Re: [Emacs-diffs] comment-cache 223d16f 2/3: Apply `comment-depth' text properties when calling `back_comment'.

[Dmitry Gutov <dgutov@yandex.ru>]

On 03/13/2016 07:59 PM, Alan Mackenzie wrote:

>> That severely depends on your usage. Have you tried it?
>
> Have now, yes.

Any performance numbers on using this approach in C/l mode?

>> Second, I'm sure you can decrease syntax-ppss-max-span, if your language
>> is expensive to parse.
>
> I would think the speed of parse-partial-sexp is pretty much constant,
> independent of the language.

Well, it seems like that you would be calling it much more frequently 
than other packages. So you could adjust for that.

> It will be much faster.  The only looping involved is over the levels of
> an optimised binary tree (the text property intervals), not linearly
> over 10k characters.  By the time syntax-ppss has finished consing down
> its cache list, the text property stuff would be done.

Yes, it would be fast. But like we've noticed, calling syntax-ppss is 
also quick, and it basically has a constant upper bound, on average. 
It's hard to be *significantly* faster than that.

So I have to wonder why the "get out of a comment" feature is used in 
C/l mode so much that it becomes a bottleneck, and you get significant 
improvement in performance by dropping the caching logic to C. That is, 
of course, not a nice thing to ask considering the overall complexity of 
CC Mode, but still.

I don't see anything comparable to 10 second waiting described in 
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=22884, when doing a 
comparable operation in a 5000-line Ruby file.

>> But it comes with a set of drawbacks that has already been brought up
>> in the discussions.
>
> That set has one member: the partial overlap in functionality with
> syntax-ppss.  No intrinsic drawbacks have yet been pointed out.  But you
> do so below (thanks!).

Aside from everything that's been mentioned already, it's written in C, 
reducing the pool of developers willing to work on it. And no, it 
doesn't make it "simple and maintainable", at least from my perspective.