Re: git grep with leading inverted bracket expression
- Date: Thu, 07 Jun 2018 21:29:48 +0200
- From: Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx>
- Subject: Re: git grep with leading inverted bracket expression
On Thu, Jun 07 2018, Matthew Wilcox wrote:
> On Thu, Jun 07, 2018 at 09:09:25PM +0200, Ævar Arnfjörð Bjarmason wrote:
>> On Thu, Jun 07 2018, Matthew Wilcox wrote:
>> > If the first atom of a regex is a bracket expression with an inverted range,
>> > git grep is very slow.
>> I have some WIP patches to fix all of this, which I'll hopefully submit
>> before 2.19 is out the door.
>> What you've discovered here is how shitty your libc regex engine is,
>> because unless you provide -P and compile with a reasonably up-to-date
>> libpcre (preferably v2) with JIT that's what you'll get.
> I'm using Debian's build, and it is linked against a recent libpcre2:
> $ ldd /usr/lib/git-core/git
> libpcre2-8.so.0 => /usr/lib/x86_64-linux-gnu/libpcre2-8.so.0 (0x00007f59ad5f2000)
> $ dpkg --status libpcre2-8-0
> Version: 10.31-3
> But I wasn't using -P. If I do, then I see the performance numbers you do:
> $ time git grep -P '[^t]truct_size' >/dev/null
> real 0m0.354s
> user 0m0.340s
> sys 0m0.639s
> $ time git grep -P 'struct_size' >/dev/null
> real 0m0.336s
> user 0m0.552s
> sys 0m0.457s
> $ time git grep 'struct_size' >/dev/null
> real 0m0.335s
> user 0m0.535s
> sys 0m0.474s
>> So you need to just use an up-to-date libpcre2 & -P and performance
>> won't suck.
Yeah that's recent enough & will get you all the benefits.
> I don't tend to use terribly advanced regexps, so I'll just set
> grep.patternType to 'perl' and then it'll automatically be fast for me
> without your patches ;-)
Indeed, if you're happy with that that'll do it.
>> My WIP patches will make us use PCRE for all grep modes, using an API it
>> has to convert basic & extended regexp syntax to its own syntax, so
>> we'll be able to do that transparently.
> That's clearly the right answer. Thanks!
Yeah, unfortunately git-grep's default is "basic" regexp which has a
really atrocious syntax that's different enough from extended & Perl's
that we probably couldn't just switch it over.
That won't be needed with my patches, but maybe I'll follow-up with
something to s/basic/extended/g by default, because on side effect of
having the pattern converter is that we could have a warning whenever
the user has a pattern that would be different under extended/perl, so
we can see how common that is.