Re: [PATCH v2 0/9] grep: move from kwset to optional PCRE v2

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Fri, Jun 28 2019, Ævar Arnfjörð Bjarmason wrote:

> A non-RFC since it seem people like this approach.
>
> This should fix the test failure noted by Johannes, there's two new
> patches at the start of this series. They address a bug that was there
> for a long time, but I happened to trip over since PCRE is more strict
> about UTF-8 validation than kwset (which doesn't care at all).
>
> I also added performance numbers to the relevant commit messages, took
> brian's suggestion of saying "NUL-byte" instead of "\0", and did some
> other copyediting of my own.
>
> The rest of the code changes are all just comments & rewording of
> previously added comments.

Junio. I thought I'd submit this in before your merge to "next", but I
see that happened. Are you OK with rewinding it for this (& maybe
something else) or should I submit a v3 rebased on "next"?

I'd really prefer the improved commit messages with performance numbers,
and thought I'd have time to work on those details since it was an
RFC/PATCH :)

> Ævar Arnfjörð Bjarmason (9):
>   log tests: test regex backends in "--encode=<enc>" tests
>   grep: don't use PCRE2?_UTF8 with "log --encoding=<non-utf8>"
>   grep: inline the return value of a function call used only once
>   grep tests: move "grep binary" alongside the rest
>   grep tests: move binary pattern tests into their own file
>   grep: make the behavior for NUL-byte in patterns sane
>   grep: drop support for \0 in --fixed-strings <pattern>
>   grep: remove the kwset optimization
>   grep: use PCRE v2 for optimized fixed-string search
>
>  Documentation/git-grep.txt                    |  17 +++
>  grep.c                                        | 115 +++++++---------
>  grep.h                                        |   3 +-
>  revision.c                                    |   3 +
>  t/t4210-log-i18n.sh                           |  39 +++++-
>  ...a1.sh => t7008-filter-branch-null-sha1.sh} |   0
>  ...08-grep-binary.sh => t7815-grep-binary.sh} | 101 --------------
>  t/t7816-grep-binary-pattern.sh                | 127 ++++++++++++++++++
>  8 files changed, 233 insertions(+), 172 deletions(-)
>  rename t/{t7009-filter-branch-null-sha1.sh => t7008-filter-branch-null-sha1.sh} (100%)
>  rename t/{t7008-grep-binary.sh => t7815-grep-binary.sh} (55%)
>  create mode 100755 t/t7816-grep-binary-pattern.sh
>
> Range-diff:
>  -:  ---------- >  1:  cfc01f49d3 log tests: test regex backends in "--encode=<enc>" tests
>  -:  ---------- >  2:  4b59eb32f0 grep: don't use PCRE2?_UTF8 with "log --encoding=<non-utf8>"
>  1:  ad55d3be7e =  3:  cc4d3b50d5 grep: inline the return value of a function call used only once
>  2:  650bcc8582 =  4:  d9b29bdd89 grep tests: move "grep binary" alongside the rest
>  3:  ef10a8820d !  5:  f85614f435 grep tests: move binary pattern tests into their own file
>     @@ -2,9 +2,10 @@
>
>          grep tests: move binary pattern tests into their own file
>
>     -    Move the tests for "-f <file>" where "<file>" contains a "\0" pattern
>     -    into their own file. I added most of these tests in 966be95549 ("grep:
>     -    add tests to fix blind spots with \0 patterns", 2017-05-20).
>     +    Move the tests for "-f <file>" where "<file>" contains a NUL byte
>     +    pattern into their own file. I added most of these tests in
>     +    966be95549 ("grep: add tests to fix blind spots with \0 patterns",
>     +    2017-05-20).
>
>          Whether a regex engine supports matching binary content is very
>          different from whether it matches binary patterns. Since
>     @@ -14,8 +15,8 @@
>          engine can sensibly match binary patterns.
>
>          Since 9eceddeec6 ("Use kwset in grep", 2011-08-21) we've been punting
>     -    patterns containing "\0" and considering them fixed, except in cases
>     -    where "--ignore-case" is provided and they're non-ASCII, see
>     +    patterns containing NUL-byte and considering them fixed, except in
>     +    cases where "--ignore-case" is provided and they're non-ASCII, see
>          5c1ebcca4d ("grep/icase: avoid kwsset on literal non-ascii strings",
>          2016-06-25). Subsequent commits will change this behavior.
