Re: git grep with leading inverted bracket expression
- Date: Thu, 7 Jun 2018 12:22:13 -0700
- From: Matthew Wilcox <willy@xxxxxxxxxxxxx>
- Subject: Re: git grep with leading inverted bracket expression
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
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
$ time git grep -P 'struct_size' >/dev/null
$ time git grep 'struct_size' >/dev/null
> So you need to just use an up-to-date libpcre2 & -P and performance
> won't suck.
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 ;-)
> 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!