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Re: address@hidden: Emacs very slow opening file]

From: Sascha Wilde
Subject: Re: address@hidden: Emacs very slow opening file]
Date: Thu, 29 Sep 2005 11:51:57 +0200
User-agent: Mutt/1.5.9i

On Wed, Sep 28, 2005 at 01:11:10PM -0400, Richard M. Stallman wrote:
> Would someone please investigate this bug report,
> and then ack?
> I tried opening a debian 'patch' file in Emacs. The Emacs process
> appeared to hang, using all my CPU. It wouldn't respond to C-g.
> After chopping down my .emacs to see what triggered the problem, it
> turns out that just using a minimal .emacs file like this triggers it:
> ;; -------
> (custom-set-variables '(global-font-lock-mode t nil (font-lock)))
> (find-file "bigfile")
> ;; -------
> "bigfile" is the patch file, which I've cut down to a reasonable size
> (1.3 Mb), and uploaded here:
> http://s89213869.onlinehome.us/bigfile
> It takes the latest CVS Emacs on my 2.2GHz P4 PC 15 seconds to open that
> file.

I can confirm, that it takes more than 10 seconds to open the given
file, using the latest emacs from cvs.

After short investigation I found the following:
What takes so long is a combination of c-mode fontification and
interpretation of file-local variables.

What happens is that: 

- running through the file emacs finds the last valid "Local
  Variables:" block starting on line 42634,

- in the first line of this block the mode gets switched to c-mode.

- After that emacs finds an "eval:" in the next line of the block, and
  wants to ask the user whether to process it, 

- so the part of the file with the eval: variable in question is
  displayed -- this is starting with line 42635(!)

- fontification is enabled, and the c-mode fontification rules seem to 
  do lots of parsing on the whole file preceding the displayed part
  _that_ is what takes so long.

To verify my theory, simply change line 42635 from
-/* mode:c */
-/* mode:diff */
(or fundamental or lisp or simply anything, which does less expensive

So, I would conclude, that there is no bug at all.  Emacs can't
possibly know, that the "Local Variables:" block isn't meant to be
interpreted in the diff file.  

And I wouldn't consider the fact, that c-mode fontification takes so
long on a big file, which isn't valid c-code at all, a bug.

Workaround: turn of global-font-lock-mode or set
enable-local-variables to nil, before opening a file like that.

Sascha Wilde
"C++ : an octopus made by nailing extra legs onto a dog"

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