bug#56682: feature/improved-locked-narrowing 9dee6df39c: Reworked locked narrowing.

[Dmitry Gutov]

On 31/01/2023 23:46, Stefan Monnier wrote:
> > Speaking of additional slowdowns, though, at least in theory -- both
> > syntax-ppss and tree-sitter drop their current parse result when point-min
> > changes.
> FWIW, `syntax-ppss` keeps 2 caches: one for point-min=1 and one for the
> other cases.  So narrowing doesn't quite "drop the current parse", it
> just temporarily ignores it, but recovers it when we exit the narrowing.

Take the scenario for scrolling through a whole file. When the file 
reaches a certain length, the current scheme is to narrow, linearly, to 
successive text chunks of size N, and fontify them.

And since for every chunk point-min will have a new value, it will have 
to re-parse the text from the beginning of the chunk. Even if the cache 
exists already for the whole (unrestricted) buffer.

And even if syntax-ppss-max-span is lower than N (which it currently is 
by default, 20000 vs 500000).