grep: cache userdiff_driver in grep_source

Right now, grep only uses the userdiff_driver for one thing:
looking up funcname patterns for "-p" and "-W".  As new uses
for userdiff drivers are added to the grep code, we want to
minimize attribute lookups, which can be expensive.

It might seem at first that this would also optimize multiple
lookups when the funcname pattern for a file is needed
multiple times. However, the compiled funcname pattern is
already cached in struct grep_opt's "priv" member, so
multiple lookups are already suppressed.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -841,12 +841,9 @@ static int match_funcname(struct grep_opt *opt, struct grep_source *gs, char *bo
 {
 	xdemitconf_t *xecfg = opt->priv;
 	if (xecfg && !xecfg->find_func) {
-		struct userdiff_driver *drv;
-		grep_attr_lock();
-		drv = userdiff_find_by_path(gs->name);
-		grep_attr_unlock();
-		if (drv && drv->funcname.pattern) {
-			const struct userdiff_funcname *pe = &drv->funcname;
+		grep_source_load_driver(gs);
+		if (gs->driver->funcname.pattern) {
+			const struct userdiff_funcname *pe = &gs->driver->funcname;
 			xdiff_set_find_func(xecfg, pe->pattern, pe->cflags);
 		} else {
 			xecfg = opt->priv = NULL;
@@ -1237,6 +1234,7 @@ void grep_source_init(struct grep_source *gs, enum grep_source_type type,
 	gs->name = name ? xstrdup(name) : NULL;
 	gs->buf = NULL;
 	gs->size = 0;
+	gs->driver = NULL;
 
 	switch (type) {
 	case GREP_SOURCE_FILE:
@@ -1340,3 +1338,15 @@ int grep_source_load(struct grep_source *gs)
 	}
 	die("BUG: invalid grep_source type");
 }
+
+void grep_source_load_driver(struct grep_source *gs)
+{
+	if (gs->driver)
+		return;
+
+	grep_attr_lock();
+	gs->driver = userdiff_find_by_path(gs->name);
+	if (!gs->driver)
+		gs->driver = userdiff_find_by_name("default");
+	grep_attr_unlock();
+}

grep.h

@@ -9,6 +9,7 @@ typedef int pcre_extra;
 #endif
 #include "kwset.h"
 #include "thread-utils.h"
+#include "userdiff.h"
 
 enum grep_pat_token {
 	GREP_PATTERN,
@@ -141,13 +142,16 @@ struct grep_source {
 
 	char *buf;
 	unsigned long size;
+
+	struct userdiff_driver *driver;
 };
 
 void grep_source_init(struct grep_source *gs, enum grep_source_type type,
 		      const char *name, const void *identifier);
 int grep_source_load(struct grep_source *gs);
 void grep_source_clear_data(struct grep_source *gs);
 void grep_source_clear(struct grep_source *gs);
+void grep_source_load_driver(struct grep_source *gs);
 
 int grep_source(struct grep_opt *opt, struct grep_source *gs);