1 files changed, 123 insertions, 36 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index 0b190529404..fa411e6e8dc 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -3482,7 +3482,7 @@ init_iterator (struct it *it, struct window *w,
 
       /* This is set only when long_line_optimizations_p is non-zero
 	 for the current buffer.  */
-      it->narrowed_begv = 0;
+      it->medium_narrowing_begv = 0;
 
       /* Compute faces etc.  */
       reseat (it, it->current.pos, true);
@@ -3491,9 +3491,91 @@ init_iterator (struct it *it, struct window *w,
   CHECK_IT (it);
 }
 
-/* Compute a suitable alternate value for BEGV and ZV that may be used
-   temporarily to optimize display if the buffer in window W contains
-   long lines.  */
+/* How Emacs deals with long lines.
+
+   (1) When a buffer is about to be (re)displayed, 'redisplay_window'
+   detects, with a heuristic, whether it contains long lines.
+
+   This happens in 'redisplay_window' because it is only displaying
+   buffers with long lines that is problematic.  In other words, none
+   of the optimizations described below is ever used in buffers that
+   are never displayed.
+
+   This happens with a heuristic, which checks whether a buffer
+   contains long lines, each time its contents have changed "enough"
+   between two redisplay cycles, because a buffer without long lines
+   can become a buffer with long lines at any time, for example after
+   a yank command, or after a replace command, or while the output of
+   an external process is inserted in a buffer.
+
+   When Emacs has detected that a buffer contains long lines, the
+   buffer-local variable 'long_line_optimizations_p' (in 'struct
+   buffer') is set, and Emacs does not try to detect whether the
+   buffer does or does not contain long lines anymore.
+
+   What a long line is depends on the variable 'long-line-threshold',
+   whose default value is 50000 (characters).
+
+   (2) When a buffer with long lines is (re)displayed, the amount of
+   data that the display routines consider is, in a few well-chosen
+   places, limited with a temporary restriction, whose bounds are
+   calculated with the functions below.
+
+   (2.1) 'get_small_narrowing_begv' is used to create a restriction
+   which starts a few hundred characters before point.  The exact
+   number of characters depends on the width of the window in which
+   the buffer is displayed.
+
+   There is no corresponding 'get_small_narrowing_zv' function,
+   because it is not necessary to set the end limit of that
+   restriction.
+
+   This restriction is used in four places, namely:
+   'back_to_previous_line_start' and 'move_it_vertically_backward'
+   (with the 'SET_WITH_NARROWED_BEGV' macro), and in
+   'composition_compute_stop_pos' and 'find_automatic_composition' (in
+   a conditional statement depending on 'long_line_optimizations_p').
+
+   (2.2) 'get_medium_narrowing_begv' is used to create a restriction
+   which starts a few thousand characters before point.  The exact
+   number of characters depends on the size (width and height) of the
+   window in which the buffer is displayed.  For performance reasons,
+   the return value of that function is cached in 'struct it', in the
+   'medium_narrowing_begv' field.
+
+   The corresponding function 'get_medium_narrowing_zv' (and
+   'medium_narrowing_zv' field in 'struct it') is not used to set the
+   end limit of a the restriction, which is again unnecessary, but to
+   determine, in 'reseat', whether the iterator has moved far enough
+   from its original position, and whether the start position of the
+   restriction must be computed anew.
+
+   This restriction is used in a single place:
+   'get_visually_first_element', with the 'SET_WITH_NARROWED_BEGV'
+   macro.
+
+   (2.3) 'get_large_narrowing_begv' and 'get_large_narrowing_zv' are
+   used to create a restriction which starts a few hundred thousand
+   characters before point and ends a few hundred thousand characters
+   after point.  The size of that restriction depends on the variable
+   'long-line-optimizations-region-size', whose default value is
+   500000 (characters); it can be adjusted by a few hundred characters
+   depending on 'long-line-optimizations-bol-search-limit', whose
+   default value is 128 (characters).
+
+   For performance reasons again, the return values of these functions
+   are stored in the 'large_narrowing_begv' and 'large_narrowing_zv'
+   fields in 'struct it'.
+
+   The restriction defined by these values is used around three
+   low-level hooks: around 'fontification-functions', in
+   'handle_fontified_prop', and around 'pre-command-hook' and
+   'post-command-hook', in 'safe_run_hooks_maybe_narrowed', which is
+   called in 'command_loop_1'.  These restrictions are set around
+   these hooks with 'labeled_narrow_to_region'; the restrictions are
+   labeled, and cannot be removed with a call to 'widen', but can be
+   removed with 'without-restriction' with a :label argument.
+*/
 
