bug#16526: 24.3.50; scroll-conservatively & c-mode regression

[Eli Zaretskii]

> Date: Sat, 25 Jan 2014 17:48:02 +0100
> From: martin rudalics <rudalics@gmx.at>
> CC: 16526@debbugs.gnu.org, acm@muc.de
> 
>  >> How should I know?  I suppose redisplay_window eventually winds up
>  >> calling the fontification function and sooner or later the c-code calls
>  >> back_comment.
>  >
>  > Yes, that's what happens.  And it cannot be avoided, AFAICS, when
>  > scroll-conservatively is on.
> 
> Well ... so you know why it calls back_comment around the END of the
> buffer?

It starts at the end of the buffer, but then moves all the way back
to the beginning.

And yes, I know why: it's because scroll-conservatively causes
redisplay to examine buffer text around point, when it decides where
to place window-start.  This is triggered by redisplay after the move
to the end of the buffer.

>  > What I see is that find_defun_start is called many times,
> 
> ... 530 times as I mentioned earlier ...
> 
>  > with its
>  > first argument moving from _end_ of the buffer backwards.
> 
> Not monotonously.  Sometimes it's called from the same position (for
> example 948653 is at least three times on my list) again.

True.  But I'm not sure this is relevant to the slow redisplay.

>  > This
>  > happens when Emacs needs to redisplay the last portion of the buffer,
>  > immediately after the call to end-of-buffer.
> 
> Hmm ... but the problem is when going to BOB.

No, going to BOB is instantaneous.  The problem happens in redisplay
after going to EOB.

>  > JIT Lock is triggered also when font-lock is turned on after the move
>  > to end of the buffer.  But the difference seems to come from the fact
>  > that under scroll-conservatively, we examine the buffer a little bit
>  > above/below the window, when we decide where to put window-start.
> 
> And somehow a "current position" is still near the end of the buffer at
> that time.

Yes, because Emacs is at EOB.