Re: State of the overlay tree branch?

[Sebastian Sturm]

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.)