GNU bug report logs - #38104
27.0.50; elixir-mode fontification is very slow

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Dmitry Gutov <dgutov <at> yandex.ru>
Subject: bug#38104: closed (Re: bug#38104: 27.0.50; elixir-mode
 fontification is very slow)
Date: Wed, 27 Nov 2019 21:59:01 +0000

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Your bug report

#38104: 27.0.50; elixir-mode fontification is very slow

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 38104 <at> debbugs.gnu.org.

-- 
38104: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=38104
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Dmitry Gutov <dgutov <at> yandex.ru>
To: Mattias Engdegård <mattiase <at> acm.org>
Cc: 38104-done <at> debbugs.gnu.org
Subject: Re: bug#38104: 27.0.50; elixir-mode fontification is very slow
Date: Wed, 27 Nov 2019 23:58:46 +0200

Hi Mattias,

On 26.11.2019 21:32, Mattias Engdegård wrote:

> As it turned out, rx is fine (now); elixir-mode, not quite. In elixir-mode.el, we have
> 
>        (identifiers . ,(rx (one-or-more (any "A-Z" "a-z" "_"))
>                            (zero-or-more (any "A-Z" "a-z" "0-9" "_"))
>                            (optional (or "?" "!"))))
> 
> First, this regex is suboptimal: the first character of an identifier should occur exactly once, or you get bad backtracking behaviour. Just remove the one-or-more construct:
> 
>        (identifiers . ,(rx (any "A-Z" "a-z" "_")
>                            (zero-or-more (any "A-Z" "a-z" "0-9" "_"))
>                            (optional (or "?" "!"))))
> 
> This definition is then used in several places, but two in particular are of interest to us:
> 
>      ;; Module attributes
>      (,(elixir-rx (and "@" (1+ identifiers)))
> 
> The construct (1+ identifiers) was perhaps meant to match multiple identifiers, but it doesn't (no separator); it just matches an identifier in several ways, which again leads to bad backtracking behaviour.
> The same problem here:
> 
>      ;; Map keys
>      (,(elixir-rx (group (and (one-or-more identifiers) ":")) space)
> 
> Remove the 1+ and one-or-more and it's fast again.

That makes a lot of sense. I removed these one-or-more's and 1+ (and a 
few others), and it became fast again.

I'll send a patch upstream. Thanks for your help!

(Looking at the tracker, they have a minor version of this change 
submitted already).

> Why did this "work" with the old rx implementation? Because that code had a nasty bug: it does not bracket definitions in rx-constituents properly. Example:
> 
> (let ((rx-constituents (cons '(hello . "HELLO") rx-constituents)))
>    (rx-to-string '(1+ hello) t))
> => "HELLO+"
> 
> The new rx implementation does not suffer from this bug.
> 
> The result in your case is that the old rx, when translating (1+ identifiers), only tacked the "+" onto whatever regexp 'identifiers' produced, resulting in
> 
> "[A-Z_a-z]+[0-9A-Z_a-z]*[!?]?+"
> 
> which is a lot faster, since only the final [!?] is repeated twice (and it probably doesn't match very often).

It's funny to think how someone probably beaten the current code into 
submission by trial and error.

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From: Dmitry Gutov <dgutov <at> yandex.ru>
To: bug-gnu-emacs <at> gnu.org
Subject: 27.0.50; elixir-mode fontification is very slow
Date: Thu, 7 Nov 2019 17:40:11 +0200

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I haven't been able to track this to a particular component (e.g. a
regexp) for now, but font-lock-fontify-region is now considerably slower
than it was in Emacs 26 (at least at revision cb8fb597e5bf4f14).

To reproduce: install elixir-mode (e.g. from MELPA Stable):

(add-to-list 'package-archives
             '("melpa-stable" . "https://stable.melpa.org/packages/") t)

M-x list-packages, install elixir-mode.

Savet the attached tiny.__ex__ as tiny.ex.

Visit tiny.ex.

Eval: (benchmark 1 '(font-lock-fontify-region (point-min) (point-max))).

"Elapsed time: 0.158824s"

With larger files, the times are much longer.

I had a break from Elixir, so I noticed this only now.

In GNU Emacs 27.0.50 (build 11, x86_64-pc-linux-gnu, GTK+ Version 3.24.8)
 of 2019-11-05 built on potemkin
Repository revision: dd19cc3aa16ccc441a8a2bfcdeb3005a6eef2543
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Ubuntu 19.04

[tiny.__ex__ (text/plain, attachment)]

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