grep: add support for the PCRE v1 JIT API

grep: add support for the PCRE v1 JIT API

Change the grep PCRE v1 code to use JIT when available. When PCRE
support was initially added in commit 63e7e9d8b6 ("git-grep: Learn
PCRE", 2011-05-09) PCRE had no JIT support, it was integrated into
8.20 released on 2011-10-21.

Enabling JIT support usually improves performance by more than
40%. The pattern compilation times are relatively slower, but those
relative numbers are tiny, and are easily made back in all but the
most trivial cases of grep. Detailed benchmarks & overview of
compilation times is at: http://sljit.sourceforge.net/pcre.html

With this change the difference in a t/perf/p7820-grep-engines.sh run
is, with just the /perl/ tests shown:

    $ GIT_PERF_REPEAT_COUNT=30 GIT_PERF_LARGE_REPO=~/g/linux GIT_PERF_MAKE_OPTS='-j8 USE_LIBPCRE=YesPlease CC=~/perl5/installed/bin/gcc NO_R_TO_GCC_LINKER=YesPlease CFLAGS=-O3 LIBPCREDIR=/home/avar/g/pcre/inst LDFLAGS=-Wl,-rpath,/home/avar/g/pcre/inst/lib' ./run HEAD~ HEAD p7820-grep-engines.sh
    Test                                           HEAD~             HEAD
    ---------------------------------------------------------------------------------------
    7820.3: perl grep 'how.to'                      0.35(1.11+0.43)   0.23(0.42+0.46) -34.3%
    7820.7: perl grep '^how to'                     0.64(2.71+0.36)   0.27(0.66+0.44) -57.8%
    7820.11: perl grep '[how] to'                   0.63(2.51+0.42)   0.33(0.98+0.39) -47.6%
    7820.15: perl grep '(e.t[^ ]*|v.ry) rare'       1.17(5.61+0.35)   0.34(1.08+0.46) -70.9%
    7820.19: perl grep 'm(ú|u)lt.b(æ|y)te'          0.43(1.52+0.44)   0.30(0.88+0.42) -30.2%

The conditional support for JIT is implemented as suggested in the
pcrejit(3) man page. E.g. defining PCRE_STUDY_JIT_COMPILE to 0 if it's
not present.

The implementation is relatively verbose because even if
PCRE_CONFIG_JIT is defined only a call to pcre_config() can determine
if the JIT is available, and if so the faster pcre_jit_exec() function
should be called instead of pcre_exec(), and a different (but not
complimentary!) function needs to be called to free pcre1_extra_info.

There's no graceful fallback if pcre_jit_stack_alloc() fails under
PCRE_CONFIG_JIT, instead the program will simply abort. I don't think
this is worth handling gracefully, it'll only fail in cases where
malloc() doesn't work, in which case we're screwed anyway.

That there's no assignment of `p->pcre1_jit_on = 0` when
PCRE_CONFIG_JIT isn't defined isn't a bug. The create_grep_pat()
function allocates the grep_pat allocates it with calloc(), so it's
guaranteed to be 0 when PCRE_CONFIG_JIT isn't defined.

I you're bisecting and find this change, check that your PCRE isn't
older than 8.32. This change intentionally broke really old versions
of PCRE, but that's fixed in follow-up commits.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>