diff options
Diffstat (limited to 'include/linux/hash.h')
| -rw-r--r-- | include/linux/hash.h | 87 |
1 files changed, 34 insertions, 53 deletions
diff --git a/include/linux/hash.h b/include/linux/hash.h index f967dedb10e2..613cfde3a1e0 100644 --- a/include/linux/hash.h +++ b/include/linux/hash.h @@ -3,85 +3,65 @@ /* 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 - +/* + * 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_PRIME_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_PRIME_64 +#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_64 #else #error Wordsize not 32 or 64 #endif /* - * The above primes are actively bad for hashing, since they are - * too sparse. The 32-bit one is mostly ok, the 64-bit one causes - * real problems. Besides, the "prime" part is pointless for the - * multiplicative hash. + * 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. + * properties. (See Knuth vol 3, section 6.4, exercise 9.) * - * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2. - * (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 -static __always_inline u32 hash_64(u64 val, unsigned int bits) -{ - u64 hash = val; - -#if BITS_PER_LONG == 64 - hash = hash * GOLDEN_RATIO_64; -#else - /* 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; -#endif - /* High bits are more random, so use them. */ - return (u32)(hash >> (64 - bits)); +static inline u32 __hash_32(u32 val) +{ + return val * GOLDEN_RATIO_32; } 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); + return __hash_32(val) >> (32 - bits); +} + +static __always_inline u32 hash_64(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) @@ -89,6 +69,7 @@ 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; |