C syntax highlighting, faster syntax based folding and a bug

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C syntax highlighting, faster syntax based folding and a bug

Lech Lorens
I have recently suffered very much from the problem reported by
Charles Campbell and dman described in todo.txt as:

7   C syntax highlighting gets a lot slower after ":set foldmethod=syntax".
    (Charles Campbell)  Inserting a "{" is very slow. (dman)

The problem stems from the fact that inserting '{' into a C file causes
all the folds from the insertion place to the end of the file to become
invalid.
The same goes for inserting '(' or '['. I tried to fix the problem and
came up with a solution (see attached c.vim.patch). I modified C syntax
highlighting in such a way that inserting '(' or '[' causes a new region
to be created but this time the region does not extend to the end of the
file and as a consequence not so many folds have to be recalculated.

I would appreciate feedback on my modifications a lot and really hope
for some comments. You might want to follow my test procedure:

Enter the directory with Vim sources. Execute Vim:
$ vim -p eval.c eval.c eval.c
:tabdo set fdm=syntax fdc=5 number
:normal 1gt
:820
:normal zO

And then type the following (observe the fold column):
Ofor (i =

After inserting '(' with the official syntax highlighting file I need to
wait a few seconds (sic!) before the screen is updated and "i ="
appears. With my modifications of c.vim Vim responds instantaneously
(again: observe the difference in the fold column).

This comes at a cost, however. While in case of the original syntax
highlighting all the braces from the modification place to the end of
the file would become highlighted as error, after my modifications only
braces inside the current block get such highlighting. This is still
quite visible if you open a parenthesis at line 820, but is not so any
more if you do it at line 843.

I am ready to accept such a trade-off as I rely very much on
C-syntax-based folding and recently more and more often have been
finding myself in situations where I input some code without getting any
visual feedback from Vim (this happens on my new machine at work, even
more on my old computer at home). I presume that there are others who
are bothered by the same problem. Perhaps the behaviour I introduced to
c.vim could be made into an option like c_no_curly_error or
c_no_bracket_error.





I would like also to describe the (erroneous in my opinion) behaviour of
Vim I discovered while trying to improve c.vim. The version of c.vim
that allows the problem to be reproduced can be obtained by applying the
attached patch c.vim.patch.bad.

If I follow the test procedure described in the first part of my email
I get the most desirable result: highlighting of all the braces to the
end of file without the folds being recomputed. This is in my opinion an
error as the cBadBlock region starting with '{' at line 827 introduces
a new fold level. cBadBlock is not closed by '}' at line 843 but the
fold - as indicated by the fold column - ends at this point. You can
verify my claim by going to line 844 and performing the following:

:for id in synstack(line("."), col("."))
: echo synIDattr(id, "name")
:endfor

The output is:
cBlock
cParen
cBadBlock

Is this a correct behaviour?

Thank you in advance for any comments!

--
Cheers,
Lech

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c.vim.patch (3K) Download Attachment
c.vim.patch.bad (4K) Download Attachment
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Re: C syntax highlighting, faster syntax based folding and a bug

Dominique Pellé

Lech Lorens wrote:

> I would appreciate feedback on my modifications a lot and really hope
> for some comments. You might want to follow my test procedure:
>
> Enter the directory with Vim sources. Execute Vim:
> $ vim -p eval.c eval.c eval.c
> :tabdo set fdm=syntax fdc=5 number
> :normal 1gt
> :820
> :normal zO
>
> And then type the following (observe the fold column):
> Ofor (i =
>
> After inserting '(' with the official syntax highlighting file I need to
> wait a few seconds (sic!) before the screen is updated and "i ="
> appears.

On my machine, I had to wait 1 minute and 50 seconds (!) before I
could see "i = " (and Vim was using 100% of the CPU during that
time).  That's with Vim-7.2.284 built with -O0 on a Core-2, 1.7Ghz.


> With my modifications of c.vim Vim responds instantaneously
> (again: observe the difference in the fold column).

After applying your patch (c.vim.patch), I could see "i = "
instantaneously.

I normally don't use syntax folding, but if I did, I would most certainly
want to do it with your patch given the major speed up.


