Simplified and improved heuristic for long lines detection.

* src/lisp.h (modiff_incr): Add a parameter to 'modiff_incr' to record
the extent of the modification.

* src/insdel.c (insert_1_both, insert_from_string_1, insert_from_gap,
insert_from_buffer_1, adjust_after_replace, replace_range,
replace_range_2, del_range_2, modify_text): Add an argument to each
call to 'modiff_incr'.

* src/textprop.c (modify_text_properties): Add an argument to the call
to 'modiff_incr'.

* src/buffer.c (Frestore_buffer_modified_p, Fbuffer_swap_text,
modify_overlay): Add an argument to each call to 'modiff_incr'.

* src/xdisp.c (redisplay_window): Use the improved version of 'MODIFF'
for the heuristic.

5 files changed, 23 insertions, 22 deletions

diff --git a/src/buffer.c b/src/buffer.c
index a777668e44b..edd85d8ef63 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -1501,7 +1501,7 @@ state of the current buffer.  Use with care.  */)
 	 decrease SAVE_MODIFF and auto_save_modified or increase
 	 MODIFF.  */
       if (SAVE_MODIFF >= MODIFF)
-	SAVE_MODIFF = modiff_incr (&MODIFF);
+	SAVE_MODIFF = modiff_incr (&MODIFF, 1);
       if (EQ (flag, Qautosaved))
 	BUF_AUTOSAVE_MODIFF (b) = MODIFF;
     }
@@ -2466,12 +2466,12 @@ results, see Info node `(elisp)Swapping Text'.  */)
   bset_point_before_scroll (current_buffer, Qnil);
   bset_point_before_scroll (other_buffer, Qnil);
 
-  modiff_incr (&current_buffer->text->modiff);
-  modiff_incr (&other_buffer->text->modiff);
-  modiff_incr (&current_buffer->text->chars_modiff);
-  modiff_incr (&other_buffer->text->chars_modiff);
-  modiff_incr (&current_buffer->text->overlay_modiff);
-  modiff_incr (&other_buffer->text->overlay_modiff);
+  modiff_incr (&current_buffer->text->modiff, 1);
+  modiff_incr (&other_buffer->text->modiff, 1);
+  modiff_incr (&current_buffer->text->chars_modiff, 1);
+  modiff_incr (&other_buffer->text->chars_modiff, 1);
+  modiff_incr (&current_buffer->text->overlay_modiff, 1);
+  modiff_incr (&other_buffer->text->overlay_modiff, 1);
   current_buffer->text->beg_unchanged = current_buffer->text->gpt;
   current_buffer->text->end_unchanged = current_buffer->text->gpt;
   other_buffer->text->beg_unchanged = other_buffer->text->gpt;
@@ -4010,7 +4010,7 @@ modify_overlay (struct buffer *buf, ptrdiff_t start, ptrdiff_t end)
 
   bset_redisplay (buf);
 
-  modiff_incr (&BUF_OVERLAY_MODIFF (buf));
+  modiff_incr (&BUF_OVERLAY_MODIFF (buf), end - start);
 }
 
 /* Remove OVERLAY from LIST.  */
diff --git a/src/insdel.c b/src/insdel.c
index 6f180ac5800..309ca728a4c 100644
--- a/src/insdel.c
+++ b/src/insdel.c
@@ -909,7 +909,7 @@ insert_1_both (const char *string,
      the insertion.  This, together with recording the insertion,
      will add up to the right stuff in the undo list.  */
   record_insert (PT, nchars);
-  modiff_incr (&MODIFF);
+  modiff_incr (&MODIFF, nchars);
   CHARS_MODIFF = MODIFF;
 
   memcpy (GPT_ADDR, string, nbytes);
@@ -1037,7 +1037,7 @@ insert_from_string_1 (Lisp_Object string, ptrdiff_t pos, ptrdiff_t pos_byte,
 #endif
 
   record_insert (PT, nchars);
-  modiff_incr (&MODIFF);
+  modiff_incr (&MODIFF, nchars);
   CHARS_MODIFF = MODIFF;
 
