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

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Bastian Beischer <bastian.beischer <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 26.0.91; CC-Mode: Major slowdown when isearching big C++ file.
Date: Tue, 06 Feb 2018 11:34:43 +0100

After updating CC-Mode to the latest version in the standalone mercurial
repository I noticed that searching large C++ files has become a lot
slower than before (factor 20 or so). I traced the problem back to this
commit:

Author: Alan Mackenzie <bug-cc-mode <at> gnu.org>
Date:   Sun Jan 21 18:05:41 2018 +0000

    Handle C99 Compound Literals in return statements and argument lists.
    
    * cc-engine.el (c-looking-at-or-maybe-in-bracelist): Recognize a brace list
    when preceded by "return" or inside parentheses, either immediately after the
    "(" or following a comma.
    (c-looking-at-inexpr-block): Test c-has-compound-literals rather than hard
    coded C++ Mode.
    (c-guess-basic-syntax, CASE 7B): Test additionally for not being just inside a
    parenthesis or being at a Java "new" keyword.  CASE 9: Remove the simple
    minded test on the contents of a block to determine a brace list.
    
    * cc-langs.el (c-has-compound-literals): New lang const and lang var.
    
    * bracelist-30.{c,res}: New test files.

I am attaching the result of a run of the emacs profiler before
("fast.profile") and after the commit ("slow.profile"). In this run I
started emacs as:

emacs -Q /path/to/big/file.C -e "(add-to-list 'load-path ~/cc-mode)" -e "(require 'cc-mode)"
M-x profiler-start
C-s searchString
M-x profiler-report

And write the result to 'slow.profile' and 'fast.profile'
respectively. It appears that 93% of the time is spent in
c-looking-at-or-maybe-in-bracelist.

Thanks for looking into this issue!


In GNU Emacs 26.0.91 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.26)
 of 2018-02-06 built on beischer-w520
Repository revision: c787a4968273027960a20ced6d63bae0d1ffa87e
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
Recent messages:
Checking 105 files in /usr/share/emacs/26.0.91/lisp/obsolete...
Checking for load-path shadows...done
Auto-saving...done
Mark set
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Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
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 --with-xwidgets --without-compress-install 'CFLAGS=-march=native -O2
 -pipe -fstack-protector-strong -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

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/home/beischer/.emacs.d/elpa/org-20180205/ob-java hides /usr/share/emacs/26.0.91/lisp/org/ob-java
/home/beischer/.emacs.d/elpa/org-20180205/ob-stan hides /usr/share/emacs/26.0.91/lisp/org/ob-stan

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Message #8 received at 30367 <at> debbugs.gnu.org (full text, mbox):

From: Bastian Beischer <bastian.beischer <at> gmail.com>
To: 30367 <at> debbugs.gnu.org
Subject: Re: bug#30367: 26.0.91;
 CC-Mode: Major slowdown when isearching big C++ file.
Date: Tue, 06 Feb 2018 11:40:31 +0100

[Message part 1 (text/plain, inline)]

I forgot to attach the profile files. Here they are.

[fast.profile (application/octet-stream, attachment)]

[slow.profile (application/octet-stream, attachment)]

Message #11 received at 30367 <at> debbugs.gnu.org (full text, mbox):

From: Alan Mackenzie <acm <at> muc.de>
To: Bastian Beischer <bastian.beischer <at> gmail.com>
Cc: 30367 <at> debbugs.gnu.org
Subject: Re: bug#30367: 26.0.91;
 CC-Mode: Major slowdown when isearching big C++ file.
Date: 7 Feb 2018 15:24:35 -0000

Hello, Bastian.

In article <mailman.8662.1517913372.27995.bug-gnu-emacs <at> gnu.org> you wrote:

> After updating CC-Mode to the latest version in the standalone mercurial
> repository I noticed that searching large C++ files has become a lot
> slower than before (factor 20 or so).

Thanks for taking the trouble to report this bug.

How large is large?  Any chance you could supply me with such a sample
C++ file that is slow on searching.  Then, in case something else
besides the size is slowing it down, I'll have a "working" file to
diagnose the bug with.