>
>  4:  03e5637efc !  6:  90afca8707 grep: make the behavior for \0 in patterns sane
>     @@ -1,12 +1,13 @@
>      Author: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
>
>     -    grep: make the behavior for \0 in patterns sane
>     +    grep: make the behavior for NUL-byte in patterns sane
>
>     -    The behavior of "grep" when patterns contained "\0" has always been
>     -    haphazard, and has served the vagaries of the implementation more than
>     -    anything else. A "\0" in a pattern can only be provided via "-f
>     -    <file>", and since pickaxe (log search) has no such flag "\0" in
>     -    patterns has only ever been supported by "grep".
>     +    The behavior of "grep" when patterns contained a NUL-byte has always
>     +    been haphazard, and has served the vagaries of the implementation more
>     +    than anything else. A pattern containing a NUL-byte can only be
>     +    provided via "-f <file>". Since pickaxe (log search) has no such flag
>     +    the NUL-byte in patterns has only ever been supported by "grep" (and
>     +    not "log --grep").
>
>          Since 9eceddeec6 ("Use kwset in grep", 2011-08-21) patterns containing
>          "\0" were considered fixed. In 966be95549 ("grep: add tests to fix
>     @@ -14,9 +15,9 @@
>          behavior.
>
>          Change the behavior to do the obvious thing, i.e. don't silently
>     -    discard a regex pattern and make it implicitly fixed just because it
>     -    contains a \0. Instead die if e.g. --basic-regexp is combined with
>     -    such a pattern.
>     +    discard a regex pattern and make it implicitly fixed just because they
>     +    contain a NUL-byte. Instead die if the backend in question can't
>     +    handle them, e.g. --basic-regexp is combined with such a pattern.
>
>          This is desired because from a user's point of view it's the obvious
>          thing to do. Whether we support BRE/ERE/Perl syntax is different from
>  5:  b9aad3ec1c !  7:  526b925fdc grep: drop support for \0 in --fixed-strings <pattern>
>     @@ -2,15 +2,14 @@
>
>          grep: drop support for \0 in --fixed-strings <pattern>
>
>     -    Change "-f <file>" to not support patterns with "\0" in them under
>     -    --fixed-strings, we'll now only support these under --perl-regexp with
>     -    PCRE v2.
>     +    Change "-f <file>" to not support patterns with a NUL-byte in them
>     +    under --fixed-strings. We'll now only support these under
>     +    "--perl-regexp" with PCRE v2.
>
>     -    A previous change to Documentation/git-grep.txt changed the
>     -    description of "-f <file>" to be vague enough as to not promise that
>     -    this would work, and by dropping support for this we make it a whole
>     -    lot easier to move away from the kwset backend, which a subsequent
>     -    change will try to do.
>     +    A previous change to grep's documentation changed the description of
>     +    "-f <file>" to be vague enough as to not promise that this would work.
>     +    By dropping support for this we make it a whole lot easier to move
>     +    away from the kwset backend, which we'll do in a subsequent change.
>
>          Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
>
>  6:  3587be009a !  8:  14269bb295 grep: remove the kwset optimization
>     @@ -2,9 +2,99 @@
>
>          grep: remove the kwset optimization
>
>     -    A later change will replace this optimization with a different one,
>     -    but as removing it and running the tests demonstrates no grep
>     -    semantics depend on this backend anymore.
>     +    A later change will replace this optimization with optimistic use of
>     +    PCRE v2. I'm completely removing it as an intermediate step, as
>     +    opposed to replacing it with PCRE v2, to demonstrate that no grep
>     +    semantics depend on this (or any other) optimization for the fixed
>     +    backend anymore.
