Speed up display of line numbers for very large buffers

* src/xdisp.c (maybe_produce_line_number): Speed up line counting
using values cached by mode-line display of line numbers.
(Bug#27622)

diff --git a/src/xdisp.c b/src/xdisp.c
index 5df2dd183118d54385ae5daa4397e494fd872e7c..28ed768523626ff63ff333845012ba953855250b 100644 (file)
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -20823,7 +20823,6 @@ maybe_produce_line_number (struct it *it)
   ptrdiff_t start_from, bytepos;
   ptrdiff_t this_line;
   bool first_time = false;
-  ptrdiff_t beg = display_line_numbers_widen ? BEG : BEGV;
   ptrdiff_t beg_byte = display_line_numbers_widen ? BEG_BYTE : BEGV_BYTE;
   ptrdiff_t z_byte = display_line_numbers_widen ? Z_BYTE : ZV_BYTE;
   void *itdata = bidi_shelve_cache ();
@@ -20834,8 +20833,21 @@ maybe_produce_line_number (struct it *it)
     {
       if (!last_line)
        {
-         /* FIXME: Maybe reuse the data in it->w->base_line_number.  */
-         start_from = beg;
+         /* If possible, reuse data cached by line-number-mode.  */
+         if (it->w->base_line_number > 0
+             && it->w->base_line_pos > 0
+             && it->w->base_line_pos <= IT_CHARPOS (*it)
+             /* line-number-mode always displays narrowed line
+                numbers, so we cannot use its data if the user wants
+                line numbers that disregard narrowing.  */
+             && !(display_line_numbers_widen
+                  && (BEG_BYTE != BEGV_BYTE || Z_BYTE != ZV_BYTE)))
+           {
+             start_from = CHAR_TO_BYTE (it->w->base_line_pos);
+             last_line = it->w->base_line_number - 1;
+           }
+         else
+           start_from = beg_byte;
          if (!it->lnum_bytepos)
            first_time = true;
        }
@@ -20845,7 +20857,7 @@ maybe_produce_line_number (struct it *it)
       /* Paranoia: what if someone changes the narrowing since the
         last time display_line was called?  Shouldn't really happen,
         but who knows what some crazy Lisp invoked by :eval could do?  */
-      if (!(beg_byte <= start_from && start_from < z_byte))
+      if (!(beg_byte <= start_from && start_from <= z_byte))
        {
          last_line = 0;
          start_from = beg_byte;