Subversion Repositories Kolibri OS

Rev

Rev 5270 | Blame | Compare with Previous | Last modification | View Log | Download | RSS feed

  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 */
  103.