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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 18:42:32 +0000
User-agent: Mutt/1.7.2 (2016-11-26)

On Thu, Feb 08, 2018 at 17:25:58 +0000, Alan Mackenzie wrote:
> Hello, Bastian.

[ .... ]

> 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).

Actually, that cond arm with the `c-go-up-list-backward' was merely an
"optimisation".  :-(  If I simply remove it, the code goes ~30 times
faster, without triggering any successes in the test suite.

Would you please try out the following patch on real code, and either
confirm to me that it fixes the bug, or tell me what it breaks.  Thanks!

diff -r e8b2c6141f97 cc-engine.el
--- a/cc-engine.el      Fri Feb 02 20:34:15 2018 +0000
+++ b/cc-engine.el      Thu Feb 08 18:39:40 2018 +0000
@@ -10547,13 +10547,6 @@
                      (looking-at c-return-key))
                 (setq braceassignp t)
-               ((and c-has-compound-literals
-                     (eq (char-after) ?,))
-                (save-excursion
-                  (when (and (c-go-up-list-backward nil lim)
-                             (eq (char-after) ?\())
-                    (setq braceassignp t)
-                    nil)))
                ((eq (char-after) ?=)
                 ;; We've seen a =, but must check earlier tokens so
                 ;; that it isn't something that should be ignored.

[ .... ]

> > Cheers
> > Bastian

Alan Mackenzie (Nuremberg, Germany).

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