Re: [PATCH] grep: do not do external grep on skip-worktree entries

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Jan 11, 2010, 21:24 UTC
Message-ID: <alpine.LFD.2.00.1001111315470.17145@localhost.localdomain>
In-Reply-To: <4c8ef71001111307q6679039ajbef22f2e1748df56@mail.gmail.com>

On Mon, 11 Jan 2010, Fredrik Kuivinen wrote:

Show 5 quoted lines

> 
> Another option is to use memmem instead. As we know the length of the
> buffer already it should be a slight improvement over strstr for
> everyone. memmem may cause some portability problems though as it is a
> GNU extension.

I'd almost prefer to just drop the strstr entirely.

It's not actually all *that* big a win, even on my machine. I get

 - strstr:

        real    0m0.309s
        user    0m0.168s
        sys     0m0.136s

 - regexec:

	real	0m0.410s
	user	0m0.220s
	sys	0m0.116s

so yeah, it's slower, but not by a huge degree. With strstr, "git grep" actually beats the external grep for me, but I don't really care. It's already way better than it used to be - and clearly strstr has a lot of potential problems.

Sure memmem() might be better for you than strstr, but on the other hand, it might easily be worse than strstr for others - and not just from a portability standpoint. Is memmem() optimized to take advantage of SSE4.2? I suspect it is not, exactly _because_ it's a GNU extension, so Intel hasn't published optimized sample code for people to use.

So I would argue against even bothering to try memmem. Especially since in your case, regexec() is apparently faster than memmem _anyway_. I expect that it is for me too, but I'm too lazy to check.

			Linus