~jpsamaroo/pahole

ref: 4cfd420f7ef13af4c6f7afee3c4c4a21b49377bc pahole/hash.h -rw-r--r-- 1.9 KiB
4cfd420f — Arnaldo Carvalho de Melo README: Add instructions to do a cross build 1 year, 9 months ago
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```#ifndef _LINUX_HASH_H
#define _LINUX_HASH_H
/* Fast hashing routine for ints,  longs and pointers.
(C) 2002 William Lee Irwin III, 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 <stdint.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 __WORDSIZE == 32
#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
#define hash_long(val, bits) hash_32(val, bits)
#elif __WORDSIZE == 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 inline uint64_t hash_64(const uint64_t val, const unsigned int bits)
{
uint64_t hash = val;

/*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
uint64_t 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 uint32_t hash_32(uint32_t val, unsigned int bits)
{
/* On some cpus multiply is faster, on others gcc will do shifts */
uint32_t hash = val * GOLDEN_RATIO_PRIME_32;

/* High bits are more random, so use them. */
return hash >> (32 - bits);
}

static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
{
return hash_long((unsigned long)ptr, bits);
}
#endif /* _LINUX_HASH_H */
```