0,0 → 1,81 |
#ifndef _LINUX_HASH_H |
#define _LINUX_HASH_H |
/* Fast hashing routine for ints, longs and pointers. |
(C) 2002 Nadia Yvette Chambers, IBM */ |
|
/* |
* Knuth recommends primes in approximately golden ratio to the maximum |
* integer representable by a machine word for multiplicative hashing. |
* Chuck Lever verified the effectiveness of this technique: |
* http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf |
* |
* These primes are chosen to be bit-sparse, that is operations on |
* them can use shifts and additions instead of multiplications for |
* machines where multiplications are slow. |
*/ |
|
#include <asm/types.h> |
#include <linux/compiler.h> |
|
/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */ |
#define GOLDEN_RATIO_PRIME_32 0x9e370001UL |
/* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */ |
#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL |
|
#if BITS_PER_LONG == 32 |
#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_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_PRIME_64 |
#else |
#error Wordsize not 32 or 64 |
#endif |
|
static __always_inline u64 hash_64(u64 val, unsigned int bits) |
{ |
u64 hash = val; |
|
/* Sigh, gcc can't optimise this alone like it does for 32 bits. */ |
u64 n = hash; |
n <<= 18; |
hash -= n; |
n <<= 33; |
hash -= n; |
n <<= 3; |
hash += n; |
n <<= 3; |
hash -= n; |
n <<= 4; |
hash += n; |
n <<= 2; |
hash += n; |
|
/* High bits are more random, so use them. */ |
return hash >> (64 - bits); |
} |
|
static inline u32 hash_32(u32 val, unsigned int bits) |
{ |
/* On some cpus multiply is faster, on others gcc will do shifts */ |
u32 hash = val * GOLDEN_RATIO_PRIME_32; |
|
/* High bits are more random, so use them. */ |
return hash >> (32 - bits); |
} |
|
static inline unsigned long hash_ptr(const void *ptr, unsigned int bits) |
{ |
return hash_long((unsigned long)ptr, bits); |
} |
|
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 */ |