bug#23327: 25.0.92; show-trailing-whitespace uses too much cpu

[Mohammed Sadik <sadiq@sadiqpk.org>]

Enabling show-trailing-whitespace and navigating through buffer uses too
much CPU.  An increase of about 4-6% of CPU on my 2.4 GHz Quad-core
system.

How to reproduce:

1. Open a large source code file (I opened cc-engine.el from Emacs source).
2. Enable show-trailing-whitespace (setq show-trailing-whitespace t)
3. Press and hold the arrow keys to navigate. See the processor usage
   (use top/htop or similar program).
4. Now disable show-trailing-whitespace (setq show-trailing-whitespace
   nil)
5. Repeat 3

This also happens when pressing any other key (eg:while typing).

Also this creates minor glitches in displaying text on buffer
(sudden disappearance and appearence of some text) when CPU is already
enough busy.



In GNU Emacs 25.0.92.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.18.9)
 of 2016-04-20 built on fedora.localdomain
Windowing system distributor 'Fedora Project', version 11.0.11800000
Configured features:
XPM JPEG TIFF GIF PNG SOUND DBUS GSETTINGS NOTIFY ACL LIBSELINUX GNUTLS
LIBXML2 FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LC_MONETARY: en_IN.UTF-8
  value of $LC_NUMERIC: en_IN.UTF-8
  value of $LC_TIME: en_IN.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Emacs-Lisp

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-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 messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
t
Mark set [2 times]
nil
Making completion list...

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec epg epg-config gnus-util mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util help-fns help-mode easymenu cl-loaddefs pcase
cl-lib mail-prsvr mail-utils time-date mule-util tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list newcomment elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core frame cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese charscript case-table epa-hook jka-cmpr-hook help
simple abbrev minibuffer cl-preloaded nadvice loaddefs button faces
cus-face macroexp files text-properties overlay sha1 md5 base64 format
env code-pages mule custom widget hashtable-print-readable backquote
dbusbind inotify dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 89437 9618)
 (symbols 48 19612 0)
 (miscs 40 83 185)
 (strings 32 14290 4341)
 (string-bytes 1 410796)
 (vectors 16 11670)
 (vector-slots 8 421100 7008)
 (floats 8 166 119)
 (intervals 56 962 0)
 (buffers 976 13)
 (heap 1024 35720 1005))