Re: [RFC PATCH v2] grep: allow for run time disabling of JIT in PCRE

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Jul 31, 2019, 12:36 UTC
Message-ID: <nycvar.QRO.7.76.6.1907311434080.21907@tvgsbejvaqbjf.bet>
In-Reply-To: <CAPUEspjo3Uo8KtSYQ=uh+_gPEjb+dBdSPgsEVE1j1bOMPF=0DA@mail.gmail.com>

Hi Carlo,

On Mon, 29 Jul 2019, Carlo Arenas wrote:

Show 19 quoted lines

> On Mon, Jul 29, 2019 at 10:47 AM Junio C Hamano <gitster@pobox.com> wrote:
> > René Scharfe <l.s.r@web.de> writes:
> > >> +pcre.jit::
> > >> +    If set to false, disable JIT when using PCRE.  Defaults to
> > >> +    true.
> > >> +    if set to -1 will try first to use JIT and fallback to the
> > >> +    interpreter instead of returning an error.
> > >
> > > Why not implement only -1, without adding this config setting?
> >
> > ... nor command line option.  If we have an auto-fallback, I would
> > think that makes the most sense.  IIRC the first iteration with only
> > the configuration was really about working around the (non-working)
> > pcre-jit---if we can self-detect and skip a non-working case, that
> > would allow us to drop end-user facing knobs, which is ideal.
>
> because that was proposed earlier[1] and wasn't accepted ;)
> [...]
> [1] https://public-inbox.org/git/20181209230024.43444-3-carenas@gmail.com/

For the record, I read https://public-inbox.org/git/xmqqh8flkgs2.fsf@gitster-ct.c.googlers.com/ as encouraging a slightly more powerful argument in favor. Junio seemed to hope that PCRE2's own `pcre2grep` would behave that way, and that would give us plenty reason to just imitate it.

I don't know whether `pcre2grep` behaves that way, even if it does not, I think the benefits of the auto fallback to the end user are considerable.

Ciao, Dscho