> I traced the problem back to this commit:

> Author: Alan Mackenzie <bug-cc-mode <at> gnu.org>
> Date:   Sun Jan 21 18:05:41 2018 +0000

>     Handle C99 Compound Literals in return statements and argument lists.
>     
>     * cc-engine.el (c-looking-at-or-maybe-in-bracelist): Recognize a brace list
>     when preceded by "return" or inside parentheses, either immediately after the
>     "(" or following a comma.
>     (c-looking-at-inexpr-block): Test c-has-compound-literals rather than hard
>     coded C++ Mode.
>     (c-guess-basic-syntax, CASE 7B): Test additionally for not being just inside a
>     parenthesis or being at a Java "new" keyword.  CASE 9: Remove the simple
>     minded test on the contents of a block to determine a brace list.
>     
>     * cc-langs.el (c-has-compound-literals): New lang const and lang var.
>     
>     * bracelist-30.{c,res}: New test files.

Thanks for getting this information.  This should help track down the
problem.

> I am attaching the result of a run of the emacs profiler before
> ("fast.profile") and after the commit ("slow.profile"). In this run I
> started emacs as:

> emacs -Q /path/to/big/file.C -e "(add-to-list 'load-path ~/cc-mode)" -e "(require 'cc-mode)"
> M-x profiler-start
> C-s searchString
> M-x profiler-report

> And write the result to 'slow.profile' and 'fast.profile'
> respectively. It appears that 93% of the time is spent in
> c-looking-at-or-maybe-in-bracelist.

> Thanks for looking into this issue!

Just as a matter of interest, you can post CC Mode bugs direct to
bug-cc-mode <at> gnu.org.  Sometimes a CC Mode dump produced by C-c C-b can
be useful, though I don't think it would help much here.


> In GNU Emacs 26.0.91 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.26)
>  of 2018-02-06 built on beischer-w520
> Repository revision: c787a4968273027960a20ced6d63bae0d1ffa87e
> Windowing system distributor 'The X.Org Foundation', version 11.0.11906000

[ .... ]

-- 
Alan Mackenzie (Nuremberg, Germany).

Message #14 received at 30367 <at> debbugs.gnu.org (full text, mbox):

From: Bastian Beischer <bastian.beischer <at> gmail.com>
To: Alan Mackenzie <acm <at> muc.de>
Cc: 30367 <at> debbugs.gnu.org
Subject: Re: bug#30367: 26.0.91; CC-Mode: Major slowdown when isearching big
 C++ file.
Date: Wed, 7 Feb 2018 16:27:35 +0100

[Message part 1 (text/plain, inline)]

Hello Alan,

On Wed, Feb 7, 2018 at 4:24 PM, Alan Mackenzie <acm <at> muc.de> wrote:

> Hello, Bastian.
>
> In article <mailman.8662.1517913372.27995.bug-gnu-emacs <at> gnu.org> you
> wrote:
>
> > After updating CC-Mode to the latest version in the standalone mercurial
> > repository I noticed that searching large C++ files has become a lot
> > slower than before (factor 20 or so).
>
> Thanks for taking the trouble to report this bug.
>
> How large is large?  Any chance you could supply me with such a sample
> C++ file that is slow on searching.  Then, in case something else
> besides the size is slowing it down, I'll have a "working" file to
> diagnose the bug with.
>

Sure, but ​I will send you the file off-list, since I don't want it to
become public.
​

