
   unsigned i = idx;
+  unsigned old_idx = idx;
 
   if (sync ())
     idx = out_i;
@@ -435,6 +436,8 @@ hb_buffer_t::sync_so_far ()
   }
 
   assert (idx <= len);
+
+  return idx - old_idx;
 }
 
 bool
diff --git a/src/hb-buffer.hh b/src/hb-buffer.hh
index 1b89a32db..a05f1662e 100644
--- a/src/hb-buffer.hh
+++ b/src/hb-buffer.hh
@@ -289,7 +289,7 @@ struct hb_buffer_t
   HB_INTERNAL void guess_segment_properties ();
 
   HB_INTERNAL bool sync ();
-  HB_INTERNAL void sync_so_far ();
+  HB_INTERNAL int sync_so_far ();
   HB_INTERNAL void clear_output ();
   HB_INTERNAL void clear_positions ();