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bug#30367: 26.0.91; CC-Mode: Major slowdown when isearching big C++ file

From: Alan Mackenzie
Subject: bug#30367: 26.0.91; CC-Mode: Major slowdown when isearching big C++ file.
Date: Thu, 8 Feb 2018 17:25:58 +0000
User-agent: Mutt/1.7.2 (2016-11-26)

Hello, Bastian.

On Wed, Feb 07, 2018 at 23:45:03 +0100, Bastian Beischer wrote:
> On Wed, Feb 7, 2018 at 11:26 PM, Alan Mackenzie <address@hidden> wrote:
> > Hello, Bastian.

[ .... ]

> >> then search for 'isinsha'.

> > Yes, searching for "isinsha", I see the slowdown.  Thanks!  I also see
> > c-looking-at-or-maybe-in-bracelist as taking an excessive amount of
> > processing.

> > So I can reproduce it!

> Great, so our results are consistent.

I think I understand what's happening, now.

c-looking-at-or-maybe-in-bracelist attempts to go back over the innards
of a brace list to the enclosing "{".  At L+122, in new handling for a
comma, I rather rashly inserted a `c-go-up-list-backward' to test whether
the comma is inside a brace.  This was in a loop, once for each sexp.

In your sample file there is, at L10,653 a construct at top level with
_lots_ of commas.  My loop is trying to go to the non-existent containing
brace for each of these commas, so ends up scanning to BOB for each of
them.  This entire activity will be being done several times.

I timed a single `c-go-up-list-backward' from one of these commas at
0.033 seconds.  30 of these take about a second, and my machine is
anything but slow.

So, I just need to work out a way of fixing this, which may take some
while (sorry).

Thanks again for reporting the bug.

[ .... ]

> Cheers
> Bastian

Alan Mackenzie (Nuremberg, Germany).

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