Code cleanup for long line optimizations

This commit does not change any code, it merely renames functions
and clarifies the documentation, to make the code hopefully easier
to grasp.

* src/dispextern.h (struct it): Rename the 'narrowed_begv',
'narrowed_zv', 'locked_narrowing_begv', 'locked_narrowing_zv' to
'medium_narrowing_begv', 'medium_narrowing_zv',
'large_narrowing_begv', 'large_narrowing_zv'.  Clarify the
comments.
Update the prototypes of the functions renamed in xdisp.c.

* src/lisp.h: Update the prototypes of the functions renamed in
editfns.c.  Remove the prototype of
'safe_run_hooks_maybe_narrowed', which is used only in keyboard.c.

* src/xdisp.c
(get_small_narrowing_begv): Renamed from
'get_closer_narrowed_begv'.
(get_medium_narrowing_begv): Renamed from 'get_narrowed_begv'.
(get_medium_narrowing_zv): Renamed from 'get_narrowed_zv'.
(get_large_narrowing_begv): Renamed from 'get_locked_narrowing_begv'.
(get_large_narrowing_zv): Renamed from 'get_locked_narrowing_zv'.
(SET_WITH_NARROWED_BEGV): Use the new field names.
(handle_fontified_prop): Use the new function and new field names.
(back_to_previous_line_start): Use the new field name.
(back_to_previous_visible_line_start): Use the new field name.
(reseat): Use the new function and new field names.
(get_visually_first_element): Use the new field name.
(move_it_vertically_backward): Use the new function name.
(redisplay_internal): Use the new function name.
Also add a large comment to explain how Emacs deals with long lines.

* src/keyboard.c:
(safe_run_hooks_maybe_narrowed): Use the new function names from
xdisp.c and editfns.c.  Make the function static, and add a
prototype.

* src/editfns.c:
(labeled_restrictions): Renamed from 'narrowing_locks'.
(labeled_restrictions_add): Renamed from 'narrowing_locks_add'.
(labeled_restrictions_remove): Renamed from
'narrowing_locks_remove'.
(labeled_restrictions_get_bound): Renamed from
'narrowing_lock_get_bound'.
(labeled_restrictions_peek_label): Renamed from
'narrowing_lock_peek_tag'.
(labeled_restrictions_push): Renamed from 'narrowing_lock_push'.
(labeled_restrictions_pop): Renamed from 'narrowing_lock_pop'.
(unwind_reset_outermost_restriction): Renamed from
'unwind_reset_outermost_narrowing'.
(reset_outermost_restrictions): Renamed from
'reset_outermost_narrowings'.
(labeled_restrictions_save): Renamed from 'narrowing_locks_save'.
(labeled_restrictions_restore): Renamed from
'narrowing_locks_restore'.
(unwind_labeled_narrow_to_region): Renamed from
'unwind_narrow_to_region_locked'.
(labeled_narrow_to_region): Renamed from
'narrow_to_region_locked'.
(Finternal__label_restriction): Renamed from
'Finternal__lock_narrowing'.
(Finternal__unlabel_restriction): Renamed from
'Finternal__unlock_narrowing'.
(Fwiden): Use the new function names.
(Fnarrow_to_region): Use the new function names.
(save_restriction_save): Use the new function names.
(syms_of_editfns): Use the new function names.
<outermost-restriction>: Renamed from 'outermost-narrowing'.

* lisp/subr.el (internal--with-restriction): Use the new internal
function name.
(internal--without-restriction): Use the new internal function
name.

* src/composite.c (composition_compute_stop_pos):
(find_automatic_composition): Use the new function name.

* doc/lispref/positions.texi (Narrowing): Add index entry.

8 files changed, 316 insertions, 209 deletions

diff --git a/doc/lispref/positions.texi b/doc/lispref/positions.texi
index 1b32f18922c..e09fd4e7ca5 100644
--- a/doc/lispref/positions.texi
+++ b/doc/lispref/positions.texi
@@ -1154,6 +1154,7 @@ saved bounds.  In that case it is equivalent to
 @end example
 
 @cindex labeled narrowing
+@cindex labeled restriction
 When the optional argument @var{label}, a symbol, is present, the
 narrowing is @dfn{labeled}.  A labeled narrowing differs from a
 non-labeled one in several ways:
diff --git a/lisp/subr.el b/lisp/subr.el
index f82826e819c..ca1fc2886b4 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -3975,7 +3975,7 @@ same LABEL argument.
   "Helper function for `with-restriction', which see."
   (save-restriction
     (narrow-to-region start end)
-    (if label (internal--lock-narrowing label))
+    (if label (internal--label-restriction label))
     (funcall body)))
 
