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The hash_64() function historically does the multiply by the GOLDEN_RATIO_PRIME_64 number with explicit shifts and adds, because unlike the 32-bit case, gcc seems unable to turn the constant multiply into the more appropriate shift and adds when required. However, that means that we generate those shifts and adds even when the architecture has a fast multiplier, and could just do it better in hardware. Use the now-cleaned-up CONFIG_ARCH_HAS_FAST_MULTIPLIER (together with "is it a 64-bit architecture") to decide whether to use an integer multiply or the explicit sequence of shift/add instructions. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
122 lines
3.2 KiB
C
122 lines
3.2 KiB
C
#ifndef _LINUX_HASH_H
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#define _LINUX_HASH_H
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/* Fast hashing routine for ints, longs and pointers.
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(C) 2002 Nadia Yvette Chambers, IBM */
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/*
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* Knuth recommends primes in approximately golden ratio to the maximum
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* integer representable by a machine word for multiplicative hashing.
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* Chuck Lever verified the effectiveness of this technique:
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* http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
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*
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* These primes are chosen to be bit-sparse, that is operations on
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* them can use shifts and additions instead of multiplications for
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* machines where multiplications are slow.
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*/
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#include <asm/types.h>
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#include <asm/hash.h>
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#include <linux/compiler.h>
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/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
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#define GOLDEN_RATIO_PRIME_32 0x9e370001UL
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/* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
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#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
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#if BITS_PER_LONG == 32
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#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
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#define hash_long(val, bits) hash_32(val, bits)
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#elif BITS_PER_LONG == 64
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#define hash_long(val, bits) hash_64(val, bits)
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#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_64
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#else
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#error Wordsize not 32 or 64
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#endif
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static __always_inline u64 hash_64(u64 val, unsigned int bits)
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{
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u64 hash = val;
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#if defined(CONFIG_ARCH_HAS_FAST_MULTIPLIER) && BITS_PER_LONG == 64
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hash = hash * GOLDEN_RATIO_PRIME_64;
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#else
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/* Sigh, gcc can't optimise this alone like it does for 32 bits. */
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u64 n = hash;
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n <<= 18;
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hash -= n;
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n <<= 33;
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hash -= n;
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n <<= 3;
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hash += n;
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n <<= 3;
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hash -= n;
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n <<= 4;
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hash += n;
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n <<= 2;
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hash += n;
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#endif
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/* High bits are more random, so use them. */
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return hash >> (64 - bits);
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}
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static inline u32 hash_32(u32 val, unsigned int bits)
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{
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/* On some cpus multiply is faster, on others gcc will do shifts */
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u32 hash = val * GOLDEN_RATIO_PRIME_32;
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/* High bits are more random, so use them. */
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return hash >> (32 - bits);
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}
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static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
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{
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return hash_long((unsigned long)ptr, bits);
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}
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static inline u32 hash32_ptr(const void *ptr)
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{
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unsigned long val = (unsigned long)ptr;
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#if BITS_PER_LONG == 64
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val ^= (val >> 32);
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#endif
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return (u32)val;
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}
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struct fast_hash_ops {
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u32 (*hash)(const void *data, u32 len, u32 seed);
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u32 (*hash2)(const u32 *data, u32 len, u32 seed);
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};
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/**
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* arch_fast_hash - Caclulates a hash over a given buffer that can have
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* arbitrary size. This function will eventually use an
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* architecture-optimized hashing implementation if
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* available, and trades off distribution for speed.
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*
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* @data: buffer to hash
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* @len: length of buffer in bytes
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* @seed: start seed
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*
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* Returns 32bit hash.
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*/
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extern u32 arch_fast_hash(const void *data, u32 len, u32 seed);
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/**
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* arch_fast_hash2 - Caclulates a hash over a given buffer that has a
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* size that is of a multiple of 32bit words. This
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* function will eventually use an architecture-
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* optimized hashing implementation if available,
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* and trades off distribution for speed.
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*
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* @data: buffer to hash (must be 32bit padded)
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* @len: number of 32bit words
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* @seed: start seed
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*
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* Returns 32bit hash.
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*/
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extern u32 arch_fast_hash2(const u32 *data, u32 len, u32 seed);
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#endif /* _LINUX_HASH_H */
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