>
> > I traced the problem back to this commit:
>
> > Author: Alan Mackenzie <bug-cc-mode <at> gnu.org>
> > Date:   Sun Jan 21 18:05:41 2018 +0000
>
> >     Handle C99 Compound Literals in return statements and argument lists.
> >
> >     * cc-engine.el (c-looking-at-or-maybe-in-bracelist): Recognize a
> brace list
> >     when preceded by "return" or inside parentheses, either immediately
> after the
> >     "(" or following a comma.
> >     (c-looking-at-inexpr-block): Test c-has-compound-literals rather
> than hard
> >     coded C++ Mode.
> >     (c-guess-basic-syntax, CASE 7B): Test additionally for not being
> just inside a
> >     parenthesis or being at a Java "new" keyword.  CASE 9: Remove the
> simple
> >     minded test on the contents of a block to determine a brace list.
> >
> >     * cc-langs.el (c-has-compound-literals): New lang const and lang var.
> >
> >     * bracelist-30.{c,res}: New test files.
>
> Thanks for getting this information.  This should help track down the
> problem.
>
> > I am attaching the result of a run of the emacs profiler before
> > ("fast.profile") and after the commit ("slow.profile"). In this run I
> > started emacs as:
>
> > emacs -Q /path/to/big/file.C -e "(add-to-list 'load-path ~/cc-mode)" -e
> "(require 'cc-mode)"
> > M-x profiler-start
> > C-s searchString
> > M-x profiler-report
>
> > And write the result to 'slow.profile' and 'fast.profile'
> > respectively. It appears that 93% of the time is spent in
> > c-looking-at-or-maybe-in-bracelist.
>
> > Thanks for looking into this issue!
>
> Just as a matter of interest, you can post CC Mode bugs direct to
> bug-cc-mode <at> gnu.org.  Sometimes a CC Mode dump produced by C-c C-b can
> be useful, though I don't think it would help much here.
>

​Sorry, I didn't know that and will do it next time!
​

>
>
> > In GNU Emacs 26.0.91 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.26)
> >  of 2018-02-06 built on beischer-w520
> > Repository revision: c787a4968273027960a20ced6d63bae0d1ffa87e
> > Windowing system distributor 'The X.Org Foundation', version
> 11.0.11906000
>
> [ .... ]
>
> --
> Alan Mackenzie (Nuremberg, Germany).
>

​Cheers
Bastian

[Message part 2 (text/html, inline)]

Message #17 received at 30367 <at> debbugs.gnu.org (full text, mbox):

From: Alan Mackenzie <acm <at> muc.de>
To: Bastian Beischer <bastian.beischer <at> gmail.com>
Cc: 30367 <at> debbugs.gnu.org
Subject: Re: bug#30367: 26.0.91; CC-Mode: Major slowdown when isearching big
 C++ file.
Date: Wed, 7 Feb 2018 21:21:45 +0000

Hello, Bastian.

That was quick work.  :-)

On Wed, Feb 07, 2018 at 16:27:35 +0100, Bastian Beischer wrote:
> Hello Alan,

> On Wed, Feb 7, 2018 at 4:24 PM, Alan Mackenzie <acm <at> muc.de> wrote:

> > Hello, Bastian.

> > In article <mailman.8662.1517913372.27995.bug-gnu-emacs <at> gnu.org> you
> > wrote:

> > > After updating CC-Mode to the latest version in the standalone mercurial
> > > repository I noticed that searching large C++ files has become a lot
> > > slower than before (factor 20 or so).

> > Thanks for taking the trouble to report this bug.

> > How large is large?  Any chance you could supply me with such a sample
> > C++ file that is slow on searching.  Then, in case something else
> > besides the size is slowing it down, I'll have a "working" file to
> > diagnose the bug with.


> Sure, but ​I will send you the file off-list, since I don't want it to
> become public.
> ​

I've got the file you sent me, thanks very much, and I will treat it as
confidential.

Unfortunately, I cannot reproduce the slowdown in that file, at least,
not starting from emacs-26.0.91 -Q.  I think you've got quite a lot of
non-default minor modes active, and I've a suspicion that we're seeing
some interaction between CC Mode and one of these minor modes, or
possibly some other non-default setting you've got set.

Would you please try to reproduce the problem with emacs -Q, and if you
can't, try to pinpoint which one of your settings/minor modes is the one
which CC Mode is working badly with.

We'll crack this!


> ​Cheers
> Bastian

-- 
Alan Mackenzie (Nuremberg, Germany).

