Re: [PATCH v2 4/7] grep: add support for the PCRE v1 JIT API

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: May 24, 2017, 07:37 UTC
Message-ID: <CACBZZX5MPXR7Z6oSPN+UjQxZhXpe8O3XVcuC8gAGjAbofJ4YDw@mail.gmail.com>
In-Reply-To: <xmqqlgpmhm08.fsf@gitster.mtv.corp.google.com>

On Wed, May 24, 2017 at 7:17 AM, Junio C Hamano <gitster@pobox.com> wrote:

Show 26 quoted lines

> Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:
>
>> diff --git a/grep.c b/grep.c
>> index 1157529115..49e9aed457 100644
>> --- a/grep.c
>> +++ b/grep.c
>> @@ -351,6 +351,9 @@ static void compile_pcre1_regexp(struct grep_pat *p, const struct grep_opt *opt)
>>       const char *error;
>>       int erroffset;
>>       int options = PCRE_MULTILINE;
>> +#ifdef PCRE_CONFIG_JIT
>> +     int canjit;
>> +#endif
>
> Is "canjit" a property purely of the library (e.g. version and
> compilation option), or of combination of that and nature of the
> pattern, or something else like the memory pressure?
>
> I am wondering if it is worth doing something like this:
>
>         static int canjit = -1;
>         if (canjit < 0) {
>                 pcre_config(PCRE_CONFIG_JIT, &canjit);
>         }
>
> if it depends purely on the library linked to the process.

It purely depends on how the the library, was compiled. I just wrote it like that because compiling the pattern is not a hot codepath (i.e. we call this max 8 or so times or so, whereas exec will be called thousands/millions/billions of times), so trying to avoid calling this trivial function seemed pointless.

But looking at this again it would be simpler to combine what you're suggesting with just passing a pointer to *.pcre[12]_jit_on directly, skipping the canjit variables.

Show 20 quoted lines

>> @@ -365,9 +368,20 @@ static void compile_pcre1_regexp(struct grep_pat *p, const struct grep_opt *opt)
>>       if (!p->pcre1_regexp)
>>               compile_regexp_failed(p, error);
>>
>> -     p->pcre1_extra_info = pcre_study(p->pcre1_regexp, 0, &error);
>> +     p->pcre1_extra_info = pcre_study(p->pcre1_regexp, PCRE_STUDY_JIT_COMPILE, &error);
>>       if (!p->pcre1_extra_info && error)
>>               die("%s", error);
>> +
>> +#ifdef PCRE_CONFIG_JIT
>> +     pcre_config(PCRE_CONFIG_JIT, &canjit);
>> +     if (canjit == 1) {
>> +             p->pcre1_jit_stack = pcre_jit_stack_alloc(1, 1024 * 1024);
>> +             if (!p->pcre1_jit_stack)
>> +                     die("BUG: Couldn't allocate PCRE JIT stack");
>
> I agree that dying is OK, but as far as I can tell, this is not a
> BUG (there is no error a programmer can correct by a follow-up
> patch); please do not mark it as such (it is likely that we'll later
> do a tree-wide s/die("BUG:/BUG("/ and this will interfere).

Makes sense. Looks like the convention for this sort of thing is to just do s/BUG: //, e.g. the code in wrapper.c does that.

Show 20 quoted lines

>> +             pcre_assign_jit_stack(p->pcre1_extra_info, NULL, p->pcre1_jit_stack);
>> +             p->pcre1_jit_on = 1;
>
> Contrary to what I wondered about "canjit" above, I think it makes
> tons of sense to contain the "is JIT in use?" information in "struct
> grep_pat" and not rely on any global state.  Not that we are likely
> to want to be able to JIT some patterns while not doing others.  So
> I agree with the design choice of adding pcre1_jit_on field to the
> structure.
>
> But then, wouldn't it make more sense to do all of the above without
> the canjit variable at all?  i.e. something like...
>
>         #ifdef PCRE_CONFIG_JIT
>                 pcre_config(PCRE_CONFIG_JIT, &p->pcre1_jit_on);
>                 if (p->pcre1_jit_on)
>                         ... stack thing ...
>         #else
>                 p->pcre1_jit_on = 0;
>         #endif

*Nod*

Show 31 quoted lines

>> +#ifdef PCRE_CONFIG_JIT
>> +     if (p->pcre1_jit_on) {
>> +             pcre_free_study(p->pcre1_extra_info);
>> +             pcre_jit_stack_free(p->pcre1_jit_stack);
>> +     } else
>> +#endif
>> +     /* PCRE_CONFIG_JIT !p->pcre1_jit_on else branch */
>>       pcre_free(p->pcre1_extra_info);
>> +
>>       pcre_free((void *)p->pcre1_tables);
>
> It is very thoughtful to add a blank line here (and you did the same
> in another similar hunk), but I have a feeling that it is still a
> bit too subtle a hint to signal to the readers that these two
> pcre_free()s fire differently, i.e. the former does not fire if jit
> is on but the latter always fires.
>
> Would this be a bit safer while being not too ugly to live, I wonder?
>
>         #ifdef PCRE_CONFIG_JIT
>                 if (p->pcre1_jit_on) {
>                         pcre_free_study(p->pcre1_extra_info);
>                         pcre_jit_stack_free(p->pcre1_jit_stack);
>                 } else
>         #endif
>                 {
>                         /* PCRE_CONFIG_JIT !p->pcre1_jit_on else branch */
>                         pcre_free(p->pcre1_extra_info);
>                 }
>                 pcre_free((void *)p->pcre1_tables);
>

Makes sense. I'll change it.