[PATCH] lib/string.c: Improve strcasecmp speed by not lowering if chars match.

[Nathan Moinvaziri]

With strings where many characters match exactly each character is needless=
ly
converted to lowercase before comparing. This patch improves the comparison
by only converting to lowercase after checking that the characters don't ma=
tch.

The more characters that match exactly the better performance we expect ver=
sus
the old function.

When running tests using Quick Benchmark with two matching 256 character
strings these changes result in anywhere between ~6-9x speed improvement.

* We use unsigned char instead of int similar to strncasecmp.
* We only subtract c1 - c2 when they are not equal.

Reviewed-by: Sergey Markelov <sergio_nsk@xxxxxxxx>
Reviewed-by: Steve Tucker <steven.r.tucker@xxxxxxxxx>
---
 lib/string.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/lib/string.c b/lib/string.c
index 3371d26a0e39..51ad56db1b5d 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -64,13 +64,20 @@ EXPORT_SYMBOL(strncasecmp);
 #ifndef __HAVE_ARCH_STRCASECMP
 int strcasecmp(const char *s1, const char *s2)
 {
-	int c1, c2;
+	/* Yes, Virginia, it had better be unsigned */
+	unsigned char c1, c2;
=20
 	do {
-		c1 =3D tolower(*s1++);
-		c2 =3D tolower(*s2++);
-	} while (c1 =3D=3D c2 && c1 !=3D 0);
-	return c1 - c2;
+		c1 =3D *s1++;
+		c2 =3D *s2++;
+		if (c1 !=3D c2) {
+			c1 =3D tolower(c1);
+			c2 =3D tolower(c2);
+			if (c1 !=3D c2)
+				return (int)c1 - (int)c2;
+		}
+	} while (c1 !=3D 0);
+	return 0;
 }
 EXPORT_SYMBOL(strcasecmp);
 #endif
--=20
2.37.2.windows.2