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

[martin rudalics]

> AH!!!!!  GOT IT!!!!  What causes the slowdown is binding
> open-paren-in-column-0-is-defun-start to nil around the scan-lists.  In
> CC Mode's "engine" parts, that variable is bound to nil.  There are good
> reasons for this (which I can't precisely recall just at the moment).
>
> In CC Mode, "'" has string syntax.  So any C comment with an odd number
> of apostrophes looks, to back_comment, like it has an "unbalanced string",
> and the code ends up at label lossage: in back_comment.  Here
> find_defun_start is called, and this returns BOB when open-paren-..-0 is
> nil.  From here (BOB) the code scans forward to check this putative
> "unbalanced string".  It's a long way from BOB to ~900k, hence the
> sluggishness.
>
> Where do we go from here?

In every version of Emacs I'm actually doing some work I have disabled
CC mode's `open-paren-in-column-0-is-defun-start' bindings.  Otherwise,
working on a file like xdisp.c would be virtually impossible for me.  As
for the release I would simply do what I proposed earlier - make these
overriding bindings customizable.  So someone who wants to comment out
code with a paren in the first column can have the "correct" behavior
while people like me can continue work with "incorrect" but yet tame
behavior.  And obviously I could continue to work without having bzr
status tell me that I've changed CC mode files ;-)

martin