 (defmacro without-restriction (&rest rest)
@@ -3997,7 +3997,7 @@ are lifted.
 (defun internal--without-restriction (body &optional label)
   "Helper function for `without-restriction', which see."
   (save-restriction
-    (if label (internal--unlock-narrowing label))
+    (if label (internal--unlabel-restriction label))
     (widen)
     (funcall body)))
 
diff --git a/src/composite.c b/src/composite.c
index 164eeb39598..885db24df01 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -1075,7 +1075,7 @@ composition_compute_stop_pos (struct composition_it *cmp_it, ptrdiff_t charpos,
 	     with long lines, however, NL might be far away, so
 	     pretend that the buffer is smaller.  */
 	  if (current_buffer->long_line_optimizations_p)
-	    endpos = get_closer_narrowed_begv (cmp_it->parent_it->w, charpos);
+	    endpos = get_small_narrowing_begv (cmp_it->parent_it->w, charpos);
 	}
     }
   cmp_it->id = -1;
@@ -1654,7 +1654,7 @@ find_automatic_composition (ptrdiff_t pos, ptrdiff_t limit, ptrdiff_t backlim,
 	{
 	  /* In buffers with very long lines, this function becomes very
 	     slow.  Pretend that the buffer is narrowed to make it fast.  */
-	  ptrdiff_t begv = get_closer_narrowed_begv (w, window_point (w));
+	  ptrdiff_t begv = get_small_narrowing_begv (w, window_point (w));
 	  if (pos > begv)
 	    head = begv;
 	}
diff --git a/src/dispextern.h b/src/dispextern.h
index 4dcab113ea2..957e09f9ecc 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -2334,21 +2334,20 @@ struct it
      with which display_string was called.  */
   ptrdiff_t end_charpos;
 
-  /* Alternate begin position of the buffer that may be used to
-     optimize display (see the SET_WITH_NARROWED_BEGV macro).  */
-  ptrdiff_t narrowed_begv;
-
-  /* Alternate end position of the buffer that may be used to
-     optimize display.  */
-  ptrdiff_t narrowed_zv;
-
-  /* Begin position of the buffer for the locked narrowing around
-     low-level hooks.  */
-  ptrdiff_t locked_narrowing_begv;
-
-  /* End position of the buffer for the locked narrowing around
-     low-level hooks.  */
-  ptrdiff_t locked_narrowing_zv;
+  /* Alternate begin and end positions of the buffer that are used to
+     optimize display of buffers with long lines.  These two fields
+     hold the return value of the 'get_medium_narrowing_begv' and
+     'get_medium_narrowing_zv' functions.  */
+  ptrdiff_t medium_narrowing_begv;
+  ptrdiff_t medium_narrowing_zv;
+
+  /* Alternate begin and end positions of the buffer that are used for
+     labeled narrowings around low-level hooks in buffers with long
+     lines.  These two fields hold the return value of the
+     'get_large_narrowing_begv' and 'get_large_narrowing_zv'
+     functions.  */
+  ptrdiff_t large_narrowing_begv;
+  ptrdiff_t large_narrowing_zv;
 
   /* C string to iterate over.  Non-null means get characters from
      this string, otherwise characters are read from current_buffer
@@ -3410,11 +3409,11 @@ void mark_window_display_accurate (Lisp_Object, bool);
 void redisplay_preserve_echo_area (int);
 void init_iterator (struct it *, struct window *, ptrdiff_t,
                     ptrdiff_t, struct glyph_row *, enum face_id);
-ptrdiff_t get_narrowed_begv (struct window *, ptrdiff_t);
-ptrdiff_t get_narrowed_zv (struct window *, ptrdiff_t);
-ptrdiff_t get_closer_narrowed_begv (struct window *, ptrdiff_t);
-ptrdiff_t get_locked_narrowing_begv (ptrdiff_t);
-ptrdiff_t get_locked_narrowing_zv (ptrdiff_t);
+ptrdiff_t get_medium_narrowing_begv (struct window *, ptrdiff_t);
+ptrdiff_t get_medium_narrowing_zv (struct window *, ptrdiff_t);
+ptrdiff_t get_small_narrowing_begv (struct window *, ptrdiff_t);
+ptrdiff_t get_large_narrowing_begv (ptrdiff_t);
+ptrdiff_t get_large_narrowing_zv (ptrdiff_t);
 void init_iterator_to_row_start (struct it *, struct window *,
                                  struct glyph_row *);
 void start_display (struct it *, struct window *, struct text_pos);
diff --git a/src/editfns.c b/src/editfns.c
index f83c5c7259b..ff711ee2a09 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -2653,182 +2653,197 @@ DEFUN ("delete-and-extract-region", Fdelete_and_extract_region,
   return del_range_1 (XFIXNUM (start), XFIXNUM (end), 1, 1);
 }
 
