[PATCH 2/3] grep: stop "using" a custom JIT stack with PCRE v2

As reported in [1] the code I added in 94da9193a6 ("grep: add support for PCRE v2", 2017-06-01) to use a custom JIT stack has never worked. It was incorrectly copy/pasted from code I added in fbaceaac47 ("grep: add support for the PCRE v1 JIT API", 2017-05-25), which did work.

Thus our intention of starting with 1 byte of stack at a maximum of 1 MB didn't happen, we'd always use the 32 KB stack provided by PCRE v2's jit_machine_stack_exec()[2]. The reason I allocated a custom stack at all was this advice in pcrejit(3) (same in pcre2jit(3)):

    "By default, it uses 32KiB on the machine stack. However, some
    large or complicated patterns need more than this"

Since we've haven't had any reports of users running into PCRE2_ERROR_JIT_STACKLIMIT in the wild I think we can safely assume that we can just use the library defaults instead and drop this code. This won't change with the wider use of PCRE v2 in ed0479ce3d ("Merge branch 'ab/no-kwset' into next", 2019-07-15), a fixed string search is not a "large or complicated pattern".

For good measure I ran the performance test noted in 94da9193a6, although the command is simpler now due to my 0f50c8e32c ("Makefile: remove the NO_R_TO_GCC_LINKER flag", 2019-05-17):

    GIT_PERF_REPEAT_COUNT=30 GIT_PERF_LARGE_REPO=~/g/linux GIT_PERF_MAKE_OPTS='-j8 USE_LIBPCRE2=Y CFLAGS=-O3 LIBPCREDIR=/home/avar/g/pcre2/inst' ./run HEAD~ HEAD p7820-grep-engines.sh

Just the /perl/ results are:

    Test                                            HEAD~             HEAD
    ---------------------------------------------------------------------------------------
    7820.3: perl grep 'how.to'                      0.17(0.27+0.65)   0.17(0.24+0.68) +0.0%
    7820.7: perl grep '^how to'                     0.16(0.23+0.66)   0.16(0.23+0.67) +0.0%
    7820.11: perl grep '[how] to'                   0.18(0.35+0.62)   0.18(0.33+0.65) +0.0%
    7820.15: perl grep '(e.t[^ ]*|v.ry) rare'       0.17(0.45+0.54)   0.17(0.49+0.50) +0.0%
    7820.19: perl grep 'm(ú|u)lt.b(æ|y)te'          0.16(0.33+0.58)   0.16(0.29+0.62) +0.0%

So, as expected there's no change, and running with valgrind reveals that we have fewer allocations now.

1. https://public-inbox.org/git/20190721194052.15440-1-carenas@gmail.com/
2. I didn't really intend to start with 1 byte, looking at the PCRE v2
   code again what happened is that I cargo-culted some of PCRE v2's
   own test code which was meant to test re-allocations. It's more
   sane to start with say 32 KB with a max of 1 MB, as pcre2grep.c
   does.

Reported-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 grep.c | 10 ----------
 grep.h |  4 ----
 2 files changed, 14 deletions(-)