> This comes at a cost, however. While in case of the original syntax
> highlighting all the braces from the modification place to the end of
> the file would become highlighted as error, after my modifications only
> braces inside the current block get such highlighting. This is still
> quite visible if you open a parenthesis at line 820, but is not so any
> more if you do it at line 843.
>
> I am ready to accept such a trade-off as I rely very much on
> C-syntax-based folding and recently more and more often have been
> finding myself in situations where I input some code without getting any
> visual feedback from Vim (this happens on my new machine at work, even
> more on my old computer at home). I presume that there are others who
> are bothered by the same problem. Perhaps the behaviour I introduced to
> c.vim could be made into an option like c_no_curly_error or
> c_no_bracket_error.

I would also accept the trade-off, given the dramatic speed up.


> I would like also to describe the (erroneous in my opinion) behaviour of
> Vim I discovered while trying to improve c.vim. The version of c.vim
> that allows the problem to be reproduced can be obtained by applying the
> attached patch c.vim.patch.bad.

I did not check that yet.

Thanks
-- Dominique

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Re: C syntax highlighting, faster syntax based folding and a bug

Lech Lorens

On 10-Nov-2009 Dominique Pellé <[hidden email]> wrote:
>
> > After inserting '(' with the official syntax highlighting file I need to
> > wait a few seconds (sic!) before the screen is updated and "i ="
> > appears.
>
> On my machine, I had to wait 1 minute and 50 seconds (!) before I
> could see "i = " (and Vim was using 100% of the CPU during that
> time).  That's with Vim-7.2.284 built with -O0 on a Core-2, 1.7Ghz.

Bram, do you have an opinion on the subject?

--
Cheers,
Lech

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Re: C syntax highlighting, faster syntax based folding and a bug

Bram Moolenaar


Lech Lorens wrote:

> On 10-Nov-2009 Dominique Pellé <[hidden email]> wrote:
> >
> > > After inserting '(' with the official syntax highlighting file I need to
> > > wait a few seconds (sic!) before the screen is updated and "i ="
> > > appears.
> >
> > On my machine, I had to wait 1 minute and 50 seconds (!) before I
> > could see "i = " (and Vim was using 100% of the CPU during that
> > time).  That's with Vim-7.2.284 built with -O0 on a Core-2, 1.7Ghz.
>
> Bram, do you have an opinion on the subject?

I have put a remark in the todo list, I'll look into it later.

Did you try profiling to find out where the time is wasted?

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Re: C syntax highlighting, faster syntax based folding and a bug

Dominique Pellé

Bram Moolenaar wrote:

> Lech Lorens wrote:
>
>> On 10-Nov-2009 Dominique Pellé <[hidden email]> wrote:
>> >
>> > > After inserting '(' with the official syntax highlighting file I need to
>> > > wait a few seconds (sic!) before the screen is updated and "i ="
>> > > appears.
>> >
>> > On my machine, I had to wait 1 minute and 50 seconds (!) before I
>> > could see "i = " (and Vim was using 100% of the CPU during that
>> > time).  That's with Vim-7.2.284 built with -O0 on a Core-2, 1.7Ghz.
>>
>> Bram, do you have an opinion on the subject?
>
> I have put a remark in the todo list, I'll look into it later.
>
> Did you try profiling to find out where the time is wasted?


I profiled with gprof using Vim-7.2.293 built with:
   -O0 -g -pg -DWE_ARE_PROFILING
... and executed the steps described earlier by Lech:

$ vim -p eval.c eval.c eval.c
:tabdo set fdm=syntax fdc=5 number
:normal 1gt
:820
:normal zO

Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls   s/call   s/call  name
 72.81     49.62    49.62 106860477     0.00     0.00  in_id_list
  7.55     54.76     5.14  5585619     0.00     0.00  syn_current_attr
  3.19     56.94     2.17  8395304     0.00     0.00  regmatch
  2.60     58.70     1.77   270449     0.00     0.00  syn_stack_find_entry
  2.44     60.37     1.66   533472     0.00     0.00  syntax_start
  1.48     61.38     1.01 114389042     0.00     0.00  utfc_ptr2len
  1.26     62.23     0.86 60053856     0.00     0.00  regnext
  0.97     62.90     0.66  6124658     0.00     0.00  vim_strchr
  0.44     63.20     0.30  6391651     0.00     0.00  vim_regexec_both
  0.40     63.47     0.28 12652530     0.00     0.00  vim_iswordc_buf
  0.37     63.72     0.25 43505784     0.00     0.00  utf_ptr2char
...

I put the full gprof profiling result at:

  http://dominique.pelle.free.fr/prof-vim-7.2.293.txt.gz

Cheers
-- Dominique

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