-/* Alist of buffers in which locked narrowing is used.  The car of
-   each list element is a buffer, the cdr is a list of triplets (tag
-   begv-marker zv-marker).  The last element of that list always uses
-   the (uninterned) Qoutermost_narrowing tag and records the narrowing
-   bounds that were set by the user and that are visible on display.
-   This alist is used internally by narrow-to-region, widen,
-   internal--lock-narrowing, internal--unlock-narrowing and
-   save-restriction.  For efficiency reasons, an alist is used instead
-   of a buffer-local variable: otherwise reset_outermost_narrowings,
-   which is called during each redisplay cycle, would have to loop
-   through all live buffers.  */
-static Lisp_Object narrowing_locks;
-
-/* Add BUF with its LOCKS in the narrowing_locks alist.  */
+/* Alist of buffers in which labeled restrictions are used.  The car
+   of each list element is a buffer, the cdr is a list of triplets
+   (label begv-marker zv-marker).  The last triplet of that list
+   always uses the (uninterned) Qoutermost_restriction label, and
+   records the restriction bounds that were current when the first
+   labeled restriction was entered (which may be a narrowing that was
+   set by the user and is visible on display).  This alist is used
+   internally by narrow-to-region, widen, internal--label-restriction,
+   internal--unlabel-restriction and save-restriction.  For efficiency
+   reasons, an alist is used instead of a buffer-local variable:
+   otherwise reset_outermost_restrictions, which is called during each
+   redisplay cycle, would have to loop through all live buffers.  */
+static Lisp_Object labeled_restrictions;
+
+/* Add BUF with its list of labeled RESTRICTIONS in the
+   labeled_restrictions alist.  */
 static void
-narrowing_locks_add (Lisp_Object buf, Lisp_Object locks)
+labeled_restrictions_add (Lisp_Object buf, Lisp_Object restrictions)
 {
-  narrowing_locks = nconc2 (list1 (list2 (buf, locks)), narrowing_locks);
+  labeled_restrictions = nconc2 (list1 (list2 (buf, restrictions)),
+				 labeled_restrictions);
 }
 
-/* Remove BUF and its locks from the narrowing_locks alist.  Do
-   nothing if BUF is not present in narrowing_locks.  */
+/* Remove BUF and its list of labeled restrictions from the
+   labeled_restrictions alist.  Do nothing if BUF is not present in
+   labeled_restrictions.  */
 static void
-narrowing_locks_remove (Lisp_Object buf)
+labeled_restrictions_remove (Lisp_Object buf)
 {
-  narrowing_locks = Fdelq (Fassoc (buf, narrowing_locks, Qnil),
-			   narrowing_locks);
+  labeled_restrictions = Fdelq (Fassoc (buf, labeled_restrictions, Qnil),
+				labeled_restrictions);
 }
 
-/* Retrieve one of the BEGV/ZV bounds of a narrowing in BUF from the
-   narrowing_locks alist, as a pointer to a struct Lisp_Marker, or
-   NULL if BUF is not in narrowing_locks or is a killed buffer.  When
-   OUTERMOST is true, the bounds that were set by the user and that
-   are visible on display are returned.  Otherwise the innermost
-   locked narrowing bounds are returned.  */
+/* Retrieve one of the labeled restriction bounds in BUF from the
+   labeled_restrictions alist, as a pointer to a struct Lisp_Marker,
+   or return NULL if BUF is not in labeled_restrictions or is a killed
+   buffer.  When OUTERMOST is true, the restriction bounds that were
+   current when the first labeled restriction was entered are
+   returned.  Otherwise the bounds of the innermost labeled
+   restriction are returned.  */
 static struct Lisp_Marker *
-narrowing_lock_get_bound (Lisp_Object buf, bool begv, bool outermost)
+labeled_restrictions_get_bound (Lisp_Object buf, bool begv, bool outermost)
 {
   if (NILP (Fbuffer_live_p (buf)))
     return NULL;
-  Lisp_Object buffer_locks = assq_no_quit (buf, narrowing_locks);
-  if (NILP (buffer_locks))
+  Lisp_Object restrictions = assq_no_quit (buf, labeled_restrictions);
+  if (NILP (restrictions))
     return NULL;
-  buffer_locks = XCAR (XCDR (buffer_locks));
+  restrictions = XCAR (XCDR (restrictions));
   Lisp_Object bounds
     = outermost
-      ? XCDR (assq_no_quit (Qoutermost_narrowing, buffer_locks))
-      : XCDR (XCAR (buffer_locks));
+      ? XCDR (assq_no_quit (Qoutermost_restriction, restrictions))
+      : XCDR (XCAR (restrictions));
   eassert (! NILP (bounds));
   Lisp_Object marker = begv ? XCAR (bounds) : XCAR (XCDR (bounds));
   eassert (EQ (Fmarker_buffer (marker), buf));
   return XMARKER (marker);
 }
 
