Improvement for long lines in buffers with font locking.

* src/xdisp.c (get_narrowed_width): Factored out from 'get_narrowed_len'.
(get_narrowed_len): Updated to use 'get_narrowed_width'.
(get_closer_narrowed_begv): New function.
(SET_WITH_NARROWED_BEGV): Add parameter to the macro.
(back_to_previous_line_start): Use the new function.
(get_visually_first_element, move_it_vertically_backward): Update the
calls to the macro.

* src/dispextern.h: Prototype of 'get_closer_narrowed_begv'.
Fix the prototypes of 'get_narrowed_begv' and 'get_narrowed_zv'.

2 files changed, 30 insertions, 12 deletions

diff --git a/src/dispextern.h b/src/dispextern.h
index bafa98161d0..2772e8cda8d 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -3404,8 +3404,9 @@ 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 *w);
-ptrdiff_t get_narrowed_zv (struct window *w);
+ptrdiff_t get_narrowed_begv (struct window *);
+ptrdiff_t get_narrowed_zv (struct window *);
+ptrdiff_t get_closer_narrowed_begv (struct window *, 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/xdisp.c b/src/xdisp.c
index add14a084fc..bd3beef134f 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -3502,14 +3502,20 @@ init_iterator (struct it *it, struct window *w,
    long lines.  */
 
 static int
-get_narrowed_len (struct window *w)
+get_narrowed_width (struct window *w)
 {
   int fact;
   /* In a character-only terminal, only one font size is used, so we
      can use a smaller factor.  */
   fact = EQ (Fterminal_live_p (Qnil), Qt) ? 2 : 3;
-  return fact * (window_body_width (w, WINDOW_BODY_IN_CANONICAL_CHARS) *
-                 window_body_height (w, WINDOW_BODY_IN_CANONICAL_CHARS));
+  return fact * window_body_width (w, WINDOW_BODY_IN_CANONICAL_CHARS);
+}
+
+static int
+get_narrowed_len (struct window *w)
+{
+  return get_narrowed_width (w) *
+    window_body_height (w, WINDOW_BODY_IN_CANONICAL_CHARS);
 }
 
 ptrdiff_t
@@ -3528,6 +3534,13 @@ get_narrowed_zv (struct window *w)
   return min ((window_point (w) / len + 1) * len, ZV);
 }
 
+ptrdiff_t
+get_closer_narrowed_begv (struct window *w, ptrdiff_t pos)
+{
+  int len = get_narrowed_width (w);
+  return max ((pos / len - 1) * len, BEGV);
+}
+
 static void
 unwind_narrowed_begv (Lisp_Object point_min)
 {
@@ -3541,15 +3554,15 @@ unwind_narrowed_zv (Lisp_Object point_max)
 }
 
 /* Set DST to EXPR.  When IT indicates that BEGV should temporarily be
-   updated to optimize display, evaluate EXPR with an updated BEGV.  */
+   updated to optimize display, evaluate EXPR with BEGV set to BV.  */
 
-#define SET_WITH_NARROWED_BEGV(IT,DST,EXPR)                             \
+#define SET_WITH_NARROWED_BEGV(IT,DST,EXPR,BV)                          \
   do {                                                                  \
     if (IT->narrowed_begv)                                              \
       {                                                                 \
         specpdl_ref count = SPECPDL_INDEX ();                           \
         record_unwind_protect (unwind_narrowed_begv, Fpoint_min ());    \
-        SET_BUF_BEGV (current_buffer, IT->narrowed_begv);               \
+        SET_BUF_BEGV (current_buffer, BV);                              \
         DST = EXPR;                                                     \
         unbind_to (count, Qnil);                                        \
       }                                                                 \
@@ -7067,7 +7080,8 @@ 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)));
+                          find_newline_no_quit (cp, bp, -1, &IT_BYTEPOS (*it)),
+                          get_closer_narrowed_begv (it->w, IT_CHARPOS (*it)));
 }
 
 
@@ -8706,7 +8720,8 @@ get_visually_first_element (struct it *it)
 
   SET_WITH_NARROWED_BEGV (it, bob,
                           string_p ? 0 :
-                          IT_BYTEPOS (*it) < BEGV ? obegv : BEGV);
+                          IT_BYTEPOS (*it) < BEGV ? obegv : BEGV,
+                          it->narrowed_begv);
 
   if (STRINGP (it->string))
     {
@@ -8749,7 +8764,8 @@ get_visually_first_element (struct it *it)
         SET_WITH_NARROWED_BEGV (it, it->bidi_it.charpos,
                                 find_newline_no_quit (IT_CHARPOS (*it),
                                                       IT_BYTEPOS (*it), -1,
-                                                      &it->bidi_it.bytepos));
+                                                      &it->bidi_it.bytepos),
+                                it->narrowed_begv);
       bidi_paragraph_init (it->paragraph_embedding, &it->bidi_it, true);
       do
         {
@@ -10668,7 +10684,8 @@ 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));
+                                  find_newline_no_quit (cp, bp, -1, NULL),
+                                  it->narrowed_begv);
           move_it_to (it, cp, -1, -1, -1, MOVE_TO_POS);
         }
       bidi_unshelve_cache (it3data, true);