Re: The current state of the comment-cache branch

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 24 Dec 2016 11:36:20 +0000
> Cc: emacs-devel <emacs-devel@gnu.org>, Eli Zaretskii <eliz@gnu.org>
> From: Alan Mackenzie <acm@muc.de>
> 
> > Could you or someone else explain why an open paren in column 0 can affect
> > the parsing of syntax in cc-mode at all? After all, such parens have no
> > special syntactic nor semantic meaning in C nor C++.
> 
> It is a convention established in Emacs around ?40 years ago, that such
> an open paren signalled the start of a defun.  This sped Emacs up
> enormously, since the alternative was searching back to the beginning of
> the buffer to check whether the paren was at the top level.
> 
> Currently, the only piece of low-level code which uses this convention is
> back_comment in .../src/syntax.c.  This scans backward over a comment to
> its beginning.  Scanning backwards is difficult, because you can't really
> tell whether a string quote is contained within a comment, or a comment
> marker might be contained within a string, ....  back_comment is
> heuristic, and sort of works OK.  But it uses the open defun convention
> to speed itself up.

I think one missing piece of the puzzle, which is what confused Elias,
is that open-paren-in-column-0-is-defun-start is not only about '(',
it's about any character that has the "open parenthesis" syntax class
under the current syntax tables.  And in C mode, that includes '{' and
'[', with the former being the important one.  I think.