1 files changed, 6 insertions, 0 deletions
diff --git a/src/composite.c b/src/composite.c
index 4d69702171f..d8998b5a1f3 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -1576,6 +1576,7 @@ find_automatic_composition (ptrdiff_t pos, ptrdiff_t limit, ptrdiff_t backlim,
   Lisp_Object window;
   struct window *w;
   bool need_adjustment = 0;
+  ptrdiff_t narrowed_begv;
 
   window = Fget_buffer_window (Fcurrent_buffer (), Qnil);
   if (NILP (window))
@@ -1586,6 +1587,11 @@ find_automatic_composition (ptrdiff_t pos, ptrdiff_t limit, ptrdiff_t backlim,
   if (NILP (string))
     {
       head = backlim < 0 ? BEGV : backlim, tail = ZV, stop = GPT;
+      /* In buffers with very long lines, this function becomes very
+	 slow.  Pretend that the buffer is narrowed to make it fast.  */
+      narrowed_begv = get_narrowed_begv (w);
+      if (pos > narrowed_begv)
+	head = narrowed_begv;
       cur.pos_byte = CHAR_TO_BYTE (cur.pos);
       cur.p = BYTE_POS_ADDR (cur.pos_byte);
     }