Re: Very promising results with libpcre2

[Junio C Hamano <gitster@pobox.com>]

Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:

> On Sat, Apr 1, 2017 at 12:48 AM, Junio C Hamano <gitster@pobox.com> wrote:
>> Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:
>>
>>> That enables the new JIT support in pcre v2:
>>>
>>>       s/iter    rx fixed   prx
>>> rx      2.19    --  -33%  -44%
>>> fixed   1.47   49%    --  -17%
>>> prx     1.22   79%   20%    --
>>
>> The numbers with JIT does look "interesting".
>>
>> I couldn't quite tell if there are major incompatibilities in the
>> regex language itself between two versions from their documentation,
>> but assuming that there isn't (modulo bugfixes and enhancements) and
>> assuming that we are going to use their standard matcher, it may be
>> OK to just use the newer one without linking both.
>
> There's no incompatibilities in the regex language itself (modulo bugs
> etc). So yeah, I'll prepare some patch to use v2.

Just to make sure that we are on the same page.  While I do not see
the need to link with both variants and allow users to choose
between them at runtime, I do not know if the whole world is ready
to drop pcre1 and use pcre2 (the latter of which has only been
around for a bit over two years).

So we'd probably want to do 

 (1) keep USE_LIBPCRE and enable v1 when set;
 (2) add USE_LIBPCRE2 and enable v2 when set;
 (3) make sure to error out when both are set.

or something like that.  It is tempting to allow us to say

    make USE_LIBPCRE=2

but the existing builds are likely to be depending on "is it set to
anything? then use PCRE1" behaviour, so we unfortunately cannot take
that route.

Thanks.