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.

4 files changed, 9 insertions, 5 deletions

diff --git a/src/buffer.c b/src/buffer.c
index edd85d8ef63..d8964180cff 100644
--- a/src/buffer.c
+++ b/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.  */
diff --git a/src/buffer.h b/src/buffer.h
index 09daa29992a..47b4bdf749b 100644
--- a/src/buffer.h
+++ b/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
diff --git a/src/lisp.h b/src/lisp.h
index 6e8c2f3a2f9..dabf013d531 100644
--- a/src/lisp.h
+++ b/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);
diff --git a/src/xdisp.c b/src/xdisp.c
index b1c492fab3a..4701e2b2459 100644
--- a/src/xdisp.c
+++ b/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)