bug#12463: 24.2; pos-visible-in-window-p gets slower over time

[Jörg Walter]

Emacs gets arbitrarily slow over time. I was able to narrow down the
problem to `pos-visible-in-window-p', which is, unfortunately, called as
part of lots of common commands. That function call is getting slower
each time it is called, if two conditions are met.

The first condition I was able to pin is the value of
`header-line-format'. The bug only occurs when it includes an image (as
is common when using tabbar.el).

It also depends on buffer contents, although I was not able to determine
a minimal condition. It happens with the fancy splash screen, but not
with the scratch buffer. I've tried inserting an image and using face
`variable-pitch', but those two aren't enough to trigger the bug.

This code sample demonstrates the bug. Run it in an "emacs -Q" instance
via `eval-buffer' and be amazed at the unbounded (linear) growth of
execution time for each iteration:

(defun bug ()
  "trigger bug related to pos-visible-in-window-p"
  (interactive)
  (benchmark 1000 '(pos-visible-in-window-p t)))

(fancy-startup-screen)
(setq header-line-format '((#("x" 0 1 (display (image :type pbm :data "P2 1 1 
255\n"))))))
(goto-char (point-min))
(while t (bug))



In GNU Emacs 24.2.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.10)
 of 2012-09-17 on queen
Windowing system distributor `The X.Org Foundation', version 11.0.11103000
Configured using:
 `configure '--with-gif=no''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Emacs-Lisp

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
C-x C-f ~ / . e m a c s . d / b u . <backspace> g . 
e l <return> M-x e v a l - b u f f e r <return> C-x 
k <return> M-x r e p o r t - e m <tab> <return>

Recent messages:
Invalid pixel value in image `(image :type pbm :data P2 1 1 255
)'
QuitInvalid pixel value in image `(image :type pbm :data P2 1 1 255
)'
Invalid pixel value in image `(image :type pbm :data P2 1 1 255
)'
Invalid pixel value in image `(image :type pbm :data P2 1 1 255
)'
Invalid pixel value in image `(image :type pbm :data P2 1 1 255
)'

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
easymenu mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail regexp-opt rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils benchmark time-date tooltip
ediff-hook vc-hooks lisp-float-type mwheel x-win x-dnd tool-bar dnd
fontset image fringe lisp-mode register page menu-bar rfn-eshadow timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
frame cham georgian utf-8-lang misc-lang vietnamese tibetan thai
tai-viet lao korean japanese hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese case-table epa-hook jka-cmpr-hook help
simple abbrev minibuffer loaddefs button faces cus-face files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote make-network-process
dbusbind dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty emacs)

-- 
CU
  Jörg