[PATCH v2] fixdep: faster CONFIG_ search

* [PATCH v2] fixdep: faster CONFIG_ search
@ 2016-08-24 18:03 Alexey Dobriyan
  2016-08-24 22:37 ` Michal Marek
  0 siblings, 1 reply; 2+ messages in thread
From: Alexey Dobriyan @ 2016-08-24 18:03 UTC (permalink / raw)
  To: mmarek; +Cc: akpm, linux-kbuild

Do you think kernel build is 100% dominated by gcc? You are wrong!
One small utility called "fixdep" consistently manages to sneak into
profile's first page (unless you have small monitor of course).

The choke point is this clever code:

	for (; m < end; m++) {
		if (*m == INT_CONF) { p = (char *) m  ; goto conf; }
		if (*m == INT_ONFI) { p = (char *) m-1; goto conf; }
		if (*m == INT_NFIG) { p = (char *) m-2; goto conf; }
		if (*m == INT_FIG_) { p = (char *) m-3; goto conf; }

4 branches per 4 characters is not fast.

Use strstr(3), so that SSE2 etc can be used.

With this patch, fixdep is so deep at the bottom, it is hard to find it.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 scripts/basic/fixdep.c |   86 +++++++++++++++----------------------------------
 1 file changed, 28 insertions(+), 58 deletions(-)

--- a/scripts/basic/fixdep.c
+++ b/scripts/basic/fixdep.c
@@ -82,8 +82,7 @@
  * to date before even starting the recursive build, so it's too late
  * at this point anyway.
  *
- * The algorithm to grep for "CONFIG_..." is bit unusual, but should
- * be fast ;-) We don't even try to really parse the header files, but
+ * We don't even try to really parse the header files, but
  * merely grep, i.e. if CONFIG_FOO is mentioned in a comment, it will
  * be picked up as well. It's not a problem with respect to
  * correctness, since that can only give too many dependencies, thus
@@ -115,11 +114,6 @@
 #include <ctype.h>
 #include <arpa/inet.h>
 
-#define INT_CONF ntohl(0x434f4e46)
-#define INT_ONFI ntohl(0x4f4e4649)
-#define INT_NFIG ntohl(0x4e464947)
-#define INT_FIG_ ntohl(0x4649475f)
-
 int insert_extra_deps;
 char *target;
 char *depfile;
@@ -241,37 +235,22 @@ static void use_config(const char *m, int slen)
 	print_config(m, slen);
 }
 
-static void parse_config_file(const char *map, size_t len)
+static void parse_config_file(const char *p)
 {
-	const int *end = (const int *) (map + len);
-	/* start at +1, so that p can never be < map */
-	const int *m   = (const int *) map + 1;
-	const char *p, *q;
-
-	for (; m < end; m++) {
-		if (*m == INT_CONF) { p = (char *) m  ; goto conf; }
-		if (*m == INT_ONFI) { p = (char *) m-1; goto conf; }
-		if (*m == INT_NFIG) { p = (char *) m-2; goto conf; }
-		if (*m == INT_FIG_) { p = (char *) m-3; goto conf; }
-		continue;
-	conf:
-		if (p > map + len - 7)
-			continue;
-		if (memcmp(p, "CONFIG_", 7))
-			continue;
+	const char *q, *r;
+
+	while ((p = strstr(p, "CONFIG_"))) {
 		p += 7;
-		for (q = p; q < map + len; q++) {
-			if (!(isalnum(*q) || *q == '_'))
-				goto found;
-		}
-		continue;
-
-	found:
-		if (!memcmp(q - 7, "_MODULE", 7))
-			q -= 7;
-		if (q - p < 0)
-			continue;
-		use_config(p, q - p);
+		q = p;
+		while (*q && (isalnum(*q) || *q == '_'))
+			q++;
+		if (memcmp(q - 7, "_MODULE", 7) == 0)
+			r = q - 7;
+		else
+			r = q;
+		if (r > p)
+			use_config(p, r - p);
+		p = q;
 	}
 }
 
@@ -291,7 +270,7 @@ static void do_config_file(const char *filename)
 {
 	struct stat st;
 	int fd;
-	void *map;
+	char *map;
 
 	fd = open(filename, O_RDONLY);
 	if (fd < 0) {
@@ -308,18 +287,23 @@ static void do_config_file(const char *filename)
 		close(fd);
 		return;
 	}
-	map = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
-	if ((long) map == -1) {
-		perror("fixdep: mmap");
+	map = malloc(st.st_size + 1);
+	if (!map) {
+		perror("fixdep: malloc");
 		close(fd);
 		return;
 	}
+	if (read(fd, map, st.st_size) != st.st_size) {
+		perror("fixdep: read");
+		close(fd);
+		return;
+	}
+	map[st.st_size] = '\0';
+	close(fd);
 
-	parse_config_file(map, st.st_size);
-
-	munmap(map, st.st_size);
+	parse_config_file(map);
 
-	close(fd);
+	free(map);
 }
 
 /*
@@ -446,22 +430,8 @@ static void print_deps(void)
 	close(fd);
 }
 
-static void traps(void)
-{
-	static char test[] __attribute__((aligned(sizeof(int)))) = "CONF";
-	int *p = (int *)test;
-
-	if (*p != INT_CONF) {
-		fprintf(stderr, "fixdep: sizeof(int) != 4 or wrong endianness? %#x\n",
-			*p);
-		exit(2);
-	}
-}
-
 int main(int argc, char *argv[])
 {
-	traps();
-
 	if (argc == 5 && !strcmp(argv[1], "-e")) {
 		insert_extra_deps = 1;
 		argv++;

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