Re: Bug #25608 and the comment-cache branch

[Dmitry Gutov]

On 14.02.2017 18:38, Stefan Monnier wrote:

> Like all the sexp movement functions, `forward-comment` is allowed to
> assume that the starting position is outside of comments/strings, so it
> doesn't need to consult the cache to see if it's within a string.

I see, thanks. And I think that means that, ideally, it would work 
without the caller having to adjust the syntax visibility bounds, or the 
like, as long as the syntax table is correct and the beginning (or the 
end) of the currently navigated comment is within view.

> In the case we do scan forward (e.g. the case where we end up using
> parse-partial-sexp (or syntax-ppss in my patch)), we actually manually
> re-introduce that behavior: if the forward parse says that the
> end-comment-marker in inside a string (or inside another comment), we
> re-parse from the beginning of that string (or comment) to try and see
> if that end-comment-marker could be considered to close a comment nested
> within the string (or the other comment).

That indeed sounds complex.

> Calling syntax-ppss every time back_comment is invoked would probably
> result in bad performance currently: when parsing backward
> (e.g. backward-sexp), the syntax-ppss-last optimization is ineffective,
> so we'd fallback on syntax-ppss-cache which ends up scanning on the
> average syntax-ppss-max-span/2 (i.e. 10K) chars.  When \n is a comment
> ender (i.e. in most programming language modes), it would imply
> a forward scan of 10K for every line.

You're probably right, but I wonder what the benchmarks would say.

(parse-partial-sexp 1 10000) takes 0.0005 seconds here, so it'd still 
require some intensive usage to show up on user's radar.

Previously, we started from the beginning of the current defun, as 
delineated by an open paren in the first column, right?

I've seen function definitions longer than 10000 chars.

> IOW, for such an approach to work, we'd have to rework syntax-ppss to be
> faster when scanning backward (e.g. reduce syntax-ppss-max-span, which
> would have other repercussions).

Perhaps we could use the "generic comment bounds" syntax-table property 
to delineate such difficult comments. If that idea sounds similar to 
comment-cache, that is no accident.

But we should try to limit the incompatibility with mixed modes by only 
caching the beginnings of comments which contain strings, nested 
comments, etc. Better suggestion welcome (use a tree data structure 
instead of in-buffer text-properties?).

I've only recently come to the realization that our usage of the 
syntax-table text property has the same general incompatibility with 
mixed mode buffers as comment-cache does. The only reasons why it 
doesn't show as much is because we use them relatively rarely. But we 
couldn't, for instance, apply a "generic string" syntax to some literal 
in a subregion that is inside a "generic string" belonging to the 
primary major mode. Not sure what to do about that.