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 #include <asm/types.h>
   7 #include <linux/compiler.h>
   8
   9 /*
  10  * The "GOLDEN_RATIO_PRIME" is used in ifs/btrfs/brtfs_inode.h and
  11  * fs/inode.c.  It's not actually prime any more (the previous primes
  12  * were actively bad for hashing), but the name remains.
  13  */
  14 #if BITS_PER_LONG == 32
  15 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_32
  16 #define hash_long(val, bits) hash_32(val, bits)
  17 #elif BITS_PER_LONG == 64
  18 #define hash_long(val, bits) hash_64(val, bits)
  19 #define GOLDEN_RATIO_PRIME GOLDEN_RATIO_64
  20 #else
  21 #error Wordsize not 32 or 64
  22 #endif
  23
  24 /*
  25  * This hash multiplies the input by a large odd number and takes the
  26  * high bits.  Since multiplication propagates changes to the most
  27  * significant end only, it is essential that the high bits of the
  28  * product be used for the hash value.
  29  *
  30  * Chuck Lever verified the effectiveness of this technique:
  31  * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
  32  *
  33  * Although a random odd number will do, it turns out that the golden
  34  * ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice
  35  * properties.  (See Knuth vol 3, section 6.4, exercise 9.)
  36  *
  37  * These are the negative, (1 - phi) = phi**2 = (3 - sqrt(5))/2,
  38  * which is very slightly easier to multiply by and makes no
  39  * difference to the hash distribution.
  40  */
  41 #define GOLDEN_RATIO_32 0x61C88647
  42 #define GOLDEN_RATIO_64 0x61C8864680B583EBull
  43
  44 #ifdef CONFIG_HAVE_ARCH_HASH
  45 /* This header may use the GOLDEN_RATIO_xx constants */
  46 #include <asm/hash.h>
  47 #endif
  48
  49 /*
  50  * The _generic versions exist only so lib/test_hash.c can compare
  51  * the arch-optimized versions with the generic.
  52  *
  53  * Note that if you change these, any <asm/hash.h> that aren't updated
  54  * to match need to have their HAVE_ARCH_* define values updated so the
  55  * self-test will not false-positive.
  56  */
  57 #ifndef HAVE_ARCH__HASH_32
  58 #define __hash_32 __hash_32_generic
  59 #endif
  60 static inline u32 __hash_32_generic(u32 val)
  61 {
  62         return val * GOLDEN_RATIO_32;
  63 }
  64
  65 #ifndef HAVE_ARCH_HASH_32
  66 #define hash_32 hash_32_generic
  67 #endif
  68 static inline u32 hash_32_generic(u32 val, unsigned int bits)
  69 {
  70         /* High bits are more random, so use them. */
  71         return __hash_32(val) >> (32 - bits);
  72 }
  73
  74 #ifndef HAVE_ARCH_HASH_64
  75 #define hash_64 hash_64_generic
  76 #endif
  77 static __always_inline u32 hash_64_generic(u64 val, unsigned int bits)
  78 {
  79 #if BITS_PER_LONG == 64
  80         /* 64x64-bit multiply is efficient on all 64-bit processors */
  81         return val * GOLDEN_RATIO_64 >> (64 - bits);
  82 #else
  83         /* Hash 64 bits using only 32x32-bit multiply. */
  84         return hash_32((u32)val ^ __hash_32(val >> 32), bits);
  85 #endif
  86 }
  87
  88 static inline u32 hash_ptr(const void *ptr, unsigned int bits)
  89 {
  90         return hash_long((unsigned long)ptr, bits);
  91 }
  92
  93 /* This really should be called fold32_ptr; it does no hashing to speak of. */
  94 static inline u32 hash32_ptr(const void *ptr)
  95 {
  96         unsigned long val = (unsigned long)ptr;
  97
  98 #if BITS_PER_LONG == 64
  99         val ^= (val >> 32);
 100 #endif
 101         return (u32)val;
 102 }
 103
 104 #endif /* _LINUX_HASH_H */