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Re: Exposing buffer text modifications to Lisp (was: Tree-sitter integra

From: Ihor Radchenko
Subject: Re: Exposing buffer text modifications to Lisp (was: Tree-sitter integration on feature/tree-sitter)
Date: Sat, 18 Jun 2022 16:13:13 +0800

Eli Zaretskii <eliz@gnu.org> writes:

>> Because lots of markers degrade Emacs regex search performance
>> tremendously.
>> See https://list.orgmode.org/orgmode/scedec$2g0$1@ciao.gmane.io/
>> and https://orgmode.org/list/87y21wkdwu.fsf@localhost
> AFAIU, the right fix for this is to fix performance degradation when a
> buffer has many markers, not avoiding the use of markers.
> Here's one conclusion from this discussion that indicates changes
> required to be done in core (other than a low-level modification hook
> for buffer text) to take care of your AST implementation.
> We already have a TODO item for making markers more efficient; any
> takers?

This is trickier than it may appear.
Each element in Org AST has 3-7 markers.
My real-life large org buffer contains ~200k Org syntax elements
(actually more, but not all the elements are ever queried).
So, we are talking about 600k-1.4M markers in buffer if Org AST were to
use markers.

Now, imagine an edit somewhere near the beginning of Org buffer. Such
edit means that Emacs will have to shift positions of nearly all the
markers in the buffer. All the >1M markers. On every

Org parser goes around this issue by updating AST positions on idle and
maintaining asynchronous request queue. This works relatively well
because AST queries are skewed to be near the buffer region being
edited. I am not sure if similar approach (not trivial to start with)
can be efficiently utilized by Emacs. IDK the typical marker access
pattern in Emacs core.

Probably, Emacs may need to implement an alternative data structure to
store markers and allow efficient batch-shifting of the markers. Again,
not trivial.

>> The situation is third-party code doing bloody murder with
>> (with-silent-modifications
>>  (insert "Some text not triggering modification hooks))
>> Another scenario is modifying text in indirect buffers created with
>> make-indirect-buffer. (where there is no chance to install
>> before/after-change-functions via clone-indirect-buffer-hook).
> In at least the latter case the idea for a proper solution was
> outlined by Stefan.

I haven't read through his email carefully yet. A quick response is that
I have seen a lot of code in the wild that simply uses
make-indirect-buffer. Expecting compliance is unreliable in practice. (I
may need to think more about this though)


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