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 <asm/hash.h>
  19 #include <linux/compiler.h>
  20
  21 /* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
  22 #define GOLDEN_RATIO_PRIME_32 0x9e370001UL
  23 /*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
  24 #define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
  25
  26 #if BITS_PER_LONG == 32
  27 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
  28 #define hash_long(val, bits) hash_32(val, bits)
  29 #elif BITS_PER_LONG == 64
  30 #define hash_long(val, bits) hash_64(val, bits)
  31 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
  32 #else
  33 #error Wordsize not 32 or 64
  34 #endif
  35
  36 static __always_inline u64 hash_64(u64 val, unsigned int bits)
  37 {
  38         u64 hash = val;
  39
  40         /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
  41         u64 n = hash;
  42         n <<= 18;
  43         hash -= n;
  44         n <<= 33;
  45         hash -= n;
  46         n <<= 3;
  47         hash += n;
  48         n <<= 3;
  49         hash -= n;
  50         n <<= 4;
  51         hash += n;
  52         n <<= 2;
  53         hash += n;
  54
  55         /* High bits are more random, so use them. */
  56         return hash >> (64 - bits);
  57 }
  58
  59 static inline u32 hash_32(u32 val, unsigned int bits)
  60 {
  61         /* On some cpus multiply is faster, on others gcc will do shifts */
  62         u32 hash = val * GOLDEN_RATIO_PRIME_32;
  63
  64         /* High bits are more random, so use them. */
  65         return hash >> (32 - bits);
  66 }
  67
  68 static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
  69 {
  70         return hash_long((unsigned long)ptr, bits);
  71 }
  72
  73 static inline u32 hash32_ptr(const void *ptr)
  74 {
  75         unsigned long val = (unsigned long)ptr;
  76
  77 #if BITS_PER_LONG == 64
  78         val ^= (val >> 32);
  79 #endif
  80         return (u32)val;
  81 }
  82
  83 struct fast_hash_ops {
  84         u32 (*hash)(const void *data, u32 len, u32 seed);
  85         u32 (*hash2)(const u32 *data, u32 len, u32 seed);
  86 };
  87
  88 /**
  89  *      arch_fast_hash - Caclulates a hash over a given buffer that can have
  90  *                       arbitrary size. This function will eventually use an
  91  *                       architecture-optimized hashing implementation if
  92  *                       available, and trades off distribution for speed.
  93  *
  94  *      @data: buffer to hash
  95  *      @len: length of buffer in bytes
  96  *      @seed: start seed
  97  *
  98  *      Returns 32bit hash.
  99  */
 100 extern u32 arch_fast_hash(const void *data, u32 len, u32 seed);
 101
 102 /**
 103  *      arch_fast_hash2 - Caclulates a hash over a given buffer that has a
 104  *                        size that is of a multiple of 32bit words. This
 105  *                        function will eventually use an architecture-
 106  *                        optimized hashing implementation if available,
 107  *                        and trades off distribution for speed.
 108  *
 109  *      @data: buffer to hash (must be 32bit padded)
 110  *      @len: number of 32bit words
 111  *      @seed: start seed
 112  *
 113  *      Returns 32bit hash.
 114  */
 115 extern u32 arch_fast_hash2(const u32 *data, u32 len, u32 seed);
 116
 117 #endif /* _LINUX_HASH_H */