-/* Retrieve the tag of the innermost narrowing in BUF.  Return nil if
-   BUF is not in narrowing_locks or is a killed buffer.  */
+/* Retrieve the label of the innermost labeled restriction in BUF.
+   Return nil if BUF is not in labeled_restrictions or is a killed
+   buffer.  */
 static Lisp_Object
-narrowing_lock_peek_tag (Lisp_Object buf)
+labeled_restrictions_peek_label (Lisp_Object buf)
 {
   if (NILP (Fbuffer_live_p (buf)))
     return Qnil;
-  Lisp_Object buffer_locks = assq_no_quit (buf, narrowing_locks);
-  if (NILP (buffer_locks))
+  Lisp_Object restrictions = assq_no_quit (buf, labeled_restrictions);
+  if (NILP (restrictions))
     return Qnil;
-  Lisp_Object tag = XCAR (XCAR (XCAR (XCDR (buffer_locks))));
-  eassert (! NILP (tag));
-  return tag;
+  Lisp_Object label = XCAR (XCAR (XCAR (XCDR (restrictions))));
+  eassert (! NILP (label));
+  return label;
 }
 
-/* Add a LOCK for BUF in the narrowing_locks alist.  */
+/* Add a labeled RESTRICTION for BUF in the labeled_restrictions
+   alist.  */
 static void
-narrowing_lock_push (Lisp_Object buf, Lisp_Object lock)
+labeled_restrictions_push (Lisp_Object buf, Lisp_Object restriction)
 {
-  Lisp_Object buffer_locks = assq_no_quit (buf, narrowing_locks);
-  if (NILP (buffer_locks))
-    narrowing_locks_add (buf, list1 (lock));
+  Lisp_Object restrictions = assq_no_quit (buf, labeled_restrictions);
+  if (NILP (restrictions))
+    labeled_restrictions_add (buf, list1 (restriction));
   else
-    XSETCDR (buffer_locks, list1 (nconc2 (list1 (lock),
-					  XCAR (XCDR (buffer_locks)))));
+    XSETCDR (restrictions, list1 (nconc2 (list1 (restriction),
+					  XCAR (XCDR (restrictions)))));
 }
 
-/* Remove the innermost lock in BUF from the narrowing_locks alist.
-   Do nothing if BUF is not present in narrowing_locks.  */
+/* Remove the innermost labeled restriction in BUF from the
+   labeled_restrictions alist.  Do nothing if BUF is not present in
+   labeled_restrictions.  */
 static void
-narrowing_lock_pop (Lisp_Object buf)
+labeled_restrictions_pop (Lisp_Object buf)
 {
-  Lisp_Object buffer_locks = assq_no_quit (buf, narrowing_locks);
-  if (NILP (buffer_locks))
+  Lisp_Object restrictions = assq_no_quit (buf, labeled_restrictions);
+  if (NILP (restrictions))
     return;
-  if (EQ (narrowing_lock_peek_tag (buf), Qoutermost_narrowing))
-    narrowing_locks_remove (buf);
+  if (EQ (labeled_restrictions_peek_label (buf), Qoutermost_restriction))
+    labeled_restrictions_remove (buf);
   else
-    XSETCDR (buffer_locks, list1 (XCDR (XCAR (XCDR (buffer_locks)))));
+    XSETCDR (restrictions, list1 (XCDR (XCAR (XCDR (restrictions)))));
 }
 
 static void
-unwind_reset_outermost_narrowing (Lisp_Object buf)
+unwind_reset_outermost_restriction (Lisp_Object buf)
 {
-  struct Lisp_Marker *begv = narrowing_lock_get_bound (buf, true, false);
-  struct Lisp_Marker *zv = narrowing_lock_get_bound (buf, false, false);
+  struct Lisp_Marker *begv
+    = labeled_restrictions_get_bound (buf, true, false);
+  struct Lisp_Marker *zv
+    = labeled_restrictions_get_bound (buf, false, false);
   if (begv != NULL && zv != NULL)
     {
       SET_BUF_BEGV_BOTH (XBUFFER (buf), begv->charpos, begv->bytepos);
       SET_BUF_ZV_BOTH (XBUFFER (buf), zv->charpos, zv->bytepos);
     }
   else
-    narrowing_locks_remove (buf);
+    labeled_restrictions_remove (buf);
 }
 
