bug#57266: Maintaining the base_line_number cache

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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Tags: patch

Based on the recent discussion around counting lines, I proposed the
patch below.  Part of it aims to just document the way in which
the `w->base_line_number` cache is maintained, but it goes further,
fixing the problem with format-mode-line and narrowing I discovered
and simplifying some of the code based on the new understanding of how
the cache is supposed to work.


        Stefan


2022-08-17  Stefan Monnier  <monnier@iro.umontreal.ca>

    * src/xdisp.c (BASE_LINE_NUMBER_VALID_P): New macro.
    (try_scrolling): Use it.  Ignore `just_this_one_p` because it was
    too pessimistic.
    (redisplay_window): Remove `buffer_unchanged_p`, not used any more.
    Check BASE_LINE_NUMBER_VALID_P once and for all at the beginning and
    don't rely on other side-information that is only correlated with
    BASE_LINE_NUMBER_VALID_P to decide when to flush this cache.
    (decode_mode_spec): Check `BASE_LINE_NUMBER_VALID_P` before using
    the cache.

    * src/window.c (set_window_buffer): Flush the `base_line_number` cache.


In GNU Emacs 29.0.50 (build 1, i686-pc-linux-gnu, GTK+ Version 3.24.34, cairo version 1.16.0)
 of 2022-08-15 built on lechazo
Repository revision: 5564f72d5b3e2f162f52d935c077f6ea15fb60b7
Repository branch: work
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux bookworm/sid

Configured using:
 'configure -C --enable-checking --enable-check-lisp-object-type --with-modules --with-cairo --with-tiff=ifavailable
 'CFLAGS=-Wall -g3 -Og -Wno-pointer-sign'
 PKG_CONFIG_PATH=/home/monnier/lib/pkgconfig'


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diff --git a/src/window.c b/src/window.c
index c8fcb3a607f..436d4e342d7 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4093,6 +4093,8 @@ set_window_buffer (Lisp_Object window, Lisp_Object buffer,
 			     buffer);
       w->start_at_line_beg = false;
       w->force_start = false;
+      /* Flush the base_line cache since it applied to another buffer.  */
+      w->base_line_number = 0;
     }
 
   wset_redisplay (w);
diff --git a/src/xdisp.c b/src/xdisp.c
index 03c43be5bc0..ec7cc904e78 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -18341,6 +18341,14 @@ cursor_row_fully_visible_p (struct window *w, bool force_p,
    `scroll-conservatively' and the Emacs manual.  */
 #define SCROLL_LIMIT 100
 
+/* The freshness of the w->base_line_number cache is only ensured at every
+   redisplay cycle, so the cache can be used only if there's been
+   no relevant changes to the buffer since the last redisplay.  */
+#define BASE_LINE_NUMBER_VALID_P(w)                      \
+   (eassert (current_buffer == XBUFFER ((w)->contents)), \
+    !current_buffer->clip_changed                        \
+    && BEG_UNCHANGED < (w)->base_line_number)
+
 static int
 try_scrolling (Lisp_Object window, bool just_this_one_p,
 	       intmax_t arg_scroll_conservatively, intmax_t scroll_step,
@@ -18640,9 +18648,10 @@ try_scrolling (Lisp_Object window, bool just_this_one_p,
   else
     {
       /* Maybe forget recorded base line for line number display.  */
-      if (!just_this_one_p
-	  || current_buffer->clip_changed
-	  || BEG_UNCHANGED < CHARPOS (startp))
+      /* FIXME: Why do we need this?  `try_scrolling` can only be called from
+         `redisplay_window` which should have flushed this cache already when
+         eeded.  */
+      if (!BASE_LINE_NUMBER_VALID_P (w))
 	w->base_line_number = 0;
 
       /* If cursor ends up on a partially visible line,
@@ -19404,9 +19413,6 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
   /* Record it now because it's overwritten.  */
   bool current_matrix_up_to_date_p = false;
   bool used_current_matrix_p = false;
-  /* This is less strict than current_matrix_up_to_date_p.
-     It indicates that the buffer contents and narrowing are unchanged.  */
-  bool buffer_unchanged_p = false;
   bool temp_scroll_step = false;
   specpdl_ref count = SPECPDL_INDEX ();
   int rc;
@@ -19505,11 +19511,6 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
 
   specbind (Qinhibit_point_motion_hooks, Qt);
 