   GAP_SIZE -= outgoing_nbytes;
@@ -1122,7 +1122,7 @@ insert_from_gap (ptrdiff_t nchars, ptrdiff_t nbytes, bool text_at_gap_tail)
      of this dance.  */
   invalidate_buffer_caches (current_buffer, GPT, GPT);
   record_insert (GPT, nchars);
-  modiff_incr (&MODIFF);
+  modiff_incr (&MODIFF, nchars);
 
   insert_from_gap_1 (nchars, nbytes, text_at_gap_tail);
 
@@ -1250,7 +1250,7 @@ insert_from_buffer_1 (struct buffer *buf,
 #endif
 
   record_insert (PT, nchars);
-  modiff_incr (&MODIFF);
+  modiff_incr (&MODIFF, nchars);
   CHARS_MODIFF = MODIFF;
 
   GAP_SIZE -= outgoing_nbytes;
@@ -1351,7 +1351,7 @@ adjust_after_replace (ptrdiff_t from, ptrdiff_t from_byte,
 
   if (len == 0)
     evaporate_overlays (from);
-  modiff_incr (&MODIFF);
+  modiff_incr (&MODIFF, nchars_del + len);
   CHARS_MODIFF = MODIFF;
 }
 
@@ -1546,7 +1546,7 @@ replace_range (ptrdiff_t from, ptrdiff_t to, Lisp_Object new,
 
   check_markers ();
 
-  modiff_incr (&MODIFF);
+  modiff_incr (&MODIFF, nchars_del + inschars);
   CHARS_MODIFF = MODIFF;
 
   if (adjust_match_data)
@@ -1680,7 +1680,7 @@ replace_range_2 (ptrdiff_t from, ptrdiff_t from_byte,
 
   check_markers ();
 
-  modiff_incr (&MODIFF);
+  modiff_incr (&MODIFF, nchars_del + inschars);
   CHARS_MODIFF = MODIFF;
 }
 
@@ -1855,7 +1855,7 @@ del_range_2 (ptrdiff_t from, ptrdiff_t from_byte,
      at the end of the text before the gap.  */
   adjust_markers_for_delete (from, from_byte, to, to_byte);
 
-  modiff_incr (&MODIFF);
+  modiff_incr (&MODIFF, nchars_del);
   CHARS_MODIFF = MODIFF;
 
   /* Relocate point as if it were a marker.  */
@@ -1909,7 +1909,7 @@ modify_text (ptrdiff_t start, ptrdiff_t end)
   BUF_COMPUTE_UNCHANGED (current_buffer, start - 1, end);
   if (MODIFF <= SAVE_MODIFF)
     record_first_change ();
-  modiff_incr (&MODIFF);
+  modiff_incr (&MODIFF, end - start);
   CHARS_MODIFF = MODIFF;
 
   bset_point_before_scroll (current_buffer, Qnil);
diff --git a/src/lisp.h b/src/lisp.h
index 5045d49e1b6..6e8c2f3a2f9 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3911,10 +3911,11 @@ integer_to_uintmax (Lisp_Object num, uintmax_t *n)
 typedef intmax_t modiff_count;
 
 INLINE modiff_count
-modiff_incr (modiff_count *a)
+modiff_incr (modiff_count *a, ptrdiff_t len)
 {
-  modiff_count a0 = *a;
-  bool modiff_overflow = INT_ADD_WRAPV (a0, 1, a);
+  modiff_count a0 = *a; int incr = len ? 1 : 0;
+  while (len >>= 1) incr++;
+  bool modiff_overflow = INT_ADD_WRAPV (a0, incr, a);
   eassert (!modiff_overflow && *a >> 30 >> 30 == 0);
   return a0;
 }
diff --git a/src/textprop.c b/src/textprop.c
index 96d07b44be8..c91a2b729c6 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -88,7 +88,7 @@ modify_text_properties (Lisp_Object buffer, Lisp_Object start, Lisp_Object end)
   BUF_COMPUTE_UNCHANGED (buf, b - 1, e);
   if (MODIFF <= SAVE_MODIFF)
     record_first_change ();
-  modiff_incr (&MODIFF);
+  modiff_incr (&MODIFF, 1);
 
   bset_point_before_scroll (current_buffer, Qnil);
 
diff --git a/src/xdisp.c b/src/xdisp.c
index ec01375e1f8..b1c492fab3a 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)
+      && MODIFF - UNCHANGED_MODIFIED > 4)
     {
       ptrdiff_t cur, next, found, max = 0;
       for (cur = 1; cur < Z; cur = next)