-/* Restore the narrowing bounds that were set by the user, and restore
-   the bounds of the locked narrowing upon return.
+/* Restore the restriction bounds that were current when the first
+   labeled restriction was entered, and restore the bounds of the
+   innermost labeled restriction upon return.
    In particular, this function is called when redisplay starts, so
    that if a Lisp function executed during redisplay calls (redisplay)
-   while a locked narrowing is in effect, the locked narrowing will
-   not be visible on display.
+   while labeled restrictions are in effect, these restrictions will
+   not become visible on display.
    See https://debbugs.gnu.org/cgi/bugreport.cgi?bug=57207#140 and
    https://debbugs.gnu.org/cgi/bugreport.cgi?bug=57207#254 for example
    recipes that demonstrate why this is necessary.  */
 void
-reset_outermost_narrowings (void)
+reset_outermost_restrictions (void)
 {
   Lisp_Object val, buf;
-  for (val = narrowing_locks; CONSP (val); val = XCDR (val))
+  for (val = labeled_restrictions; CONSP (val); val = XCDR (val))
     {
       buf = XCAR (XCAR (val));
       eassert (BUFFERP (buf));
-      struct Lisp_Marker *begv = narrowing_lock_get_bound (buf, true, true);
-      struct Lisp_Marker *zv = narrowing_lock_get_bound (buf, false, true);
+      struct Lisp_Marker *begv
+	= labeled_restrictions_get_bound (buf, true, true);
+      struct Lisp_Marker *zv
+	= labeled_restrictions_get_bound (buf, false, true);
       if (begv != NULL && zv != NULL)
 	{
 	  SET_BUF_BEGV_BOTH (XBUFFER (buf), begv->charpos, begv->bytepos);
 	  SET_BUF_ZV_BOTH (XBUFFER (buf), zv->charpos, zv->bytepos);
-	  record_unwind_protect (unwind_reset_outermost_narrowing, buf);
+	  record_unwind_protect (unwind_reset_outermost_restriction, buf);
 	}
       else
-	narrowing_locks_remove (buf);
+	labeled_restrictions_remove (buf);
     }
 }
 
-/* Helper functions to save and restore the narrowing locks of the
-   current buffer in Fsave_restriction.  */
+/* Helper functions to save and restore the labeled restrictions of
+   the current buffer in Fsave_restriction.  */
 static Lisp_Object
-narrowing_locks_save (void)
+labeled_restrictions_save (void)
 {
   Lisp_Object buf = Fcurrent_buffer ();
-  Lisp_Object locks = assq_no_quit (buf, narrowing_locks);
-  if (!NILP (locks))
-    locks = XCAR (XCDR (locks));
-  return Fcons (buf, Fcopy_sequence (locks));
+  Lisp_Object restrictions = assq_no_quit (buf, labeled_restrictions);
+  if (! NILP (restrictions))
+    restrictions = XCAR (XCDR (restrictions));
+  return Fcons (buf, Fcopy_sequence (restrictions));
 }
 
 static void
-narrowing_locks_restore (Lisp_Object buf_and_saved_locks)
+labeled_restrictions_restore (Lisp_Object buf_and_restrictions)
 {
-  Lisp_Object buf = XCAR (buf_and_saved_locks);
-  Lisp_Object saved_locks = XCDR (buf_and_saved_locks);
-  narrowing_locks_remove (buf);
-  if (!NILP (saved_locks))
-    narrowing_locks_add (buf, saved_locks);
+  Lisp_Object buf = XCAR (buf_and_restrictions);
+  Lisp_Object restrictions = XCDR (buf_and_restrictions);
+  labeled_restrictions_remove (buf);
+  if (! NILP (restrictions))
+    labeled_restrictions_add (buf, restrictions);
 }
 
 static void
-unwind_narrow_to_region_locked (Lisp_Object tag)
+unwind_labeled_narrow_to_region (Lisp_Object label)
 {
-  Finternal__unlock_narrowing (tag);
+  Finternal__unlabel_restriction (label);
   Fwiden ();
 }
 
