include/linux/hash.h

   1 #ifndef _LINUX_HASH_H
   2 #define _LINUX_HASH_H
   3 /* Fast hashing routine for ints,  longs and pointers.
   4    (C) 2002 Nadia Yvette Chambers, IBM */
   5
   6 /*
   7  * Knuth recommends primes in approximately golden ratio to the maximum
   8  * integer representable by a machine word for multiplicative hashing.
   9  * Chuck Lever verified the effectiveness of this technique:
  10  * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
  11  *
  12  * These primes are chosen to be bit-sparse, that is operations on
  13  * them can use shifts and additions instead of multiplications for
  14  * machines where multiplications are slow.
  15  */
  16
  17 #include <asm/types.h>
  18 #include <linux/compiler.h>
  19
  20 /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
  21 #define GOLDEN_RATIO_PRIME_32 0x9e370001UL
  22 /*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
  23 #define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
  24
  25 #if BITS_PER_LONG == 32
  26 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
  27 #define hash_long(val, bits) hash_32(val, bits)
  28 #elif BITS_PER_LONG == 64
  29 #define hash_long(val, bits) hash_64(val, bits)
  30 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
  31 #else
  32 #error Wordsize not 32 or 64
  33 #endif
  34
  35 /*
  36  * The above primes are actively bad for hashing, since they are
  37  * too sparse. The 32-bit one is mostly ok, the 64-bit one causes
  38  * real problems. Besides, the "prime" part is pointless for the
  39  * multiplicative hash.
  40  *
  41  * Although a random odd number will do, it turns out that the golden
  42  * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
  43  * properties.
  44  *
  45  * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2.
  46  * (See Knuth vol 3, section 6.4, exercise 9.)
  47  */
  48 #define GOLDEN_RATIO_32 0x61C88647
  49 #define GOLDEN_RATIO_64 0x61C8864680B583EBull
  50
  51 static __always_inline u64 hash_64(u64 val, unsigned int bits)
  52 {
  53         u64 hash = val;
  54
  55 #if BITS_PER_LONG == 64
  56         hash = hash * GOLDEN_RATIO_64;
  57 #else
  58         /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
  59         u64 n = hash;
  60         n <<= 18;
  61         hash -= n;
  62         n <<= 33;
  63         hash -= n;
  64         n <<= 3;
  65         hash += n;
  66         n <<= 3;
  67         hash -= n;
  68         n <<= 4;
  69         hash += n;
  70         n <<= 2;
  71         hash += n;
  72 #endif
  73
  74         /* High bits are more random, so use them. */
  75         return hash >> (64 - bits);
  76 }
  77
  78 static inline u32 hash_32(u32 val, unsigned int bits)
  79 {
  80         /* On some cpus multiply is faster, on others gcc will do shifts */
  81         u32 hash = val * GOLDEN_RATIO_PRIME_32;
  82
  83         /* High bits are more random, so use them. */
  84         return hash >> (32 - bits);
  85 }
  86
  87 static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
  88 {
  89         return hash_long((unsigned long)ptr, bits);
  90 }
  91
  92 static inline u32 hash32_ptr(const void *ptr)
  93 {
  94         unsigned long val = (unsigned long)ptr;
  95
  96 #if BITS_PER_LONG == 64
  97         val ^= (val >> 32);
  98 #endif
  99         return (u32)val;
 100 }
 101
 102 #endif /* _LINUX_HASH_H */