| #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; |
| } |
| |
| static inline u32 hash_32(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 */ |