-/* Narrow current_buffer to BEGV-ZV with a narrowing locked with TAG.  */
+/* Narrow current_buffer to BEGV-ZV with a restriction labeled with
+   LABEL.  */
 void
-narrow_to_region_locked (Lisp_Object begv, Lisp_Object zv, Lisp_Object tag)
+labeled_narrow_to_region (Lisp_Object begv, Lisp_Object zv,
+			  Lisp_Object label)
 {
   Fnarrow_to_region (begv, zv);
-  Finternal__lock_narrowing (tag);
+  Finternal__label_restriction (label);
   record_unwind_protect (restore_point_unwind, Fpoint_marker ());
-  record_unwind_protect (unwind_narrow_to_region_locked, tag);
+  record_unwind_protect (unwind_labeled_narrow_to_region, label);
 }
 
 DEFUN ("widen", Fwiden, Swiden, 0, 0, "",
@@ -2842,11 +2857,11 @@ To gain access to other portions of the buffer, use
 `without-restriction' with the same label.  */)
   (void)
 {
-  Fset (Qoutermost_narrowing, Qnil);
+  Fset (Qoutermost_restriction, Qnil);
   Lisp_Object buf = Fcurrent_buffer ();
-  Lisp_Object tag = narrowing_lock_peek_tag (buf);
+  Lisp_Object label = labeled_restrictions_peek_label (buf);
 
-  if (NILP (tag))
+  if (NILP (label))
     {
       if (BEG != BEGV || Z != ZV)
 	current_buffer->clip_changed = 1;
@@ -2856,19 +2871,21 @@ To gain access to other portions of the buffer, use
     }
   else
     {
-      struct Lisp_Marker *begv = narrowing_lock_get_bound (buf, true, false);
-      struct Lisp_Marker *zv = narrowing_lock_get_bound (buf, false, false);
+      struct Lisp_Marker *begv
+	= labeled_restrictions_get_bound (buf, true, false);
+      struct Lisp_Marker *zv
+	= labeled_restrictions_get_bound (buf, false, false);
       eassert (begv != NULL && zv != NULL);
       if (begv->charpos != BEGV || zv->charpos != ZV)
 	current_buffer->clip_changed = 1;
       SET_BUF_BEGV_BOTH (current_buffer, begv->charpos, begv->bytepos);
       SET_BUF_ZV_BOTH (current_buffer, zv->charpos, zv->bytepos);
-      /* If the only remaining bounds in narrowing_locks for
+      /* If the only remaining bounds in labeled_restrictions for
 	 current_buffer are the bounds that were set by the user, no
-	 locked narrowing is in effect in current_buffer anymore:
-	 remove it from the narrowing_locks alist.  */
-      if (EQ (tag, Qoutermost_narrowing))
-	narrowing_lock_pop (buf);
+	 labeled restriction is in effect in current_buffer anymore:
+	 remove it from the labeled_restrictions alist.  */
+      if (EQ (label, Qoutermost_restriction))
+	labeled_restrictions_pop (buf);
     }
   /* Changing the buffer bounds invalidates any recorded current column.  */
   invalidate_current_column ();
@@ -2905,13 +2922,15 @@ argument.  To gain access to other portions of the buffer, use
     args_out_of_range (start, end);
 
   Lisp_Object buf = Fcurrent_buffer ();
-  if (! NILP (narrowing_lock_peek_tag (buf)))
+  if (! NILP (labeled_restrictions_peek_label (buf)))
     {
-      struct Lisp_Marker *begv = narrowing_lock_get_bound (buf, true, false);
-      struct Lisp_Marker *zv = narrowing_lock_get_bound (buf, false, false);
+      /* Limit the start and end positions to those of the innermost
+	 labeled restriction.  */
+      struct Lisp_Marker *begv
+	= labeled_restrictions_get_bound (buf, true, false);
+      struct Lisp_Marker *zv
+	= labeled_restrictions_get_bound (buf, false, false);
       eassert (begv != NULL && zv != NULL);
-      /* Limit the start and end positions to those of the locked
-	 narrowing.  */
       if (s < begv->charpos) s = begv->charpos;
       if (s > zv->charpos) s = zv->charpos;
       if (e < begv->charpos) e = begv->charpos;
@@ -2919,11 +2938,11 @@ argument.  To gain access to other portions of the buffer, use
     }
 
