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From: Jeff King <peff@peff.net>
To: Matthieu S <matthieu.stigler@gmail.com>
Cc: git@vger.kernel.org
Subject: Re: Fwd: git diff with “--word-diff-regex” extremely slow compared to “--word-diff”?
Date: Sun, 20 Nov 2016 15:17:44 -0500	[thread overview]
Message-ID: <20161120201744.7ym4gsmjoijw6oow@sigill.intra.peff.net> (raw)
In-Reply-To: <CAEYvigLz3muWD-QFjMZUn=H3RQoxhTYX9EwB6=aiMjWOEN3CBA@mail.gmail.com>

On Fri, Nov 18, 2016 at 03:40:22PM -0800, Matthieu S wrote:

> Why is the speed so different if one uses --word-diff instead of
> --word-diff-regex= ? Is it just because my expression is (slightly)
> more complex than the default one (split on period instead of only
> whitespace) ? Or is it that the default word-diff is implemented
> differently/more efficiently? How can I overcome this speed slowdown?

I think it's probably both.

See diff.c:find_word_boundaries(). If there's no regex, we use a simple
loop over isspace() to find the boundaries. I don't recall anybody
measuring the performance before, but I'm not surprised to hear that
matching a regex is slower.

If I look at the output of "perf", though, it looks like we also spend a
lot more time in xdl_clean_mmatch(). Which isn't surprising. Your regex
treats commas as boundaries, which is going to generate a lot more
matches for this particular data set (though the output is the same, I
think, because of the nature of the change).

I would have expected "--word-diff-regex=[^[:space:]]" to be faster than
your regex, though, and it does not seem to be.

-Peff

  reply	other threads:[~2016-11-20 20:17 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CAEYvigJ14xYDmRG2N0yTgM4spaaB7s9923w0+e9+QQEeFz0NTQ@mail.gmail.com>
2016-11-18 23:40 ` Fwd: git diff with “--word-diff-regex” extremely slow compared to “--word-diff”? Matthieu S
2016-11-20 20:17   ` Jeff King [this message]
2016-11-22 18:08     ` Matthieu S
2016-11-22 19:26       ` Jeff King

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