 static int
 get_narrowed_width (struct window *w)
@@ -3513,28 +3595,28 @@ get_narrowed_len (struct window *w)
 }
 
 ptrdiff_t
-get_narrowed_begv (struct window *w, ptrdiff_t pos)
+get_medium_narrowing_begv (struct window *w, ptrdiff_t pos)
 {
   int len = get_narrowed_len (w);
   return max ((pos / len - 1) * len, BEGV);
 }
 
 ptrdiff_t
-get_narrowed_zv (struct window *w, ptrdiff_t pos)
+get_medium_narrowing_zv (struct window *w, ptrdiff_t pos)
 {
   int len = get_narrowed_len (w);
   return min ((pos / len + 1) * len, ZV);
 }
 
 ptrdiff_t
-get_closer_narrowed_begv (struct window *w, ptrdiff_t pos)
+get_small_narrowing_begv (struct window *w, ptrdiff_t pos)
 {
   int len = get_narrowed_width (w);
   return max ((pos / len - 1) * len, BEGV);
 }
 
 ptrdiff_t
-get_locked_narrowing_begv (ptrdiff_t pos)
+get_large_narrowing_begv (ptrdiff_t pos)
 {
   if (long_line_optimizations_region_size <= 0)
     return BEGV;
@@ -3552,7 +3634,7 @@ get_locked_narrowing_begv (ptrdiff_t pos)
 }
 
 ptrdiff_t
-get_locked_narrowing_zv (ptrdiff_t pos)
+get_large_narrowing_zv (ptrdiff_t pos)
 {
   if (long_line_optimizations_region_size <= 0)
     return ZV;
@@ -3571,7 +3653,7 @@ unwind_narrowed_begv (Lisp_Object point_min)
 
 #define SET_WITH_NARROWED_BEGV(IT,DST,EXPR,BV)				\
   do {									\
-    if (IT->narrowed_begv)						\
+    if (IT->medium_narrowing_begv)						\
       {									\
 	specpdl_ref count = SPECPDL_INDEX ();				\
 	record_unwind_protect (unwind_narrowed_begv, Fpoint_min ());	\
@@ -4396,17 +4478,17 @@ handle_fontified_prop (struct it *it)
       if (current_buffer->long_line_optimizations_p
 	  && long_line_optimizations_region_size > 0)
 	{
-	  ptrdiff_t begv = it->locked_narrowing_begv;
-	  ptrdiff_t zv = it->locked_narrowing_zv;
+	  ptrdiff_t begv = it->large_narrowing_begv;
+	  ptrdiff_t zv = it->large_narrowing_zv;
 	  ptrdiff_t charpos = IT_CHARPOS (*it);
 	  if (charpos < begv || charpos > zv)
 	    {
-	      begv = get_locked_narrowing_begv (charpos);
-	      zv = get_locked_narrowing_zv (charpos);
+	      begv = get_large_narrowing_begv (charpos);
+	      zv = get_large_narrowing_zv (charpos);
 	    }
 	  if (begv != BEG || zv != Z)
-	    narrow_to_region_locked (make_fixnum (begv), make_fixnum (zv),
-				     Qlong_line_optimizations_in_fontification_functions);
+	    labeled_narrow_to_region (make_fixnum (begv), make_fixnum (zv),
+				      Qlong_line_optimizations_in_fontification_functions);
 	}
 
       /* Don't allow Lisp that runs from 'fontification-functions'
@@ -7041,7 +7123,7 @@ back_to_previous_line_start (struct it *it)
   dec_both (&cp, &bp);
   SET_WITH_NARROWED_BEGV (it, IT_CHARPOS (*it),
 			  find_newline_no_quit (cp, bp, -1, &IT_BYTEPOS (*it)),
-			  get_closer_narrowed_begv (it->w, IT_CHARPOS (*it)));
+			  get_small_narrowing_begv (it->w, IT_CHARPOS (*it)));
 }
 
