On Wed, 11 Feb 2009, wli@xxxxxxxxxxxxxxxxx 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 However, lookup2() is outdated as Bob wrote a new hash function called lookup3().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@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxx) + * Copyright (C) 2006. Bob Jenkins (bob_jenkins@xxxxxxxxxxxxxxxx) * * 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@xxxxxxxxxx) + * 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@xxxxxxxxxxxxxxxxx) * * 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@xxxxxxxxxxxxxxxxx, kadlec@xxxxxxxxxxxx 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 -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
