Subject: bug#42490: Emacs is very slow when navigating into a specific C++ file

From: Olivier Scalbert <[email-masked]>
Date: Thu, 23 Jul 2020 09:45:18 +0200

Hi !

With Emacs 26.1 or 26.3 on my Xeon Debian box, I have some problems with 
the following C++ file: 
https://github.com/mamedev/mame/blob/master/src/devices/cpu/z80/z80.cpp 
(which is part of the Mame project).

When I open it and press several times the "page down" key, Emacs seems 
to hang. In fact 100% of the cpu is consumed during 10 to 15 seconds. 
Then it works well.

My Emacs config is nearly empty.

The C++ code is a little special as is contains lot of macros, but I can
open it and navigate into it without problem with vim or VSCode.

If I turn off syntax highlighting, it run smoother.

Thanks for the help !

Olivier

In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.5)
of 2019-09-23, modified by Debian built on x86-grnet-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux 10 (buster)

Recent messages:
Loading /etc/emacs/site-start.d/50gnugo.el (source)...done
Loading /etc/emacs/site-start.d/50latex-cjk-common.el (source)...
Loading /usr/share/emacs/site-lisp/latex-cjk-common/cjk-enc.el 
(source)...done
Loading /etc/emacs/site-start.d/50latex-cjk-common.el (source)...done
Loading /etc/emacs/site-start.d/50latex-cjk-thai.el (source)...done
Loading /etc/emacs/site-start.d/50openscad.el (source)...done
Loading /etc/emacs/site-start.d/50python-docutils.el (source)...done
Loading /etc/emacs/site-start.d/50texlive-lang-english.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list... [2 times]

Configured using:
'configure --build x86_64-linux-gnu --prefix=/usr
--sharedstatedir=/var/lib --libexecdir=/usr/lib
--localstatedir=/var/lib --infodir=/usr/share/info
--mandir=/usr/share/man --enable-libsystemd --with-pop=yes
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.1/site-lisp:/usr/share/emacs/site-lisp 

--with-sound=alsa --without-gconf --with-mailutils --build
x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
--libexecdir=/usr/lib --localstatedir=/var/lib
--infodir=/usr/share/info --mandir=/usr/share/man --enable-libsystemd
--with-pop=yes
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.1/site-lisp:/usr/share/emacs/site-lisp 

--with-sound=alsa --without-gconf --with-mailutils --with-x=yes
--with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
-fdebug-prefix-map=/build/emacs-StqULU/emacs-26.1+1=. 
-fstack-protector-strong
-Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
-D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 THREADS LIBSYSTEMD LCMS2

Important settings:
value of $LANG: en_US.UTF-8
locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
tooltip-mode: t
global-eldoc-mode: t
eldoc-mode: t
electric-indent-mode: t
mouse-wheel-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
column-number-mode: t
line-number-mode: t
transient-mark-mode: t

Load-path shadows:
/usr/share/emacs/site-lisp/rst hides 
/usr/share/emacs/26.1/lisp/textmodes/rst
/usr/share/emacs/site-lisp/latex-cjk-thai/thai-word hides 
/usr/share/emacs/26.1/lisp/language/thai-word

Features:
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format-spec rfc822 mml mml-sec epa derived epg gnus-util rmail
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mm-util mail-prsvr mail-utils elec-pair solarized-dark-theme solarized
color dash finder-inf info package easymenu epg-config url-handlers
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password-cache url-vars seq byte-opt gv bytecomp byte-compile cconv
cl-loaddefs cl-lib time-date mule-util tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow isearch timer select
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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 lcms2 dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 16 282823 12891)
(symbols 48 28411 2)
(miscs 40 50 76)
(strings 32 78429 1210)
(string-bytes 1 1842487)
(vectors 16 21905)
(vector-slots 8 611952 8270)
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(intervals 56 282 0)
(buffers 992 12))