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Re: State of the overlay tree branch?

From: Sebastien Chapuis
Subject: Re: State of the overlay tree branch?
Date: Wed, 21 Mar 2018 15:14:18 +0100
User-agent: mu4e 0.9.19; emacs 27.0.50

Eli Zaretskii writes:

>> 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.)

The language server protocol defines a position in file with zero-indexed
line and column offsets [1]:

interface Position {
        line: number;
        character: number;

lsp-mode uses heavily line-number-at-pos to convert an Emacs buffer
point to a LSP position, and vice-versa [2].
This can happen thousands times on each keystroke.

If Emacs could provide a function to do the conversion very fast (or at
least faster than with line-number-at-pos), it would be great.

[2] The conversion from a LSP position to point doesn't use
line-number-at-pos, but forward-line and forward-char. It's still very slow.

Sebastien Chapuis

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