 /* Find in the current buffer the first display or overlay string
@@ -7345,7 +7427,7 @@ back_to_previous_visible_line_start (struct it *it)
   it->continuation_lines_width = 0;
 
   eassert (IT_CHARPOS (*it) >= BEGV);
-  eassert (it->narrowed_begv > 0 /* long-line optimizations: all bets off */
+  eassert (it->medium_narrowing_begv > 0 /* long-line optimizations: all bets off */
 	   || IT_CHARPOS (*it) == BEGV
 	   || FETCH_BYTE (IT_BYTEPOS (*it) - 1) == '\n');
   CHECK_IT (it);
@@ -7463,24 +7545,29 @@ reseat (struct it *it, struct text_pos pos, bool force_p)
 
   if (current_buffer->long_line_optimizations_p)
     {
-      if (!it->narrowed_begv)
+      if (!it->medium_narrowing_begv)
 	{
-	  it->narrowed_begv = get_narrowed_begv (it->w, window_point (it->w));
-	  it->narrowed_zv = get_narrowed_zv (it->w, window_point (it->w));
-	  it->locked_narrowing_begv
-	    = get_locked_narrowing_begv (window_point (it->w));
-	  it->locked_narrowing_zv
-	    = get_locked_narrowing_zv (window_point (it->w));
+	  it->medium_narrowing_begv
+	    = get_medium_narrowing_begv (it->w, window_point (it->w));
+	  it->medium_narrowing_zv
+	    = get_medium_narrowing_zv (it->w, window_point (it->w));
+	  it->large_narrowing_begv
+	    = get_large_narrowing_begv (window_point (it->w));
+	  it->large_narrowing_zv
+	    = get_large_narrowing_zv (window_point (it->w));
 	}
-      else if ((pos.charpos < it->narrowed_begv || pos.charpos > it->narrowed_zv)
+      else if ((pos.charpos < it->medium_narrowing_begv
+		|| pos.charpos > it->medium_narrowing_zv)
 		&& (!redisplaying_p || it->line_wrap == TRUNCATE))
 	{
-	  it->narrowed_begv = get_narrowed_begv (it->w, pos.charpos);
-	  it->narrowed_zv = get_narrowed_zv (it->w, pos.charpos);
-	  it->locked_narrowing_begv
-	    = get_locked_narrowing_begv (window_point (it->w));
-	  it->locked_narrowing_zv
-	    = get_locked_narrowing_zv (window_point (it->w));
+	  it->medium_narrowing_begv
+	    = get_medium_narrowing_begv (it->w, pos.charpos);
+	  it->medium_narrowing_zv
+	    = get_medium_narrowing_zv (it->w, pos.charpos);
+	  it->large_narrowing_begv
+	    = get_large_narrowing_begv (window_point (it->w));
+	  it->large_narrowing_zv
+	    = get_large_narrowing_zv (window_point (it->w));
 	}
     }
 
@@ -8789,7 +8876,7 @@ get_visually_first_element (struct it *it)
   SET_WITH_NARROWED_BEGV (it, bob,
 			  string_p ? 0 :
 			  IT_CHARPOS (*it) < BEGV ? obegv : BEGV,
-			  it->narrowed_begv);
+			  it->medium_narrowing_begv);
 
   if (STRINGP (it->string))
     {
@@ -8833,7 +8920,7 @@ get_visually_first_element (struct it *it)
 				find_newline_no_quit (IT_CHARPOS (*it),
 						      IT_BYTEPOS (*it), -1,
 						      &it->bidi_it.bytepos),
-				it->narrowed_begv);
+				it->medium_narrowing_begv);
       bidi_paragraph_init (it->paragraph_embedding, &it->bidi_it, true);
       do
 	{
@@ -10722,7 +10809,7 @@ move_it_vertically_backward (struct it *it, int dy)
 	  dec_both (&cp, &bp);
 	  SET_WITH_NARROWED_BEGV (it, cp,
 				  find_newline_no_quit (cp, bp, -1, NULL),
-				  get_closer_narrowed_begv (it->w, IT_CHARPOS (*it)));
+				  get_small_narrowing_begv (it->w, IT_CHARPOS (*it)));
 	  move_it_to (it, cp, -1, -1, -1, MOVE_TO_POS);
 	}
       bidi_unshelve_cache (it3data, true);
@@ -16394,7 +16481,7 @@ redisplay_internal (void)
   FOR_EACH_FRAME (tail, frame)
     XFRAME (frame)->already_hscrolled_p = false;
 
-  reset_outermost_narrowings ();
+  reset_outermost_restrictions ();
 
  retry:
   /* Remember the currently selected window.  */