GNU bug report logs - #24009
[PATCH] grep: use fastmap in regex

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From: "Jens Schleusener" <Jens.Schleusener <at> t-online.de>
To: 24009 <at> debbugs.gnu.org
Subject: bug#24009: [PATCH] grep: use fastmap in regex
Date: Sat, 16 Jul 2016 22:06:53 +0200 (CEST)

Hi Norihiro.

> sed and gawk use fastmap in regex, but grep does not.  By using fastmap,
> I expect that grep speeds up for patterns as regex is used.
>
> before:
> $ time -p env LC_ALL=ja_JP.eucjp src/grep '\([a-b]\)\1' k
> real 7.83
> user 7.62
> sys 0.07
>
> after:
> $ time -p env LC_ALL=ja_JP.eucjp src/grep '\([a-b]\)\1' k
> real 0.46
> user 0.38
> sys 0.07
>
> However, if grep uses fastmap, fails in case-fold-titlecase test.  It
> means that grep's behavior differ from sed and gawk, as they use fastmap,
> although it seems to be a bug in regex.

Wow, that is a spectacular speed improvement. Since I use grep with regex 
patterns heavily in some of my scripts I could not resist to make some 
first simple tests (including your example pattern with a back reference). 
The non-representative results using grep 2.25 shows a gain of a factor 
5-10 (while the unpatched self-compiled grep 2.25 itself was already a 
factor 1.4-2.8 faster than the grep 2.16 offered by the OS (OpenSUSE Leap 
42.1). At least in my tests all the grep outputs  were identical.

By the way I had to remove one of the two "=" in your patch otherwise gcc 
issued an error (but caution, I am a C-layman).

Regards

Jens

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