Re: On Tabs and Spaces

[Jeff King <peff@peff.net>]

On Wed, Oct 17, 2007 at 05:59:27PM -0700, Linus Torvalds wrote:

> It happens. We do de-spacification in the kernel occasionally when it is 
> an annoyance. Usually it shows up in patches, though - exactly because 
> code which adds spaces instead of tabs won't line up correctly in the 
> diff.

You have made this claim several times, and I really don't understand
it. If I have 8 spaces, then a diff line will have either " ", "+", or
"-" followed by 8 spaces. If I use a hard tab, then the tab will end up
only taking up 7 spaces because of the nature of tabs.

This might matter if I'm comparing non-diff code to diff code. But in a
diff, _everything_ is indented by exactly one space, so it all lines up.
Is there something I'm missing?

> So it doesn't matter *which* one you use (all spaces or all tabs) in that 
> sense.

Yes, I agree with that (even with an all-tabs policy, there are still
mangled and incorrect patches that come in -- and the maintainer rejects
or fixes them).

Which was what I was trying to point out with my question (though I was
also curious to hear your answer): all-space versus all-tab is largely a
matter of preference. And that means that people who want git to change
to _their_ preference are just being silly.

> And smaller *is* faster. Do something like this on the kernel:
> 
> 	GIT_PAGER= time git grep sched_fair
> 
> and then do the same thing with the kernel sources blown up by 20% by 
> de-tabification. Guess which one is 20% slower?

I was about to tell you that you're full of it, but there really is a
slowdown:

$ cd linux-2.6
$ GIT_PAGER= time git grep sched_fair >/dev/null
0.34user 0.94system 0:01.30elapsed 98%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+7548minor)pagefaults 0swaps

$ find . -name .git -prune -o -type f | xargs perl -pi -e 's/\t/        /g'
$ git-commit -a -m de-tabify
$ git-repack -a -d
$ GIT_PAGER= time git grep sched_fair >/dev/null
0.42user 1.06system 0:01.54elapsed 96%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+7591minor)pagefaults 0swaps

It's actually about 16%.


Gah, I can't believe I've not only been sucked into a tab vs spaces
discussion, but now I've actually wasted time doing a performance
comparison on it.

As an aside, that commit was enough to trigger a "git-gc --auto", which
was my first experience with it. It's actually kind of annoying
(especially since I was about to repack -a -d).

-Peff