Re: [PATCH 0/7] grep.c: teach --column to 'git-grep(1)'

[Jeff King <peff@peff.net>]

On Tue, Jun 19, 2018 at 07:33:39PM +0200, René Scharfe wrote:

> > The key thing about this iteration is that it doesn't regress
> > performance, because we always short-circuit where we used to. The other
> > obvious route is to stop short-circuiting only when "--column" is in
> > effect, which would have the same property (at the expense of a little
> > extra code in match_expr_eval()).
> 
> The performance impact of the exhaustive search for --color scales with
> the number of shown lines, while it would scale with the total number of
> lines for --column.  Coloring the results of highly selective patterns
> is relatively cheap, short-circuiting them still helps significantly.

I thought that at first, too, but I think we'd still scale with the
number of shown lines. We're talking about short-circuiting OR, so by
definition we stop the short-circuit because we matched the first half
of the OR.

If you stop short-circuiting AND, then yes, you incur a penalty for
every line. But I don't think --column would need to do that.

Although there are interesting cases around inversion. For example:

  git grep --not \( --not -e a --and --not -e b \)

is equivalent to:

  git grep -e a --or -e b

Do people care if we actually hunt down the exact column where we
_didn't_ match "b" in the first case?  The two are equivalent, but I
have to wonder if somebody writing the first one really cares.

> Disabling that optimization for --column wouldn't be a regression since
> it's a new option..  Picking a random result (based on the order of
> evaluation) seems sloppy and is probably going to surprise users.

I don't see it as a random result; short-circuiting logic is well
understood and we follow the user's ordering.

I think the place where it's _most_ ugly is "--column --color", where we
may color the short-circuited value in the second pass.

> We could add an optimizer pass to reduce the number of regular
> expressions in certain cases if that is really too slow.  E.g. this:

Yes, we actually discussed this kind of transformation. I think it's way
out of scope for this patch series, though. If we do anything more, I
think it should be to disable short-circuiting when --column is in use.

-Peff