-  buffer_unchanged_p
-    = (w->window_end_valid
-       && !current_buffer->clip_changed
-       && !window_outdated (w));
-
   /* When windows_or_buffers_changed is non-zero, we can't rely
      on the window end being valid, so set it to zero there.  */
   if (windows_or_buffers_changed)
@@ -19648,6 +19649,10 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
 	}
     }
 
+  if (!BASE_LINE_NUMBER_VALID_P (w))
+    /* Forget any recorded base line for line number display.  */
+    w->base_line_number = 0;
+
  force_start:
 
   /* Handle case where place to start displaying has been specified,
@@ -19668,10 +19673,6 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
       w->preserve_vscroll_p = false;
       w->window_end_valid = false;
 
-      /* Forget any recorded base line for line number display.  */
-      if (!buffer_unchanged_p)
-	w->base_line_number = 0;
-
       /* Redisplay the mode line.  Select the buffer properly for that.
 	 Also, run the hook window-scroll-functions
 	 because we have scrolled.  */
@@ -20000,12 +20001,6 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
 
       if (w->cursor.vpos >= 0)
 	{
-	  if (!just_this_one_p
-	      || current_buffer->clip_changed
-	      || BEG_UNCHANGED < CHARPOS (startp))
-	    /* Forget any recorded base line for line number display.  */
-	    w->base_line_number = 0;
-
 	  if (!cursor_row_fully_visible_p (w, true, false, false))
 	    {
 	      clear_glyph_matrix (w->desired_matrix);
@@ -20076,10 +20071,6 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
   debug_method_add (w, "recenter");
 #endif
 
-  /* Forget any previously recorded base line for line number display.  */
-  if (!buffer_unchanged_p)
-    w->base_line_number = 0;
-
   /* Determine the window start relative to point.  */
   init_iterator (&it, w, PT, PT_BYTE, NULL, DEFAULT_FACE_ID);
   it.current_y = it.last_visible_y;
@@ -24211,6 +24202,13 @@ maybe_produce_line_number (struct it *it)
       if (!last_line)
 	{
 	  /* If possible, reuse data cached by line-number-mode.  */
+	  /* NOTE: We use `base_line_number` without checking
+	     BASE_LINE_NUMBER_VALID_P because we assume that `redisplay_window`
+	     has already flushed this cache for us when needed.
+	     NOTE²: IIUC checking BASE_LINE_NUMBER_VALID_P here would be
+	     overly pessimistic because it might say that the cache
+	     was invalid before entering `redisplay_window` yet the
+	     value has just been refreshed.  */
 	  if (it->w->base_line_number > 0
 	      && it->w->base_line_pos > 0
 	      && it->w->base_line_pos <= IT_CHARPOS (*it)
@@ -27949,6 +27947,17 @@ decode_mode_spec (struct window *w, register int c, int field_width,
 	startpos = marker_position (w->start);
 	startpos_byte = marker_byte_position (w->start);
 	height = WINDOW_TOTAL_LINES (w);
+
+ 	if (!BASE_LINE_NUMBER_VALID_P (w))
+ 	  /* NOTE: This check is necessary when we're called from
+ 	     `format-mode-line` but not when we're called from
+ 	     `redisplay_window`.
+ 	     FIXME: It's usually harmless in that case, except if you have
+ 	     several `%l` in your modeline, in which case it will cause the
+ 	     cache to to be recomputed as many times when it's out of
+ 	     date :-(  */
+ 	  w->base_line_number = 0;
+
 	/* We cannot cope with w->start being outside of the
 	   accessible portion of the buffer; in particular,
 	   display_count_lines call below might infloop if called with
@@ -27962,8 +27971,6 @@ decode_mode_spec (struct window *w, register int c, int field_width,
 	  {
 	    startpos = BUF_BEGV (b);
 	    startpos_byte = BUF_BEGV_BYTE (b);
-	    w->base_line_pos = 0;
-	    w->base_line_number = 0;
 	  }
 
 	/* If we decided that this buffer isn't suitable for line numbers,
@@ -28044,6 +28051,12 @@ decode_mode_spec (struct window *w, register int c, int field_width,
 		goto no_value;
 	      }
 
+	    /* NOTE: if current_buffer->clip_changed is set or
+               if BEG_UNCHANGED is < position, this new cached value
+               may be unusable, because we can't reset BEG_UNCHANGED
+               or `clip_changed` from here (since they reflect the
+               changes since the last redisplay do they can only be reset
+               from `mark_window_display_accurate_1`).  */
 	    w->base_line_number = topline - nlines;
 	    w->base_line_pos = BYTE_TO_CHAR (position);
 	  }