Further tweaks to long lines handling.

* src/xdisp.c (redisplay_window): Increase the threshold above which
long lines detection is performed in the buffer.  This should avoid
triggering that detection for most simple editing operations.

* src/lisp.h (modiff_incr): Explain why the counter is incremented
logarithmically.

* src/buffer.h (struct buffer_text): Adapt the comment about the
'modiff' field accordingly.

* src/buffer.c (modify_overlay): Increase the counter by 1 instead of
the size of the buffer section on which the overlay is placed.

* etc/NEWS: Small improvement.

etc/NEWS

@@ -330,9 +330,9 @@ The display of long lines has been optimized, and Emacs no longer
 chokes when a buffer on display contains long lines.  If you still
 experience slowdowns while editing files with long lines, this is
 either due to font locking, which you can turn off with M-x
-font-lock-mode, or to the current major mode or one of the enabled
-minor modes, in which case you should open the the file with M-x
-find-file-literally instead of C-x C-f.  The variable
+font-lock-mode or C-u C-x x f, or to the current major mode or one of
+the enabled minor modes, in which case you should open the the file
+with M-x find-file-literally instead of C-x C-f.  The variable
 'long-line-threshold' controls whether and when these display
 optimizations are used.
 

src/buffer.c

@@ -4010,7 +4010,7 @@ modify_overlay (struct buffer *buf, ptrdiff_t start, ptrdiff_t end)
 
   bset_redisplay (buf);
 
-  modiff_incr (&BUF_OVERLAY_MODIFF (buf), end - start);
+  modiff_incr (&BUF_OVERLAY_MODIFF (buf), 1);
 }
 
 /* Remove OVERLAY from LIST.  */

src/buffer.h

@@ -237,9 +237,10 @@ struct buffer_text
     ptrdiff_t z_byte;		/* Byte pos of end of buffer.  */
     ptrdiff_t gap_size;		/* Size of buffer's gap.  */
     modiff_count modiff;	/* This counts buffer-modification events
-				   for this buffer.  It is incremented for
-				   each such event, and never otherwise
-				   changed.  */
+				   for this buffer.  It is increased
+				   logarithmically to the extent of the
+				   modification for each such event,
+				   and never otherwise changed.  */
     modiff_count chars_modiff;	/* This is modified with character change
 				   events for this buffer.  It is set to
 				   modiff for each such event, and never

src/lisp.h

@@ -3914,6 +3914,9 @@ INLINE modiff_count
 modiff_incr (modiff_count *a, ptrdiff_t len)
 {
   modiff_count a0 = *a; int incr = len ? 1 : 0;
+  /* Increase the counter more for a large modification and less for a
+     small modification.  Increase it logarithmically to avoid
+     increasing it too much.  */
   while (len >>= 1) incr++;
   bool modiff_overflow = INT_ADD_WRAPV (a0, incr, a);
   eassert (!modiff_overflow && *a >> 30 >> 30 == 0);

src/xdisp.c

@@ -19293,7 +19293,7 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
   /* Check whether the buffer to be displayed contains long lines.  */
   if (!NILP (Vlong_line_threshold)
       && !current_buffer->long_line_optimizations_p
-      && MODIFF - UNCHANGED_MODIFIED > 4)
+      && MODIFF - UNCHANGED_MODIFIED > 8)
     {
       ptrdiff_t cur, next, found, max = 0;
       for (cur = 1; cur < Z; cur = next)