   /* Record the accessible range of the buffer when narrow-to-region
-     is called, that is, before applying the narrowing.  It is used
-     only by internal--lock-narrowing.  */
-  Fset (Qoutermost_narrowing, list3 (Qoutermost_narrowing,
-				     Fpoint_min_marker (),
-				     Fpoint_max_marker ()));
+     is called, that is, before applying the narrowing.  That
+     information is used only by internal--label-restriction.  */
+  Fset (Qoutermost_restriction, list3 (Qoutermost_restriction,
+				       Fpoint_min_marker (),
+				       Fpoint_max_marker ()));
 
   if (BEGV != s || ZV != e)
     current_buffer->clip_changed = 1;
@@ -2940,38 +2959,38 @@ argument.  To gain access to other portions of the buffer, use
   return Qnil;
 }
 
-DEFUN ("internal--lock-narrowing", Finternal__lock_narrowing,
-       Sinternal__lock_narrowing, 1, 1, 0,
-       doc: /* Lock the current narrowing with LABEL.
+DEFUN ("internal--label-restriction", Finternal__label_restriction,
+       Sinternal__label_restriction, 1, 1, 0,
+       doc: /* Label the current restriction with LABEL.
 
 This is an internal function used by `with-restriction'.  */)
-  (Lisp_Object tag)
+  (Lisp_Object label)
 {
   Lisp_Object buf = Fcurrent_buffer ();
-  Lisp_Object outermost_narrowing
-    = buffer_local_value (Qoutermost_narrowing, buf);
-  /* If internal--lock-narrowing is ever called without being preceded
-     by narrow-to-region, do nothing.  */
-  if (NILP (outermost_narrowing))
+  Lisp_Object outermost_restriction
+    = buffer_local_value (Qoutermost_restriction, buf);
+  /* If internal--label-restriction is ever called without being
+     preceded by narrow-to-region, do nothing.  */
+  if (NILP (outermost_restriction))
     return Qnil;
-  if (NILP (narrowing_lock_peek_tag (buf)))
-    narrowing_lock_push (buf, outermost_narrowing);
-  narrowing_lock_push (buf, list3 (tag,
-				   Fpoint_min_marker (),
-				   Fpoint_max_marker ()));
+  if (NILP (labeled_restrictions_peek_label (buf)))
+    labeled_restrictions_push (buf, outermost_restriction);
+  labeled_restrictions_push (buf, list3 (label,
+					 Fpoint_min_marker (),
+					 Fpoint_max_marker ()));
   return Qnil;
 }
 
-DEFUN ("internal--unlock-narrowing", Finternal__unlock_narrowing,
-       Sinternal__unlock_narrowing, 1, 1, 0,
-       doc: /* Unlock a narrowing locked with LABEL.
+DEFUN ("internal--unlabel-restriction", Finternal__unlabel_restriction,
+       Sinternal__unlabel_restriction, 1, 1, 0,
+       doc: /* If the current restriction is labeled with LABEL, remove its label.
 
 This is an internal function used by `without-restriction'.  */)
-  (Lisp_Object tag)
+  (Lisp_Object label)
 {
   Lisp_Object buf = Fcurrent_buffer ();
-  if (EQ (narrowing_lock_peek_tag (buf), tag))
-    narrowing_lock_pop (buf);
+  if (EQ (labeled_restrictions_peek_label (buf), label))
+    labeled_restrictions_pop (buf);
   return Qnil;
 }
 
@@ -3071,15 +3090,15 @@ save_restriction_restore_1 (Lisp_Object data)
 Lisp_Object
 save_restriction_save (void)
 {
-  Lisp_Object restr = save_restriction_save_1 ();
-  Lisp_Object locks = narrowing_locks_save ();
-  return Fcons (restr, locks);
+  Lisp_Object restriction = save_restriction_save_1 ();
+  Lisp_Object labeled_restrictions = labeled_restrictions_save ();
+  return Fcons (restriction, labeled_restrictions);
 }
 
 void
 save_restriction_restore (Lisp_Object data)
 {
-  narrowing_locks_restore (XCDR (data));
+  labeled_restrictions_restore (XCDR (data));
   save_restriction_restore_1 (XCAR (data));
 }
 
@@ -4748,7 +4767,7 @@ syms_of_editfns (void)
   DEFSYM (Qwall, "wall");
   DEFSYM (Qpropertize, "propertize");
 
-  staticpro (&narrowing_locks);
+  staticpro (&labeled_restrictions);
 
   DEFVAR_LISP ("inhibit-field-text-motion", Vinhibit_field_text_motion,
 	       doc: /* Non-nil means text motion commands don't notice fields.  */);
@@ -4809,12 +4828,12 @@ This variable is experimental; email 32252@debbugs.gnu.org if you need
 it to be non-nil.  */);
   binary_as_unsigned = false;
 
