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Re: State of the overlay tree branch?

From: Sebastian Sturm
Subject: Re: State of the overlay tree branch?
Date: Sun, 18 Mar 2018 22:04:13 +0100
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I also found it surprising that overlays would slow down line counting, but since I don't know anything about the architecture of the Emacs display engine, or its overlay implementation, I figured that overlays must be to blame because

(i) the issue went away after switching to the feature/noverlay branch

(ii) configuring the semantic highlighter to use its font-lock backend also resolved the performance issue (though with the font-lock backend, highlights are easily messed up by editing operations which makes the overlay variant far more appealing)

I also found that some other heavy users of overlays such as avy-goto-word-0-{above,below} feel faster with the feature/noverlay branch, so I'd welcome a merge of the overlay branch even if there was a technically superior alternative to line-number-at-pos that didn't suffer from overlay-related performance issues.

That being said, your suggestion sounds intriguing. What would be required to expose find_newline to Lisp? Would I simply have to wrap it in one of Emacs's DEFINE_<something> macros? Is there some documentation on the Emacs C backend?

On 03/18/2018 09:39 PM, Eli Zaretskii wrote:
>> From: Sebastian Sturm <address@hidden>
>> Date: Sun, 18 Mar 2018 21:14:53 +0100
>> [1] I'm using cquery for my C++ editing needs, which comes with an
>> overlay-based semantic highlighting mechanism. With my emacs
>> configuration, lsp-mode/lsp-ui emit 6 calls to line-number-at-pos per
>> character insertion, which consume ~20 to 25 ms each when performing
>> edits close to the bottom of a 66KB C++ file (measured using
>> (benchmark-run 1000 (line-number-at-pos (point))) on a release build of
>> emacs-27/git commit #9942734...). Using the noverlay branch, this figure
>> drops to ~160us per call.
> If lsp-mode/lsp-ui needs a fast line counter, one can easily be
> provided by exposing find_newline to Lisp.  IME, it's lightning-fast,
> and should run circles around count-lines (used by line-number-at-pos).
> (I'm not sure I even understand how overlays come into play here,
> btw.)

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