Message #20 received at 30367 <at> debbugs.gnu.org (full text, mbox):

From: Bastian Beischer <bastian.beischer <at> gmail.com>
To: Alan Mackenzie <acm <at> muc.de>
Cc: 30367 <at> debbugs.gnu.org
Subject: Re: bug#30367: 26.0.91; CC-Mode: Major slowdown when isearching big
 C++ file.
Date: Wed, 7 Feb 2018 22:53:47 +0100

Hello Alan,

On Wed, Feb 7, 2018 at 10:21 PM, Alan Mackenzie <acm <at> muc.de> wrote:
>
> Hello, Bastian.
>
> That was quick work.  :-)
>
> On Wed, Feb 07, 2018 at 16:27:35 +0100, Bastian Beischer wrote:
> > Hello Alan,
>
> > On Wed, Feb 7, 2018 at 4:24 PM, Alan Mackenzie <acm <at> muc.de> wrote:
>
> > > Hello, Bastian.
>
> > > In article <mailman.8662.1517913372.27995.bug-gnu-emacs <at> gnu.org> you
> > > wrote:
>
> > > > After updating CC-Mode to the latest version in the standalone mercurial
> > > > repository I noticed that searching large C++ files has become a lot
> > > > slower than before (factor 20 or so).
>
> > > Thanks for taking the trouble to report this bug.
>
> > > How large is large?  Any chance you could supply me with such a sample
> > > C++ file that is slow on searching.  Then, in case something else
> > > besides the size is slowing it down, I'll have a "working" file to
> > > diagnose the bug with.
>
>
> > Sure, but I will send you the file off-list, since I don't want it to
> > become public.
> >
>
> I've got the file you sent me, thanks very much, and I will treat it as
> confidential.
>
> Unfortunately, I cannot reproduce the slowdown in that file, at least,
> not starting from emacs-26.0.91 -Q.  I think you've got quite a lot of
> non-default minor modes active, and I've a suspicion that we're seeing
> some interaction between CC Mode and one of these minor modes, or
> possibly some other non-default setting you've got set.
>
> Would you please try to reproduce the problem with emacs -Q, and if you
> can't, try to pinpoint which one of your settings/minor modes is the one
> which CC Mode is working badly with.

Actually the profile runs I sent were done with 'emacs -Q'. It's just
that I used a different emacs instance to report the bug, which had
the usual modes loaded. Sorry about that. So I can reproduce the
problem from emacs -Q, but I _do_ load the latest cc-mode from it's
standalone repository. The actual command was:

emacs -Q --eval "(add-to-list 'load-path \"~/cc-mode\")" --eval
"(require 'cc-mode)" /path/to/file.C

then search for 'isinsha'.

~/cc-mode revision is as follows:

$ cd ~/cc-mode
$ hg summary
parent: 6256:e8b2c6141f97
Added tag 2018-01-29-mac-m for changeset 9785123a85b4
branch: default
commit: (clean)
update: (current)

emacs version is as in the original report (GNU Emacs 26.0.91 (build
1, x86_64-pc-linux-gnu, GTK+ Version 3.22.26) of 2018-02-06, commit
c787a4968273027960a20ced6d63bae0d1ffa87e)

>
>
> We'll crack this!

I'm sure :-)

>
>
> > Cheers
> > Bastian
>
> --
> Alan Mackenzie (Nuremberg, Germany).

Cheers
Bastian

Message #23 received at 30367 <at> debbugs.gnu.org (full text, mbox):

From: Alan Mackenzie <acm <at> muc.de>
To: Bastian Beischer <bastian.beischer <at> gmail.com>
Cc: 30367 <at> debbugs.gnu.org
Subject: Re: bug#30367: 26.0.91; CC-Mode: Major slowdown when isearching big
 C++ file.
Date: Wed, 7 Feb 2018 22:26:38 +0000

Hello, Bastian.

On Wed, Feb 07, 2018 at 22:53:47 +0100, Bastian Beischer wrote:
> Hello Alan,

> On Wed, Feb 7, 2018 at 10:21 PM, Alan Mackenzie <acm <at> muc.de> wrote:

[ .... ]

> > Unfortunately, I cannot reproduce the slowdown in that file, at least,
> > not starting from emacs-26.0.91 -Q.  I think you've got quite a lot of
> > non-default minor modes active, and I've a suspicion that we're seeing
> > some interaction between CC Mode and one of these minor modes, or
> > possibly some other non-default setting you've got set.