>     +
>     +    For now this is mostly (but not entirely) a performance regression, as
>     +    shown by this hacky one-liner:
>     +
>     +        for opt in '' ' -i'
>     +            do
>     +            GIT_PERF_7821_GREP_OPTS=$opt GIT_PERF_REPEAT_COUNT=10 GIT_PERF_LARGE_REPO=~/g/linux GIT_PERF_MAKE_OPTS='-j8 CFLAGS=-O3 USE_LIBPCRE=YesPlease' ./run origin/master HEAD -- p7821-grep-engines-fixed.sh
>     +        done &&
>     +        for opt in '' ' -i'
>     +            do GIT_PERF_4221_LOG_OPTS=$opt GIT_PERF_REPEAT_COUNT=10 GIT_PERF_LARGE_REPO=~/g/linux GIT_PERF_MAKE_OPTS='-j8 CFLAGS=-O3 USE_LIBPCRE=YesPlease' ./run origin/master HEAD -- p4221-log-grep-engines-fixed.sh
>     +        done
>     +
>     +    Which produces:
>     +
>     +    plain grep:
>     +
>     +        Test                             origin/master     HEAD
>     +        -------------------------------------------------------------------------
>     +        7821.1: fixed grep int           0.55(1.60+0.63)   0.82(3.11+0.51) +49.1%
>     +        7821.2: basic grep int           0.62(1.68+0.49)   0.85(3.02+0.52) +37.1%
>     +        7821.3: extended grep int        0.61(1.63+0.53)   0.91(3.09+0.44) +49.2%
>     +        7821.4: perl grep int            0.55(1.60+0.57)   0.41(0.93+0.57) -25.5%
>     +        7821.6: fixed grep uncommon      0.20(0.50+0.44)   0.35(1.27+0.42) +75.0%
>     +        7821.7: basic grep uncommon      0.20(0.49+0.45)   0.35(1.29+0.41) +75.0%
>     +        7821.8: extended grep uncommon   0.20(0.45+0.48)   0.35(1.25+0.44) +75.0%
>     +        7821.9: perl grep uncommon       0.20(0.53+0.41)   0.16(0.24+0.49) -20.0%
>     +        7821.11: fixed grep æ            0.35(1.27+0.40)   0.25(0.82+0.39) -28.6%
>     +        7821.12: basic grep æ            0.35(1.28+0.38)   0.25(0.75+0.44) -28.6%
>     +        7821.13: extended grep æ         0.36(1.21+0.46)   0.25(0.86+0.35) -30.6%
>     +        7821.14: perl grep æ             0.35(1.33+0.34)   0.16(0.26+0.47) -54.3%
>     +
>     +    grep with -i:
>     +
>     +        Test                                origin/master     HEAD
>     +        -----------------------------------------------------------------------------
>     +        7821.1: fixed grep -i int           0.61(1.84+0.64)   1.11(4.12+0.64) +82.0%
>     +        7821.2: basic grep -i int           0.72(1.86+0.57)   1.15(4.48+0.49) +59.7%
>     +        7821.3: extended grep -i int        0.94(1.83+0.60)   1.53(4.12+0.58) +62.8%
>     +        7821.4: perl grep -i int            0.66(1.82+0.59)   0.55(1.08+0.58) -16.7%
>     +        7821.6: fixed grep -i uncommon      0.21(0.51+0.44)   0.44(1.74+0.34) +109.5%
>     +        7821.7: basic grep -i uncommon      0.21(0.55+0.41)   0.44(1.72+0.40) +109.5%
>     +        7821.8: extended grep -i uncommon   0.21(0.57+0.39)   0.42(1.64+0.45) +100.0%
>     +        7821.9: perl grep -i uncommon       0.21(0.48+0.48)   0.17(0.30+0.45) -19.0%
>     +        7821.11: fixed grep -i æ            0.25(0.73+0.45)   0.25(0.75+0.45) +0.0%
>     +        7821.12: basic grep -i æ            0.25(0.71+0.49)   0.26(0.77+0.44) +4.0%
>     +        7821.13: extended grep -i æ         0.25(0.75+0.44)   0.25(0.74+0.46) +0.0%
>     +        7821.14: perl grep -i æ             0.17(0.26+0.48)   0.16(0.20+0.52) -5.9%
>     +
>     +    plain log:
>     +
>     +        Test                                     origin/master     HEAD
>     +        ---------------------------------------------------------------------------------
>     +        4221.1: fixed log --grep='int'           7.31(7.06+0.21)   8.11(7.85+0.20) +10.9%
>     +        4221.2: basic log --grep='int'           7.30(6.94+0.27)   8.16(7.89+0.19) +11.8%
>     +        4221.3: extended log --grep='int'        7.34(7.05+0.21)   8.08(7.76+0.25) +10.1%
>     +        4221.4: perl log --grep='int'            7.27(6.94+0.24)   7.05(6.76+0.25) -3.0%
>     +        4221.6: fixed log --grep='uncommon'      6.97(6.62+0.32)   7.86(7.51+0.30) +12.8%
>     +        4221.7: basic log --grep='uncommon'      7.05(6.69+0.29)   7.89(7.60+0.28) +11.9%
