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)

1 files changed, 16 insertions, 4 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index 5df2dd18311..28ed7685236 100644
--- 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;