Re: Tabs in commit messages - de-tabify option in strbuf_stripspace()?

From: Jeff King <peff@peff.net>
Date: Mar 16, 2016, 05:17 UTC
Message-ID: <20160316051748.GA3524@sigill.intra.peff.net>
In-Reply-To: <CA+55aFyFHmmoHMgRkuBqNUho=tiJ=VwxHWzcGw3pRjr+aGS7ZQ@mail.gmail.com>

On Tue, Mar 15, 2016 at 09:57:16PM -0700, Linus Torvalds wrote:

Show 18 quoted lines

> On Tue, Mar 15, 2016 at 9:46 PM, Junio C Hamano <gitster@pobox.com> wrote:
> >
> > It also ignores that byte counts of non-HT bytes may not necessarily
> > match display columns.  There is utf8_width() function exported from
> > utf8.c for that purpose.
> 
> Hmm. I did that to now make it horribly slower. Doing the
> per-character widths really does horrible things for performance.
> 
> That said, I think I can make it do the fast thing for lines that have
> no tabs at all, which is the big bulk of it. So it would do the width
> calculations only in the rare "yes, I found a tab" case.
> 
> I already wrote it in a way that non-tab lines end up just looping
> over the line looking for a tab and then fall back to the old code.
> 
> I might just have to make that behavior a bit more explicit. Let me
> stare at it a bit, but it won't happen until tomorrow, I think.

I wondered about performance when reading yours, and measured a straight "git log >/dev/null" on the kernel at about 3% slower (best-of-five and average). We can get most of that back by sticking a

    memchr(line, '\t', linelen);

in front and skipping the loop if it returns NULL. You can also implement your loop in terms of memchr() calls to skip to the next tab, but I don't know if it's worth it. It's really only worth spending time optimizing the non-tab case (and even then, only worth it for trivial things like "use the already optimized-as-hell memchr").

-Peff