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Message-ID: <alpine.DEB.2.00.0902121006440.18739@blackhole.kfki.hu>
Date: Thu, 12 Feb 2009 10:11:43 +0100 (CET)
From: Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
To: wli@...ementarian.org
cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
netfilter-devel@...r.kernel.org,
Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] Update jhash.h with the new version of Jenkins' hash
On Wed, 11 Feb 2009, wli@...ementarian.org wrote:
> On Wed, Feb 11, 2009 at 11:19:20AM +0100, Jozsef Kadlecsik wrote:
> > /* The golden ration: an arbitrary value */
> > -#define JHASH_GOLDEN_RATIO 0x9e3779b9
> > +#define JHASH_GOLDEN_RATIO 0xdeadbeef
>
> The golden ratio is not an arbitrary value, nor is it a food ration.
> It is (sqrt(5)+1)/2. and the original value of JHASH_GOLDEN_RATIO
> approximated its reciprocal and/or fractional part in fixed point.
>
> (double)0xdeadbeef/((unsigned long long)1 << 32) is a rather poor
> approximation of (sqrt(5)-1)/2. If this value which is not the
> golden ratio is arbitrary, calling it something different from
> JHASH_GOLDEN_RATIO would surprise the code's readers much less.
OK, here follows the patch with the arbitrary initial parameter renamed.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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However, lookup2() is outdated as Bob wrote a new hash function called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() 20-40% faster
than lookup2() depending on the key length.
The patch replaces the lookup2() implementation of the 'jhash*'
functions with that of lookup3().
You can read a longer comparison of the two and other hash functions at
http://burtleburtle.net/bob/hash/doobs.html.
Signed-off-by: Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
diff --git a/include/linux/jhash.h b/include/linux/jhash.h
index 2a2f99f..765a9d6 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@...tleburtle.net)
+ * Copyright (C) 2006. Bob Jenkins (bob_jenkins@...tleburtle.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@...hat.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@...ckhole.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
+/* An arbitrary initial parameter */
+#define JHASH_INIT_PARAM 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_INIT_PARAM + 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_INIT_PARAM + (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_INIT_PARAM + initval;
+ b += JHASH_INIT_PARAM + initval;
+ c += JHASH_INIT_PARAM + 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@...ckhole.kfki.hu, kadlec@...l.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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