code / linux / torvalds / linux / 1ed1fa5f9c311a74f031cabb18a415b4defdfa03 / . / include / linux / hash.h

#ifndef _LINUX_HASH_H | |

#define _LINUX_HASH_H | |

/* Fast hashing routine for ints, longs and pointers. | |

(C) 2002 Nadia Yvette Chambers, IBM */ | |

#include <asm/types.h> | |

#include <linux/compiler.h> | |

/* | |

* The "GOLDEN_RATIO_PRIME" is used in ifs/btrfs/brtfs_inode.h and | |

* fs/inode.c. It's not actually prime any more (the previous primes | |

* were actively bad for hashing), but the name remains. | |

*/ | |

#if BITS_PER_LONG == 32 | |

#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_32 | |

#define hash_long(val, bits) hash_32(val, bits) | |

#elif BITS_PER_LONG == 64 | |

#define hash_long(val, bits) hash_64(val, bits) | |

#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_64 | |

#else | |

#error Wordsize not 32 or 64 | |

#endif | |

/* | |

* This hash multiplies the input by a large odd number and takes the | |

* high bits. Since multiplication propagates changes to the most | |

* significant end only, it is essential that the high bits of the | |

* product be used for the hash value. | |

* | |

* Chuck Lever verified the effectiveness of this technique: | |

* http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf | |

* | |

* Although a random odd number will do, it turns out that the golden | |

* ratio phi = (sqrt(5)-1)/2, or its negative, has particularly nice | |

* properties. (See Knuth vol 3, section 6.4, exercise 9.) | |

* | |

* These are the negative, (1 - phi) = phi**2 = (3 - sqrt(5))/2, | |

* which is very slightly easier to multiply by and makes no | |

* difference to the hash distribution. | |

*/ | |

#define GOLDEN_RATIO_32 0x61C88647 | |

#define GOLDEN_RATIO_64 0x61C8864680B583EBull | |

#ifdef CONFIG_HAVE_ARCH_HASH | |

/* This header may use the GOLDEN_RATIO_xx constants */ | |

#include <asm/hash.h> | |

#endif | |

/* | |

* The _generic versions exist only so lib/test_hash.c can compare | |

* the arch-optimized versions with the generic. | |

* | |

* Note that if you change these, any <asm/hash.h> that aren't updated | |

* to match need to have their HAVE_ARCH_* define values updated so the | |

* self-test will not false-positive. | |

*/ | |

#ifndef HAVE_ARCH__HASH_32 | |

#define __hash_32 __hash_32_generic | |

#endif | |

static inline u32 __hash_32_generic(u32 val) | |

{ | |

return val * GOLDEN_RATIO_32; | |

} | |

#ifndef HAVE_ARCH_HASH_32 | |

#define hash_32 hash_32_generic | |

#endif | |

static inline u32 hash_32_generic(u32 val, unsigned int bits) | |

{ | |

/* High bits are more random, so use them. */ | |

return __hash_32(val) >> (32 - bits); | |

} | |

#ifndef HAVE_ARCH_HASH_64 | |

#define hash_64 hash_64_generic | |

#endif | |

static __always_inline u32 hash_64_generic(u64 val, unsigned int bits) | |

{ | |

#if BITS_PER_LONG == 64 | |

/* 64x64-bit multiply is efficient on all 64-bit processors */ | |

return val * GOLDEN_RATIO_64 >> (64 - bits); | |

#else | |

/* Hash 64 bits using only 32x32-bit multiply. */ | |

return hash_32((u32)val ^ __hash_32(val >> 32), bits); | |

#endif | |

} | |

static inline u32 hash_ptr(const void *ptr, unsigned int bits) | |

{ | |

return hash_long((unsigned long)ptr, bits); | |

} | |

/* This really should be called fold32_ptr; it does no hashing to speak of. */ | |

static inline u32 hash32_ptr(const void *ptr) | |

{ | |

unsigned long val = (unsigned long)ptr; | |

#if BITS_PER_LONG == 64 | |

val ^= (val >> 32); | |

#endif | |

return (u32)val; | |

} | |

#endif /* _LINUX_HASH_H */ |