[PATCH] Update jhash.h with the new version of Jenkins' hash

* [PATCH] Update jhash.h with the new version of Jenkins' hash
@ 2009-02-11 10:19 Jozsef Kadlecsik
  2009-02-11 20:19 ` Michał Mirosław
                   ` (2 more replies)
  0 siblings, 3 replies; 22+ messages in thread
From: Jozsef Kadlecsik @ 2009-02-11 10:19 UTC (permalink / raw)
  To: linux-kernel; +Cc: netdev, netfilter-devel, Rusty Russell

Hi,

The current jhash.h implements the lookup2() hash function by Bob Jenkins. 
However, lookup2() is outdated as Bob wrote a new hash function called 
lookup3(). The new hash function

- mixes better than lookup2(): it passes the check that every input bit 
  changes every output bit 50% of the time, while lookup2() failed it.
- performs better: compiled with -O2 on Core2 Duo, lookup3() is 20-40% 
  faster than lookup2() depending on the key length.

The patch replaces the lookup2() implementation of the 'jhash*' 
functions with that of lookup3() in linux/jhash.h.

You can read a longer comparison of the Jenkins and other hash functions 
at http://burtleburtle.net/bob/hash/doobs.html.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>

diff --git a/include/linux/jhash.h b/include/linux/jhash.h
index 2a2f99f..2000b9f 100644
--- a/include/linux/jhash.h
+++ b/include/linux/jhash.h
@@ -3,80 +3,95 @@
 
 /* jhash.h: Jenkins hash support.
  *
- * Copyright (C) 1996 Bob Jenkins (bob_jenkins@burtleburtle.net)
+ * Copyright (C) 2006. Bob Jenkins (bob_jenkins@burtleburtle.net)
  *
  * http://burtleburtle.net/bob/hash/
  *
  * These are the credits from Bob's sources:
  *
- * lookup2.c, by Bob Jenkins, December 1996, Public Domain.
- * hash(), hash2(), hash3, and mix() are externally useful functions.
- * Routines to test the hash are included if SELF_TEST is defined.
- * You can use this free for any purpose.  It has no warranty.
+ * lookup3.c, by Bob Jenkins, May 2006, Public Domain.
  *
- * Copyright (C) 2003 David S. Miller (davem@redhat.com)
+ * These are functions for producing 32-bit hashes for hash table lookup.
+ * hashword(), hashlittle(), hashlittle2(), hashbig(), mix(), and final() 
+ * are externally useful functions.  Routines to test the hash are included 
+ * if SELF_TEST is defined.  You can use this free for any purpose.  It's in
+ * the public domain.  It has no warranty.
+ *
+ * Copyright (C) 2009 Jozsef Kadlecsik (kadlec@blackhole.kfki.hu)
  *
  * I've modified Bob's hash to be useful in the Linux kernel, and
- * any bugs present are surely my fault.  -DaveM
+ * any bugs present are my fault.  Jozsef
  */
 
-/* NOTE: Arguments are modified. */
-#define __jhash_mix(a, b, c) \
+#define __rot(x,k) (((x)<<(k)) | ((x)>>(32-(k))))
+
+/* __jhash_mix - mix 3 32-bit values reversibly. */
+#define __jhash_mix(a,b,c) \
+{ \
+  a -= c;  a ^= __rot(c, 4);  c += b; \
+  b -= a;  b ^= __rot(a, 6);  a += c; \
+  c -= b;  c ^= __rot(b, 8);  b += a; \
+  a -= c;  a ^= __rot(c,16);  c += b; \
+  b -= a;  b ^= __rot(a,19);  a += c; \
+  c -= b;  c ^= __rot(b, 4);  b += a; \
+}
+
+/* __jhash_final - final mixing of 3 32-bit values (a,b,c) into c */
+#define __jhash_final(a,b,c) \
 { \
-  a -= b; a -= c; a ^= (c>>13); \
-  b -= c; b -= a; b ^= (a<<8); \
-  c -= a; c -= b; c ^= (b>>13); \
-  a -= b; a -= c; a ^= (c>>12);  \
-  b -= c; b -= a; b ^= (a<<16); \
-  c -= a; c -= b; c ^= (b>>5); \
-  a -= b; a -= c; a ^= (c>>3);  \
-  b -= c; b -= a; b ^= (a<<10); \
-  c -= a; c -= b; c ^= (b>>15); \
+  c ^= b; c -= __rot(b,14); \
+  a ^= c; a -= __rot(c,11); \
+  b ^= a; b -= __rot(a,25); \
+  c ^= b; c -= __rot(b,16); \
+  a ^= c; a -= __rot(c,4);  \
+  b ^= a; b -= __rot(a,14); \
+  c ^= b; c -= __rot(b,24); \
 }
 
 /* The golden ration: an arbitrary value */
-#define JHASH_GOLDEN_RATIO	0x9e3779b9
+#define JHASH_GOLDEN_RATIO	0xdeadbeef
 
 /* The most generic version, hashes an arbitrary sequence
  * of bytes.  No alignment or length assumptions are made about
- * the input key.
+ * the input key. The result depends on endianness.
  */
 static inline u32 jhash(const void *key, u32 length, u32 initval)
 {
-	u32 a, b, c, len;
+	u32 a,b,c;
 	const u8 *k = key;
 
