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TOMOYO Linux Cross Reference
Linux/include/linux/hash.h

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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 #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 */
105 

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