Re: follow-mode: extremely slow in combination with org-mode

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 17 Jun 2018 11:13:33 +0000
> Cc: Gerald Wildgruber <wildgruber@tu-berlin.de>, emacs-devel@gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> On Sun, Jun 17, 2018 at 11:12:39 +0300, Eli Zaretskii wrote:
> > > From: Gerald Wildgruber <wildgruber@tu-berlin.de>
> > > CC: <emacs-devel@gnu.org>
> > > Date: Sun, 17 Jun 2018 08:57:22 +0200
> 
> > >           - follow-windows-start-end                            17606  57%
> > >            - if                                                 17606  57%
> > >             - let                                               17603  57%
> > >              - let                                              17603  57%
> > >               - while                                           17603  57%
> > >                - setq                                           17599  57%
> > >                 - cons                                          17599  57%
> > >                  - cons                                         17599  57%
> > >                   - cons                                        17599  57%
> > >                    - follow-calc-win-end                        17591  57%
> > >                     - let*                                      17591  57%
> > >                      - if                                        9229  30%
> > >                       + let                                      1115   3%
> > >                       + pos-visible-in-window-p                    26   0%
> > >                        posn-point                                8350  27%
> > >                      + window-inside-pixel-edges                   12   0%
> 
> > Alan, why doesn't follow-calc-win-end doesn't just use window-end with
> > last argument non-nil?  AFAU, that should allow you to compute the
> > result using 1/4 of the processing time you now invest.  Why do you
> > need all the rest in that function?  What am I missing?
> 
> I honestly don't know for sure.  But on 2014-04-27 I made a note in my
> personal log:
> 
>     "In follow-calc-win-end, we check for EOB being visible in the
>     window, rather than being the window-end.  There needs to be a
>     comment here about what the last element means."
> 
> .  I wish I could decipher what I meant, then.  I think I might have
> encountered a corner case, where EOB is _inside_ a window, possibly not
> even the last window, and using window-end failed for some reason.
> 
> But I think follow-calc-win-end was in the original follow.el written by
> Anders Lindgren.  Maybe, back in the mists of time, window-end didn't
> work for some reason - maybe it hadn't yet acquired it's UPDATE
> argument, or something like that.

OK, thanks for the explanations.

Gerald, could you try the following patch to follow-calc-win-end?
Please first see if this makes it significantly faster in your use
cases, and if so, please run with it for a while and see if there are
some adverse effects.  (If this is not faster, it isn't worth testing
it more.)

diff --git a/lisp/follow.el b/lisp/follow.el
index fd397c0..c99fc93 100644
--- a/lisp/follow.el
+++ b/lisp/follow.el
@@ -925,14 +925,17 @@ follow-calc-win-end
 the last fully-visible line in WIN.  END-OF-BUFFER is t when EOB
 is fully-visible in WIN.  If WIN is nil, the selected window is
 used."
-  (let* ((win (or win (selected-window)))
-	 (edges (window-inside-pixel-edges win))
-	 (ht (- (nth 3 edges) (nth 1 edges)))
-	 (last-line-pos (posn-point (posn-at-x-y 0 (1- ht) win))))
-    (if (pos-visible-in-window-p last-line-pos win)
-	(let ((end (window-end win t)))
-	  (list end (pos-visible-in-window-p (point-max) win)))
-      (list last-line-pos nil))))
+  (or win (setq win (selected-window)))
+  (with-selected-window win
+    (let* ((wend (window-end win t))
+           (endp (= wend (point-max))))
+      (and (> wend (point-min))
+           (setq wend (1- wend)))
+      (setq wend
+            (save-excursion
+              (goto-char wend)
+              (line-beginning-position)))
+      (list wend endp))))
 
 (defun follow-calc-win-start (windows pos win)
   "Determine the start of window WIN in a Follow mode window chain.