2 files changed, 15 insertions, 5 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 883c0d84bf6..dc81626038d 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,7 +1,13 @@
-2011-07-24  Paul Eggert  <eggert@cs.ucla.edu>
+2011-07-25  Paul Eggert  <eggert@cs.ucla.edu>
 
         * Makefile.in (gl-stamp): move-if-change is now in build-aux.
 
+2011-07-25  Eli Zaretskii  <eliz@gnu.org>
+
+        * xdisp.c (compute_display_string_pos): Fix logic of caching
+        previous display string position.  Initialize cached_prev_pos to
+        -1.  Fixes slow-down at the beginning of a buffer.
+
 2011-07-24  Eli Zaretskii  <eliz@gnu.org>
 
         * xfaces.c (check_lface_attrs) [HAVE_WINDOW_SYSTEM]: Allow `nil'
diff --git a/src/xdisp.c b/src/xdisp.c
index 8b164008c3f..7493fbff008 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -3137,7 +3137,7 @@ next_overlay_change (EMACS_INT pos)
 /* Record one cached display string position found recently by
    compute_display_string_pos.  */
 static EMACS_INT cached_disp_pos;
-static EMACS_INT cached_prev_pos;
+static EMACS_INT cached_prev_pos = -1;
 static struct buffer *cached_disp_buffer;
 static int cached_disp_modiff;
 static int cached_disp_overlay_modiff;
@@ -3184,18 +3184,22 @@ compute_display_string_pos (struct text_pos *position,
           && BUF_MODIFF (b) == cached_disp_modiff
           && BUF_OVERLAY_MODIFF (b) == cached_disp_overlay_modiff)
         {
-          if (cached_prev_pos
+          if (cached_prev_pos >= 0
               && cached_prev_pos < charpos && charpos <= cached_disp_pos)
             return cached_disp_pos;
           /* Handle overstepping either end of the known interval.  */
           if (charpos > cached_disp_pos)
             cached_prev_pos = cached_disp_pos;
           else  /* charpos <= cached_prev_pos */
-            cached_prev_pos = max (charpos - 1, BEGV);
+            cached_prev_pos = max (charpos - 1, 0);
         }
 
       /* Record new values in the cache.  */
-      cached_disp_buffer = b;
+      if (b != cached_disp_buffer)
+        {
+          cached_disp_buffer = b;
+          cached_prev_pos = max (charpos - 1, 0);
+        }
       cached_disp_modiff = BUF_MODIFF (b);
       cached_disp_overlay_modiff = BUF_OVERLAY_MODIFF (b);
     }