-	len = length;
-	a = b = JHASH_GOLDEN_RATIO;
-	c = initval;
-
-	while (len >= 12) {
-		a += (k[0] +((u32)k[1]<<8) +((u32)k[2]<<16) +((u32)k[3]<<24));
-		b += (k[4] +((u32)k[5]<<8) +((u32)k[6]<<16) +((u32)k[7]<<24));
-		c += (k[8] +((u32)k[9]<<8) +((u32)k[10]<<16)+((u32)k[11]<<24));
-
-		__jhash_mix(a,b,c);
+	/* Set up the internal state */
+	a = b = c = JHASH_GOLDEN_RATIO + length + initval;
 
+	/* all but the last block: affect some 32 bits of (a,b,c) */
+	while (length > 12) {
+    		a += (k[0] + ((u32)k[1]<<8) + ((u32)k[2]<<16) + ((u32)k[3]<<24));
+		b += (k[4] + ((u32)k[5]<<8) + ((u32)k[6]<<16) + ((u32)k[7]<<24));
+		c += (k[8] + ((u32)k[9]<<8) + ((u32)k[10]<<16) + ((u32)k[11]<<24));
+		__jhash_mix(a, b, c);
+		length -= 12;
 		k += 12;
-		len -= 12;
 	}
 
-	c += length;
-	switch (len) {
-	case 11: c += ((u32)k[10]<<24);
-	case 10: c += ((u32)k[9]<<16);
-	case 9 : c += ((u32)k[8]<<8);
-	case 8 : b += ((u32)k[7]<<24);
-	case 7 : b += ((u32)k[6]<<16);
-	case 6 : b += ((u32)k[5]<<8);
+	/* last block: affect all 32 bits of (c) */
+	/* all the case statements fall through */
+	switch (length) {
+	case 12: c += (u32)k[11]<<24;
+	case 11: c += (u32)k[10]<<16;
+	case 10: c += (u32)k[9]<<8;
+	case 9 : c += k[8];
+	case 8 : b += (u32)k[7]<<24;
+	case 7 : b += (u32)k[6]<<16;
+	case 6 : b += (u32)k[5]<<8;
 	case 5 : b += k[4];
-	case 4 : a += ((u32)k[3]<<24);
-	case 3 : a += ((u32)k[2]<<16);
-	case 2 : a += ((u32)k[1]<<8);
+	case 4 : a += (u32)k[3]<<24;
+	case 3 : a += (u32)k[2]<<16;
+	case 2 : a += (u32)k[1]<<8;
 	case 1 : a += k[0];
-	};
-
-	__jhash_mix(a,b,c);
+		__jhash_final(a, b, c);
+	case 0 :
+		break;
+	}
 
 	return c;
 }
@@ -86,58 +101,57 @@ static inline u32 jhash(const void *key, u32 length, u32 initval)
  */
 static inline u32 jhash2(const u32 *k, u32 length, u32 initval)
 {
-	u32 a, b, c, len;
+	u32 a, b, c;
 
-	a = b = JHASH_GOLDEN_RATIO;
-	c = initval;
-	len = length;
+	/* Set up the internal state */
+	a = b = c = JHASH_GOLDEN_RATIO + (length<<2) + initval;
 
-	while (len >= 3) {
+	/* handle most of the key */
+	while (length > 3) {
 		a += k[0];
 		b += k[1];
 		c += k[2];
 		__jhash_mix(a, b, c);
-		k += 3; len -= 3;
+		length -= 3;
+		k += 3;
 	}
 
-	c += length * 4;
-
-	switch (len) {
-	case 2 : b += k[1];
-	case 1 : a += k[0];
-	};
-
-	__jhash_mix(a,b,c);
+	/* handle the last 3 u32's */
+	/* all the case statements fall through */ 
+	switch (length) {
+	case 3: c += k[2];
+	case 2: b += k[1];
+	case 1: a += k[0];
+		__jhash_final(a, b, c);
+	case 0:     /* case 0: nothing left to add */
+		break;
+	}
 
 	return c;
 }
 
-
 /* A special ultra-optimized versions that knows they are hashing exactly
  * 3, 2 or 1 word(s).
- *
- * NOTE: In partilar the "c += length; __jhash_mix(a,b,c);" normally
- *       done at the end is not done here.
  */
 static inline u32 jhash_3words(u32 a, u32 b, u32 c, u32 initval)
 {
-	a += JHASH_GOLDEN_RATIO;
-	b += JHASH_GOLDEN_RATIO;
-	c += initval;
+	a += JHASH_GOLDEN_RATIO + initval;
+	b += JHASH_GOLDEN_RATIO + initval;
+	c += JHASH_GOLDEN_RATIO + initval;
 
-	__jhash_mix(a, b, c);
+	__jhash_final(a, b, c);
 
 	return c;
 }
 
 static inline u32 jhash_2words(u32 a, u32 b, u32 initval)
 {
-	return jhash_3words(a, b, 0, initval);
+	return jhash_3words(0, a, b, initval);
 }
 
 static inline u32 jhash_1word(u32 a, u32 initval)
 {
-	return jhash_3words(a, 0, 0, initval);
+	return jhash_3words(0, 0, a, initval);
 }
 
 #endif /* _LINUX_JHASH_H */


Best regards,
Jozsef
-
E-mail  : kadlec@blackhole.kfki.hu, kadlec@mail.kfki.hu
PGP key : http://www.kfki.hu/~kadlec/pgp_public_key.txt
Address : KFKI Research Institute for Particle and Nuclear Physics
          H-1525 Budapest 114, POB. 49, Hungary

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