>     +        4221.8: extended log --grep='uncommon'   6.89(6.56+0.32)   7.99(7.66+0.24) +16.0%
>     +        4221.9: perl log --grep='uncommon'       7.02(6.66+0.33)   6.97(6.54+0.36) -0.7%
>     +        4221.11: fixed log --grep='æ'            7.37(7.03+0.33)   7.67(7.30+0.31) +4.1%
>     +        4221.12: basic log --grep='æ'            7.41(7.00+0.31)   7.60(7.28+0.26) +2.6%
>     +        4221.13: extended log --grep='æ'         7.35(6.96+0.38)   7.73(7.31+0.34) +5.2%
>     +        4221.14: perl log --grep='æ'             7.43(7.10+0.32)   6.95(6.61+0.27) -6.5%
>     +
>     +    log with -i:
>     +
>     +        Test                                        origin/master     HEAD
>     +        ------------------------------------------------------------------------------------
>     +        4221.1: fixed log -i --grep='int'           7.40(7.05+0.23)   8.66(8.38+0.20) +17.0%
>     +        4221.2: basic log -i --grep='int'           7.39(7.09+0.23)   8.67(8.39+0.20) +17.3%
>     +        4221.3: extended log -i --grep='int'        7.29(6.99+0.26)   8.69(8.31+0.26) +19.2%
>     +        4221.4: perl log -i --grep='int'            7.42(7.16+0.21)   7.14(6.80+0.24) -3.8%
>     +        4221.6: fixed log -i --grep='uncommon'      6.94(6.58+0.35)   8.43(8.04+0.30) +21.5%
>     +        4221.7: basic log -i --grep='uncommon'      6.95(6.62+0.31)   8.34(7.93+0.32) +20.0%
>     +        4221.8: extended log -i --grep='uncommon'   7.06(6.75+0.25)   8.32(7.98+0.31) +17.8%
>     +        4221.9: perl log -i --grep='uncommon'       6.96(6.69+0.26)   7.04(6.64+0.32) +1.1%
>     +        4221.11: fixed log -i --grep='æ'            7.92(7.55+0.33)   7.86(7.44+0.34) -0.8%
>     +        4221.12: basic log -i --grep='æ'            7.88(7.49+0.32)   7.84(7.46+0.34) -0.5%
>     +        4221.13: extended log -i --grep='æ'         7.91(7.51+0.32)   7.87(7.48+0.32) -0.5%
>     +        4221.14: perl log -i --grep='æ'             7.01(6.59+0.35)   6.99(6.64+0.28) -0.3%
>     +
>     +    Some of those, as noted in [1] are because PCRE is faster at finding
>     +    fixed strings. This looks bad for some engines, but in the next change
>     +    we'll optimistically use PCRE v2 for all of these, so it'll look
>     +    better.
>     +
>     +    1. https://public-inbox.org/git/87v9x793qi.fsf@evledraar.gmail.com/
>
>          Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
>
>  7:  5bc25c03b8 !  9:  c0fd75d102 grep: use PCRE v2 for optimized fixed-string search
>     @@ -7,19 +7,95 @@
>          slower than PCRE v1 and v2 JIT with the kwset backend, so that
>          optimization was counterproductive.
>
>     -    This brings back the optimization for "-F", without changing the
>     -    semantics of "\0" in patterns. As seen in previous commits in this
>     -    series we could support it now, but I'd rather just leave that
>     -    edge-case aside so the tests don't need to do one thing or the other
>     -    depending on what --fixed-strings backend we're using.
>     -
>     -    I could also support the v1 backend here, but that would make the code
>     -    more complex, and I'd rather aim for simplicity here and in future
>     +    This brings back the optimization for "--fixed-strings", without
>     +    changing the semantics of having a NUL-byte in patterns. As seen in
>     +    previous commits in this series we could support it now, but I'd
>     +    rather just leave that edge-case aside so we don't have one behavior
>     +    or the other depending what "--fixed-strings" backend we're using. It
>     +    makes the behavior harder to understand and document, and makes tests
>     +    for the different backends more painful.
>     +
>     +    I could also support the PCRE v1 backend here, but that would make the
>     +    code more complex. I'd rather aim for simplicity here and in future
>          changes to the diffcore. We're not going to have someone who
>          absolutely must have faster search, but for whom building PCRE v2
>          isn't acceptable.
