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

[Dmitry Gutov]

Hi Alan,

On 03/14/2016 08:46 PM, Alan Mackenzie wrote:

> > > CC Mode doesn't use syntax-ppss.  It would be too much work to put it in,
> > > particularly as that function's future is unclear.
>
> > Could you please avoid FUD like that?
>
> There's no FUD in my last paragraph, it's an accurate representation of
> the facts.

Not at all. syntax-ppss's future is rather clear, and it's here to stay, 
after years of successfully being in use.

> I am not prepared to spend the time needed to adapt CC Mode
> to use syntax-ppss, and wouldn't be even if it worked properly.

That's a separate issue. Right now we're discussing how to best 
implement "comment cache", and if it's needed at all.

> I'm still trying to work out the point you're trying to make.

Apparently, the point is that you've been offered a simpler solution for 
the same problem, and that you basically ignored it.

> Ah, so by "it" in "please try it already" you meant "apply Stefan's
> patch".  That wasn't clear.  Sorry.

Isn't what this subthread is about? The alternative approach?

> Yes, applying that patch and doing measurements would be a lot of work.
> You're welcome to do it if you're interested enough.

I might, but you'd first have to let me know what to test. If the 
comment cache patch doesn't actually help with 22884, why are we even 
discussing it?

> I have tried out Stefan's patch, and for the vast majority of
> comments on which it actually works, I've no doubt it will work fast
> enough.  Its speed isn't the issue.

Ah, so the speed advantage of using text properties is not that much of 
advantage. Correct?

> Stefan's patch doesn't actually fix what I see as the root cause of bug
> #22884, namely that comments get scanned backwards.  At some time in the
> future this backward scanning will cause more bugs.

Can you produce a test case where it fails? This time without involving 
narrowing, please.

> What has been tried is Martin Rudalics's `foo' and `bar' functions
> (which repeatedly perform `c-end-of-defun' and `c-beginning-of-defun'
> respectively).  I'm not the only person who has noticed a dramatic
> increase in speed for the comment-cache branch in these
> (unrepresentative) tests.  If you're interested, look at some of the
> timings in the posts branching off of Martin's first post in this
> thread.

Do you have any non-synthetic tests? (while (not (eobp)) (end-of-defun)) 
is not something that is likely to occur is a user's interation with 
Emacs with any regularity.

For a synthetic test, by the way, a 60%-80% increase in performance is 
not that impressive.

> It will make no difference to the scenario in #22884, no, and it was
> never envisaged that it would.  What all these things have in common is
> problems caused by scanning comments backwards.

syntax-ppss helps with avoiding that.