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  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. static __always_inline u64 hash_64(u64 val, unsigned int bits)
  36. {
  37.         u64 hash = val;
  38.  
  39.         /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
  40.         u64 n = hash;
  41.         n <<= 18;
  42.         hash -= n;
  43.         n <<= 33;
  44.         hash -= n;
  45.         n <<= 3;
  46.         hash += n;
  47.         n <<= 3;
  48.         hash -= n;
  49.         n <<= 4;
  50.         hash += n;
  51.         n <<= 2;
  52.         hash += n;
  53.  
  54.         /* High bits are more random, so use them. */
  55.         return hash >> (64 - bits);
  56. }
  57.  
  58. static inline u32 hash_32(u32 val, unsigned int bits)
  59. {
  60.         /* On some cpus multiply is faster, on others gcc will do shifts */
  61.         u32 hash = val * GOLDEN_RATIO_PRIME_32;
  62.  
  63.         /* High bits are more random, so use them. */
  64.         return hash >> (32 - bits);
  65. }
  66.  
  67. static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
  68. {
  69.         return hash_long((unsigned long)ptr, bits);
  70. }
  71.  
  72. static inline u32 hash32_ptr(const void *ptr)
  73. {
  74.         unsigned long val = (unsigned long)ptr;
  75.  
  76. #if BITS_PER_LONG == 64
  77.         val ^= (val >> 32);
  78. #endif
  79.         return (u32)val;
  80. }
  81. #endif /* _LINUX_HASH_H */
  82.