>
>     -    1. https://public-inbox.org/git/87v9x793qi.fsf@evledraar.gmail.com/
>     +    The difference between this series of commits and the current "master"
>     +    is, using the same t/perf commands shown in the last commit:
>     +
>     +    plain grep:
>     +
>     +        Test                             origin/master     HEAD
>     +        -------------------------------------------------------------------------
>     +        7821.1: fixed grep int           0.55(1.67+0.56)   0.41(0.98+0.60) -25.5%
>     +        7821.2: basic grep int           0.58(1.65+0.52)   0.41(0.96+0.57) -29.3%
>     +        7821.3: extended grep int        0.57(1.66+0.49)   0.42(0.93+0.60) -26.3%
>     +        7821.4: perl grep int            0.54(1.67+0.50)   0.43(0.88+0.65) -20.4%
>     +        7821.6: fixed grep uncommon      0.21(0.52+0.42)   0.16(0.24+0.51) -23.8%
>     +        7821.7: basic grep uncommon      0.20(0.49+0.45)   0.17(0.28+0.47) -15.0%
>     +        7821.8: extended grep uncommon   0.20(0.54+0.39)   0.16(0.25+0.50) -20.0%
>     +        7821.9: perl grep uncommon       0.20(0.58+0.36)   0.16(0.23+0.50) -20.0%
>     +        7821.11: fixed grep æ            0.35(1.24+0.43)   0.16(0.23+0.50) -54.3%
>     +        7821.12: basic grep æ            0.36(1.29+0.38)   0.16(0.20+0.54) -55.6%
>     +        7821.13: extended grep æ         0.35(1.23+0.44)   0.16(0.24+0.50) -54.3%
>     +        7821.14: perl grep æ             0.35(1.33+0.34)   0.16(0.28+0.46) -54.3%
>     +
>     +    grep with -i:
>     +
>     +        Test                                origin/master     HEAD
>     +        ----------------------------------------------------------------------------
>     +        7821.1: fixed grep -i int           0.62(1.81+0.70)   0.47(1.11+0.64) -24.2%
>     +        7821.2: basic grep -i int           0.67(1.90+0.53)   0.46(1.07+0.62) -31.3%
>     +        7821.3: extended grep -i int        0.62(1.92+0.53)   0.53(1.12+0.58) -14.5%
>     +        7821.4: perl grep -i int            0.66(1.85+0.58)   0.45(1.10+0.59) -31.8%
>     +        7821.6: fixed grep -i uncommon      0.21(0.54+0.43)   0.17(0.20+0.55) -19.0%
>     +        7821.7: basic grep -i uncommon      0.20(0.52+0.45)   0.17(0.29+0.48) -15.0%
>     +        7821.8: extended grep -i uncommon   0.21(0.52+0.44)   0.17(0.26+0.50) -19.0%
>     +        7821.9: perl grep -i uncommon       0.21(0.53+0.44)   0.17(0.20+0.56) -19.0%
>     +        7821.11: fixed grep -i æ            0.26(0.79+0.44)   0.16(0.29+0.46) -38.5%
>     +        7821.12: basic grep -i æ            0.26(0.79+0.42)   0.16(0.20+0.54) -38.5%
>     +        7821.13: extended grep -i æ         0.26(0.84+0.39)   0.16(0.24+0.50) -38.5%
>     +        7821.14: perl grep -i æ             0.16(0.24+0.49)   0.17(0.25+0.51) +6.3%
>     +
>     +    plain log:
>     +
>     +        Test                                     origin/master     HEAD
>     +        --------------------------------------------------------------------------------
>     +        4221.1: fixed log --grep='int'           7.24(6.95+0.28)   7.20(6.95+0.18) -0.6%
>     +        4221.2: basic log --grep='int'           7.31(6.97+0.22)   7.20(6.93+0.21) -1.5%
>     +        4221.3: extended log --grep='int'        7.37(7.04+0.24)   7.22(6.91+0.25) -2.0%
>     +        4221.4: perl log --grep='int'            7.31(7.04+0.21)   7.19(6.89+0.21) -1.6%
>     +        4221.6: fixed log --grep='uncommon'      6.93(6.59+0.32)   7.04(6.66+0.37) +1.6%
>     +        4221.7: basic log --grep='uncommon'      6.92(6.58+0.29)   7.08(6.75+0.29) +2.3%