> > Would you please try to reproduce the problem with emacs -Q, and if you
> > can't, try to pinpoint which one of your settings/minor modes is the one
> > which CC Mode is working badly with.

> Actually the profile runs I sent were done with 'emacs -Q'. It's just
> that I used a different emacs instance to report the bug, which had
> the usual modes loaded. Sorry about that. So I can reproduce the
> problem from emacs -Q, but I _do_ load the latest cc-mode from it's
> standalone repository. The actual command was:

> emacs -Q --eval "(add-to-list 'load-path \"~/cc-mode\")" --eval
> "(require 'cc-mode)" /path/to/file.C

As a matter of interest, you can run

    emacs -Q -L ~/cc-mode /path/to/file.C

, this being less effort to type.  (I don't understand why you've got the
`require' form in there - maybe you really do need it.)  :-)

> 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!

> ~/cc-mode revision is as follows:

> $ cd ~/cc-mode
> $ hg summary
> parent: 6256:e8b2c6141f97
> Added tag 2018-01-29-mac-m for changeset 9785123a85b4
> branch: default
> commit: (clean)
> update: (current)

Thanks.  I actually run the tip of the standalone CC Mode as my default
CC Mode anyway, since I'm so often working on it.

> emacs version is as in the original report (GNU Emacs 26.0.91 (build
> 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.26) of 2018-02-06, commit
> c787a4968273027960a20ced6d63bae0d1ffa87e)



> > We'll crack this!

> I'm sure :-)

Just not tonight.  ;-)

> Cheers
> Bastian

-- 
Alan Mackenzie (Nuremberg, Germany).

Message #26 received at 30367 <at> debbugs.gnu.org (full text, mbox):

From: Bastian Beischer <bastian.beischer <at> gmail.com>
To: Alan Mackenzie <acm <at> muc.de>
Cc: 30367 <at> debbugs.gnu.org
Subject: Re: bug#30367: 26.0.91; CC-Mode: Major slowdown when isearching big
 C++ file.
Date: Wed, 7 Feb 2018 23:45:03 +0100

On Wed, Feb 7, 2018 at 11:26 PM, Alan Mackenzie <acm <at> muc.de> wrote:
> Hello, Bastian.
>
> On Wed, Feb 07, 2018 at 22:53:47 +0100, Bastian Beischer wrote:
>> Hello Alan,
>
>> On Wed, Feb 7, 2018 at 10:21 PM, Alan Mackenzie <acm <at> muc.de> wrote:
>
> [ .... ]
>
>> > Unfortunately, I cannot reproduce the slowdown in that file, at least,
>> > not starting from emacs-26.0.91 -Q.  I think you've got quite a lot of
>> > non-default minor modes active, and I've a suspicion that we're seeing
>> > some interaction between CC Mode and one of these minor modes, or
>> > possibly some other non-default setting you've got set.
>
>> > Would you please try to reproduce the problem with emacs -Q, and if you
>> > can't, try to pinpoint which one of your settings/minor modes is the one
>> > which CC Mode is working badly with.
>
>> Actually the profile runs I sent were done with 'emacs -Q'. It's just
>> that I used a different emacs instance to report the bug, which had
>> the usual modes loaded. Sorry about that. So I can reproduce the
>> problem from emacs -Q, but I _do_ load the latest cc-mode from it's
>> standalone repository. The actual command was:
>
>> emacs -Q --eval "(add-to-list 'load-path \"~/cc-mode\")" --eval
>> "(require 'cc-mode)" /path/to/file.C
>
> As a matter of interest, you can run
>
>     emacs -Q -L ~/cc-mode /path/to/file.C
>
> , this being less effort to type.  (I don't understand why you've got the
> `require' form in there - maybe you really do need it.)  :-)

Yes the command you sent works just as well, so the require form is
not needed. Thanks!

>
>> 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.

