Re: /srv/bzr/emacs/trunk r101338: * lisp/emacs-lisp/syntax.el (syntax-ppss): More sanity check to catch

[Dmitry Gutov <dgutov@yandex.ru>]

On 12.02.2014 16:23, Stefan Monnier wrote:
> In 99% of the cases, syntax-ppss is only called once during font-lock.
> So, while in theory, yes, we might still get some benefits, in practice
> we don't.

`syntax-propertize' can call it multiple times, though (at least in the 
case of ruby-mode), and that's called as often as font-lock.

> Of course, another issue is "which syntax-table and syntax-propertize
> function to use".  How does mmm-mode handle that currently?

It has a function called `mmm-update-submode-region', which restores the 
saved buffer-local values, syntax table, etc, for the mode at point. 
It's called:

* From post-command-hook.
* From `mmm-syntax-propertize-function', which iterates over regions, 
calls the aforementioned function and then the submode's 
syntax-propertize-function.
* From `mmm-fontify-region', which is similar, but for font-lock.
* From `mmm-indent-line' and the specialized `mmm-indent-line', because 
we usually want to indent by the rules of the submode at indentation, 
and it might be different in the middle of the line.

IOW, various stuff works, as long as mmm-mode is aware of the respective 
facility. Each has to be supported explicitly, though.

By the way, AFAICT, the juggling of local variables between modes (save, 
restore, rinse, repeat) is one of the major factors in mmm-mode performance.

There was an old `multi-mode' by Dave Love that used indirect buffers to 
keep the buffer-local values around, and switched between them in 
post-command-hook (which might or might not have been a good idea) and 
in `multi-fontify-region', taking advantage of the fact that 
fontification in an indirect buffer translates into the base buffer.

Now I hear that we shouldn't use indirect buffers. Would there be a fast 
way to save and restore all local variables in a buffer? This would cut 
down on both mmm-mode overhead and complexity.