>     +        4221.8: extended log --grep='uncommon'   6.92(6.55+0.31)   7.00(6.68+0.31) +1.2%
>     +        4221.9: perl log --grep='uncommon'       7.03(6.59+0.33)   7.12(6.73+0.34) +1.3%
>     +        4221.11: fixed log --grep='æ'            7.41(7.08+0.28)   7.05(6.76+0.29) -4.9%
>     +        4221.12: basic log --grep='æ'            7.39(6.99+0.33)   7.00(6.68+0.25) -5.3%
>     +        4221.13: extended log --grep='æ'         7.34(7.00+0.25)   7.15(6.81+0.31) -2.6%
>     +        4221.14: perl log --grep='æ'             7.43(7.13+0.26)   7.01(6.60+0.36) -5.7%
>     +
>     +    log with -i:
>     +
>     +        Test                                        origin/master     HEAD
>     +        ------------------------------------------------------------------------------------
>     +        4221.1: fixed log -i --grep='int'           7.31(7.07+0.24)   7.23(7.00+0.22) -1.1%
>     +        4221.2: basic log -i --grep='int'           7.40(7.08+0.28)   7.19(6.92+0.20) -2.8%
>     +        4221.3: extended log -i --grep='int'        7.43(7.13+0.25)   7.27(6.99+0.21) -2.2%
>     +        4221.4: perl log -i --grep='int'            7.34(7.10+0.24)   7.10(6.90+0.19) -3.3%
>     +        4221.6: fixed log -i --grep='uncommon'      7.07(6.71+0.32)   7.11(6.77+0.28) +0.6%
>     +        4221.7: basic log -i --grep='uncommon'      6.99(6.64+0.28)   7.12(6.69+0.38) +1.9%
>     +        4221.8: extended log -i --grep='uncommon'   7.11(6.74+0.32)   7.10(6.77+0.27) -0.1%
>     +        4221.9: perl log -i --grep='uncommon'       6.98(6.60+0.29)   7.05(6.64+0.34) +1.0%
>     +        4221.11: fixed log -i --grep='æ'            7.85(7.45+0.34)   7.03(6.68+0.32) -10.4%
>     +        4221.12: basic log -i --grep='æ'            7.87(7.49+0.29)   7.06(6.69+0.31) -10.3%
>     +        4221.13: extended log -i --grep='æ'         7.87(7.54+0.31)   7.09(6.69+0.31) -9.9%
>     +        4221.14: perl log -i --grep='æ'             7.06(6.77+0.28)   6.91(6.57+0.31) -2.1%
>     +
>     +    So as with e05b027627 ("grep: use PCRE v2 for optimized fixed-string
>     +    search", 2019-06-26) there's a huge improvement in performance for
>     +    "grep", but in "log" most of our time is spent elsewhere, so we don't
>     +    notice it that much.
>
>          Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
>
>     @@ -81,15 +157,19 @@
>      +		} else {
>      +			/*
>      +			 * E.g. t7811-grep-open.sh relies on the
>     -+			 * pattern being restored, and unfortunately
>     -+			 * there's no PCRE compile flag for "this is
>     -+			 * fixed", so we need to munge it to
>     -+			 * "\Q<pat>\E".
>     ++			 * pattern being restored.
>      +			 */
>      +			char *old_pattern = p->pattern;
>      +			size_t old_patternlen = p->patternlen;
>      +			struct strbuf sb = STRBUF_INIT;
>      +
>     ++			/*
>     ++			 * There is the PCRE2_LITERAL flag, but it's
>     ++			 * only in PCRE v2 10.30 and later. Needing to
>     ++			 * ifdef our way around that and dealing with
>     ++			 * it + PCRE2_MULTILINE being an error is more
>     ++			 * complex than just quoting this ourselves.
>     ++			*/
>      +			strbuf_add(&sb, "\\Q", 2);
>      +			strbuf_add(&sb, p->pattern, p->patternlen);
>      +			strbuf_add(&sb, "\\E", 2);
>     @@ -101,9 +181,9 @@
>      +			p->patternlen = old_patternlen;
>      +			strbuf_release(&sb);
>      +		}
>     -+#else
>     ++#else /* !USE_LIBPCRE2 */
>       		compile_fixed_regexp(p, opt);
>     -+#endif
>     ++#endif /* !USE_LIBPCRE2 */
>       		return;
>       	}