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hash.h
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#pragma once
#include <osmocom/core/log2.h>
/* Fast hashing routine for ints, longs and pointers.
(C) 2002 Nadia Yvette Chambers, IBM */
#include <limits.h>
#if ULONG_MAX == 4294967295
#define BITS_PER_LONG 32
#else
#define BITS_PER_LONG 64
#endif
/*
* 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
/*
* 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 uint32_t __hash_32_generic(uint32_t val)
{
return val * GOLDEN_RATIO_32;
}
#ifndef HAVE_ARCH_HASH_32
#define hash_32 hash_32_generic
#endif
static inline uint32_t hash_32_generic(uint32_t 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 uint32_t hash_64_generic(uint64_t 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((uint32_t)val ^ __hash_32(val >> 32), bits);
#endif
}
static inline uint32_t 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 uint32_t hash32_ptr(const void *ptr)
{
unsigned long val = (unsigned long)ptr;
#if BITS_PER_LONG == 64
val ^= (val >> 32);
#endif
return (uint32_t)val;
}
__always_inline
#define __always_inline
Definition conv_acc_neon_impl.h:26
hash_32
#define hash_32
Definition hash.h:65
hash_long
#define hash_long(val, bits)
Definition hash.h:22
__hash_32_generic
static uint32_t __hash_32_generic(uint32_t val)
Definition hash.h:59
hash_32_generic
static uint32_t hash_32_generic(uint32_t val, unsigned int bits)
Definition hash.h:67
hash32_ptr
static uint32_t hash32_ptr(const void *ptr)
Definition hash.h:93
GOLDEN_RATIO_32
#define GOLDEN_RATIO_32
Definition hash.h:45
__hash_32
#define __hash_32
Definition hash.h:57
GOLDEN_RATIO_64
#define GOLDEN_RATIO_64
Definition hash.h:46
hash_ptr
static uint32_t hash_ptr(const void *ptr, unsigned int bits)
Definition hash.h:87
hash_64_generic
static __always_inline uint32_t hash_64_generic(uint64_t val, unsigned int bits)
Definition hash.h:76
log2.h