Speed up redisplay of long truncated lines

* src/xdisp.c (forward_to_next_line_start): Fix logic of
interpreting the result of Fnext_single_property_change.
(reseat_at_next_visible_line_start): When ON_NEWLINE_P is zero,
pass NULL to 'forward_to_next_line_start', to avoid costly bidi
iteration when none is needed.  This speeds up redisplay of very
long lines under 'truncate-lines'.

1 files changed, 6 insertions, 4 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index 215a6d561ea..690f10b8403 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -7153,10 +7153,10 @@ forward_to_next_line_start (struct it *it, bool *skipped_p,
           || ((pos = Fnext_single_property_change (make_fixnum (start),
                                                    Qdisplay, Qnil,
                                                    make_fixnum (limit)),
-               NILP (pos))
+               (NILP (pos) || XFIXNAT (pos) == limit))
               && next_overlay_change (start) == ZV))
         {
-          if (!it->bidi_p)
+          if (!it->bidi_p || !bidi_it_prev)
             {
               IT_CHARPOS (*it) = limit;
               IT_BYTEPOS (*it) = bytepos;
@@ -7319,7 +7319,8 @@ reseat_at_next_visible_line_start (struct it *it, bool on_newline_p)
   bool skipped_p = false;
   struct bidi_it bidi_it_prev;
   bool newline_found_p
-    = forward_to_next_line_start (it, &skipped_p, &bidi_it_prev);
+    = forward_to_next_line_start (it, &skipped_p,
+                                  on_newline_p ? &bidi_it_prev : NULL);
 
   /* Skip over lines that are invisible because they are indented
      more than the value of IT->selective.  */
@@ -7331,7 +7332,8 @@ reseat_at_next_visible_line_start (struct it *it, bool on_newline_p)
         eassert (IT_BYTEPOS (*it) == BEGV
                  || FETCH_BYTE (IT_BYTEPOS (*it) - 1) == '\n');
         newline_found_p =
-          forward_to_next_line_start (it, &skipped_p, &bidi_it_prev);
+          forward_to_next_line_start (it, &skipped_p,
+                                      on_newline_p ? &bidi_it_prev : NULL);
       }
 
   /* Position on the newline if that's what's requested.  */