Subject: [RFC/PATCH 7/7] grep: use PCRE v2 for optimized fixed-string search

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Wed, 26 Jun 2019 02:03:29 +0200

Bring back optimized fixed-string search for "grep", this time with
PCRE v2 as an optional backend. As noted in [1] with kwset we were
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
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/[email-masked]/

Signed-off-by: Ævar Arnfjörð Bjarmason <[email-masked]>
---
 grep.c | 47 +++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 45 insertions(+), 2 deletions(-)

diff --git a/grep.c b/grep.c
index 4716217837..6b75d5be68 100644
--- a/grep.c
+++ b/grep.c
@@ -356,6 +356,18 @@ static NORETURN void compile_regexp_failed(const struct grep_pat *p,
 	die("%s'%s': %s", where, p->pattern, error);
 }
 
+static int is_fixed(const char *s, size_t len)
+{
+	size_t i;
+
+	for (i = 0; i < len; i++) {
+		if (is_regex_special(s[i]))
+			return 0;
+	}
+
+	return 1;
+}
+
 #ifdef USE_LIBPCRE1
 static void compile_pcre1_regexp(struct grep_pat *p, const struct grep_opt *opt)
 {
@@ -602,7 +614,6 @@ static int pcre2match(struct grep_pat *p, const char *line, const char *eol,
 static void free_pcre2_pattern(struct grep_pat *p)
 {
 }
-#endif /* !USE_LIBPCRE2 */
 
 static void compile_fixed_regexp(struct grep_pat *p, struct grep_opt *opt)
 {
@@ -623,11 +634,13 @@ static void compile_fixed_regexp(struct grep_pat *p, struct grep_opt *opt)
 		compile_regexp_failed(p, errbuf);
 	}
 }
+#endif /* !USE_LIBPCRE2 */
 
 static void compile_regexp(struct grep_pat *p, struct grep_opt *opt)
 {
 	int err;
 	int regflags = REG_NEWLINE;
+	int pat_is_fixed;
 
 	p->word_regexp = opt->word_regexp;
 	p->ignore_case = opt->ignore_case;
@@ -636,8 +649,38 @@ static void compile_regexp(struct grep_pat *p, struct grep_opt *opt)
 	if (memchr(p->pattern, 0, p->patternlen) && !opt->pcre2)
 		die(_("given pattern contains NULL byte (via -f <file>). This is only supported with -P under PCRE v2"));
 
-	if (opt->fixed) {
+	pat_is_fixed = is_fixed(p->pattern, p->patternlen);
+	if (opt->fixed || pat_is_fixed) {
+#ifdef USE_LIBPCRE2
+		opt->pcre2 = 1;
+		if (pat_is_fixed) {
+			compile_pcre2_pattern(p, opt);
+		} 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".
+			 */
+			char *old_pattern = p->pattern;
+			size_t old_patternlen = p->patternlen;
+			struct strbuf sb = STRBUF_INIT;
+
+			strbuf_add(&sb, "\\Q", 2);
+			strbuf_add(&sb, p->pattern, p->patternlen);
+			strbuf_add(&sb, "\\E", 2);
+
+			p->pattern = sb.buf;
+			p->patternlen = sb.len;
+			compile_pcre2_pattern(p, opt);
+			p->pattern = old_pattern;
+			p->patternlen = old_patternlen;
+			strbuf_release(&sb);
+		}
+#else
 		compile_fixed_regexp(p, opt);
+#endif
 		return;
 	}
 
-- 
2.22.0.455.g172b71a6c5