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

From: Junio C Hamano <[email-masked]>
Date: Wed, 24 May 2017 14:17:11 +0900

Ævar Arnfjörð Bjarmason  <[email-masked]> 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.

> @@ -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).

> +		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

> +#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);

Thanks.