>
>> ~/cc-mode revision is as follows:
>
>> $ cd ~/cc-mode
>> $ hg summary
>> parent: 6256:e8b2c6141f97
>> Added tag 2018-01-29-mac-m for changeset 9785123a85b4
>> branch: default
>> commit: (clean)
>> update: (current)
>
> Thanks.  I actually run the tip of the standalone CC Mode as my default
> CC Mode anyway, since I'm so often working on it.
>
>> emacs version is as in the original report (GNU Emacs 26.0.91 (build
>> 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.26) of 2018-02-06, commit
>> c787a4968273027960a20ced6d63bae0d1ffa87e)
>
>
>
>> > We'll crack this!
>
>> I'm sure :-)
>
> Just not tonight.  ;-)
>
>> Cheers
>> Bastian
>
> --
> Alan Mackenzie (Nuremberg, Germany).

Message #29 received at 30367 <at> debbugs.gnu.org (full text, mbox):

From: Alan Mackenzie <acm <at> muc.de>
To: Bastian Beischer <bastian.beischer <at> gmail.com>
Cc: 30367 <at> debbugs.gnu.org
Subject: Re: bug#30367: 26.0.91; CC-Mode: Major slowdown when isearching big
 C++ file.
Date: Thu, 8 Feb 2018 17:25:58 +0000

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 <acm <at> muc.de> 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).

Message #32 received at 30367 <at> debbugs.gnu.org (full text, mbox):

From: Alan Mackenzie <acm <at> muc.de>
To: Bastian Beischer <bastian.beischer <at> gmail.com>
Cc: 30367 <at> debbugs.gnu.org
Subject: Re: bug#30367: 26.0.91; CC-Mode: Major slowdown when isearching big
 C++ file.
Date: Thu, 8 Feb 2018 18:42:32 +0000

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)
 		 nil)
-		((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).



> ------------------------------------------------------------------------------
> Check out the vibrant tech community on one of the world's most
> engaging tech sites, Slashdot.org! http://sdm.link/slashdot

Message #35 received at 30367 <at> debbugs.gnu.org (full text, mbox):

From: Bastian Beischer <bastian.beischer <at> gmail.com>
To: Alan Mackenzie <acm <at> muc.de>
Cc: 30367 <at> debbugs.gnu.org
Subject: Re: bug#30367: 26.0.91;
 CC-Mode: Major slowdown when isearching big C++ file.
Date: Fri, 09 Feb 2018 10:16:53 +0100

Hello Alan,

Alan Mackenzie <acm <at> muc.de> writes:

> 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)
>  		 nil)
> -		((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.
>
>
>
> [ .... ]
>

Yes, this patch fixes the slowdown. But since I don't understand what
the original commit which introduced the slowdown tried to fix I cannot
comment on whether or not its the right thing to do within the context
of that commit.

I will run emacs with the patch included for a while and will let you
know in case I find any problems!

Thanks for tackling this so quickly!

>> > Cheers
>> > Bastian

Message #40 received at 30367-done <at> debbugs.gnu.org (full text, mbox):

From: Alan Mackenzie <acm <at> muc.de>
To: Bastian Beischer <bastian.beischer <at> gmail.com>
Cc: 30367-done <at> debbugs.gnu.org
Subject: Re: bug#30367: 26.0.91; CC-Mode: Major slowdown when isearching big
 C++ file.
Date: Tue, 20 Mar 2018 18:49:47 +0000

Hello, Bastian.

On Fri, Feb 09, 2018 at 10:16:53 +0100, Bastian Beischer wrote:
> Hello Alan,

> Alan Mackenzie <acm <at> muc.de> writes:

[ .... ]

> > 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)
> >  		 nil)
> > -		((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.



> > [ .... ]


> Yes, this patch fixes the slowdown. But since I don't understand what
> the original commit which introduced the slowdown tried to fix I cannot
> comment on whether or not its the right thing to do within the context
> of that commit.

Thanks for the prompt, positive reply.  I'm afraid I can't remember
either, what the original patch was for.

> I will run emacs with the patch included for a while and will let you
> know in case I find any problems!

Nothing heard from you in the meantime.  Good!

> Thanks for tackling this so quickly!

Sorry I've not maintained this speed in committing it.  I have now
committed the fix, in revision 16559146f9db1b36d5e8b6c92edb5bb36fafdb85,
and I'm closing the bug.

> >> > Cheers
> >> > Bastian

-- 
Alan Mackenzie (Nuremberg, Germany).

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