bug#30735: 25.3; slow comment c++-mode

[ngeiswei@googlemail.com]

1. Open the following file (make sure you have write privilege) 

https://raw.githubusercontent.com/opencog/atomspace/master/tests/unify/
UnifyUTest.cxxtest

2. Enable c++-mode (M-x c++-mode)

3. Mark a large region of that file (say l.175 to the end)

4. Comment out that region (M-;)

5. Meditate in the church of Emacs for a few minutes

If c++-mode is disabled (using fundamental-mode for instance)
commenting
that same region is instantaneous. After running some profiling it
seems
most of the CPU resources is spent in c-syntactic-skip-backward.

The same slowness occurs with Emacs 25.1. I couldn't try older versions
due to temacs Segfault compiling issue (unresolved by disabling
randomize_va_space).


In GNU Emacs 25.3.1 (x86_64-redhat-linux-gnu, GTK+ Version 3.22.19)
 of 2017-09-15 built on buildvm-31.phx2.fedoraproject.org
Windowing system distributor 'Fedora Project', version 11.0.11906000
Configured using:
 'configure --build=x86_64-redhat-linux-gnu
 --host=x86_64-redhat-linux-gnu --program-prefix=
 --disable-dependency-tracking --prefix=/usr --exec-prefix=/usr
 --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc
 --datadir=/usr/share --includedir=/usr/include --libdir=/usr/lib64
 --libexecdir=/usr/libexec --localstatedir=/var
 --sharedstatedir=/var/lib --mandir=/usr/share/man
 --infodir=/usr/share/info --with-dbus --with-gif --with-jpeg --with-
png
 --with-rsvg --with-tiff --with-xft --with-xpm --with-x-toolkit=gtk3
 --with-gpm=no --with-xwidgets --with-modules
 build_alias=x86_64-redhat-linux-gnu host_alias=x86_64-redhat-linux-gnu
 'CFLAGS=-DMAIL_USE_LOCKF -O2 -g -pipe -Wall -Werror=format-security
 -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong
 --param=ssp-buffer-size=4 -grecord-gcc-switches
 -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -m64 -mtune=generic'
 LDFLAGS=-Wl,-z,relro
 PKG_CONFIG_PATH=:/usr/lib64/pkgconfig:/usr/share/pkgconfig'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND DBUS GCONF GSETTINGS
NOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 MODULES XWIDGETS

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

Major mode: C++/l

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
  abbrev-mode: t

Recent messages:
Saved text until ", ts_expected));
}

#undef al
#undef an
"
Mark set [2 times]
Saved text from "#include <opencog/util/Logger.h>

#inclu"

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec password-cache 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 mail-prsvr mail-utils
cl-extra help-mode cc-mode cc-fonts easymenu cc-guess cc-menus cc-cmds
cc-styles cc-align cc-engine cc-vars cc-defs cl-loaddefs pcase cl-lib
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 xwidget-internal move-toolbar
gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 136237 5145)
 (symbols 48 21890 0)
 (miscs 40 56 157)
 (strings 32 19783 4769)
 (string-bytes 1 778551)
 (vectors 16 14190)
 (vector-slots 8 458261 5239)
 (floats 8 167 43)
 (intervals 56 9646 10)
 (buffers 976 18))