-  DEFVAR_LISP ("outermost-narrowing", Voutermost_narrowing,
+  DEFVAR_LISP ("outermost-restriction", Voutermost_restriction,
 	       doc: /* Outermost narrowing bounds, if any.  Internal use only.  */);
-  Voutermost_narrowing = Qnil;
-  Fmake_variable_buffer_local (Qoutermost_narrowing);
-  DEFSYM (Qoutermost_narrowing, "outermost-narrowing");
-  Funintern (Qoutermost_narrowing, Qnil);
+  Voutermost_restriction = Qnil;
+  Fmake_variable_buffer_local (Qoutermost_restriction);
+  DEFSYM (Qoutermost_restriction, "outermost-restriction");
+  Funintern (Qoutermost_restriction, Qnil);
 
   defsubr (&Spropertize);
   defsubr (&Schar_equal);
@@ -4907,8 +4926,8 @@ it to be non-nil.  */);
   defsubr (&Sdelete_and_extract_region);
   defsubr (&Swiden);
   defsubr (&Snarrow_to_region);
-  defsubr (&Sinternal__lock_narrowing);
-  defsubr (&Sinternal__unlock_narrowing);
+  defsubr (&Sinternal__label_restriction);
+  defsubr (&Sinternal__unlabel_restriction);
   defsubr (&Ssave_restriction);
   defsubr (&Stranspose_regions);
 }
diff --git a/src/keyboard.c b/src/keyboard.c
index f7aa496bb81..b1ccf4acde4 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -318,6 +318,8 @@ static Lisp_Object command_loop (void);
 static void echo_now (void);
 static ptrdiff_t echo_length (void);
 
+static void safe_run_hooks_maybe_narrowed (Lisp_Object, struct window *);
+
 /* Incremented whenever a timer is run.  */
 unsigned timers_run;
 
@@ -1909,7 +1911,7 @@ safe_run_hooks (Lisp_Object hook)
   unbind_to (count, Qnil);
 }
 
-void
+static void
 safe_run_hooks_maybe_narrowed (Lisp_Object hook, struct window *w)
 {
   specpdl_ref count = SPECPDL_INDEX ();
@@ -1919,11 +1921,11 @@ safe_run_hooks_maybe_narrowed (Lisp_Object hook, struct window *w)
   if (current_buffer->long_line_optimizations_p
       && long_line_optimizations_region_size > 0)
     {
-      ptrdiff_t begv = get_locked_narrowing_begv (PT);
-      ptrdiff_t zv = get_locked_narrowing_zv (PT);
+      ptrdiff_t begv = get_large_narrowing_begv (PT);
+      ptrdiff_t zv = get_large_narrowing_zv (PT);
       if (begv != BEG || zv != Z)
-	narrow_to_region_locked (make_fixnum (begv), make_fixnum (zv),
-				 Qlong_line_optimizations_in_command_hooks);
+	labeled_narrow_to_region (make_fixnum (begv), make_fixnum (zv),
+				  Qlong_line_optimizations_in_command_hooks);
     }
 
   run_hook_with_args (2, ((Lisp_Object []) {hook, hook}),
diff --git a/src/lisp.h b/src/lisp.h
index 1276285e2f2..a3d06c3b45d 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -4687,8 +4687,8 @@ extern void save_restriction_restore (Lisp_Object);
 extern Lisp_Object make_buffer_string (ptrdiff_t, ptrdiff_t, bool);
 extern Lisp_Object make_buffer_string_both (ptrdiff_t, ptrdiff_t, ptrdiff_t,
 					    ptrdiff_t, bool);
-extern void narrow_to_region_locked (Lisp_Object, Lisp_Object, Lisp_Object);
-extern void reset_outermost_narrowings (void);
+extern void labeled_narrow_to_region (Lisp_Object, Lisp_Object, Lisp_Object);
+extern void reset_outermost_restrictions (void);
 extern void init_editfns (void);
 extern void syms_of_editfns (void);
 
@@ -4857,7 +4857,6 @@ extern bool detect_input_pending (void);
 extern bool detect_input_pending_ignore_squeezables (void);
 extern bool detect_input_pending_run_timers (bool);
 extern void safe_run_hooks (Lisp_Object);
-extern void safe_run_hooks_maybe_narrowed (Lisp_Object, struct window *);
 extern void safe_run_hooks_2 (Lisp_Object, Lisp_Object, Lisp_Object);
 extern void cmd_error_internal (Lisp_Object, const char *);
 extern Lisp_Object command_loop_2 (Lisp_Object);
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.  */