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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
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.

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!