#include <inttypes.h>
#include "arch/arch.h"
+#include "compiler/compiler.h"
/* Fast hashing routine for a long.
(C) 2002 William Lee Irwin III, IBM */
#error Define GOLDEN_RATIO_PRIME for your wordsize.
#endif
-#define GR_PRIME_64 0x9e37fffffffc0001ULL
+/*
+ * 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.
+ *
+ * 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.
+ *
+ * These are the negative, (1 - phi) = (phi^2) = (3 - sqrt(5))/2.
+ * (See Knuth vol 3, section 6.4, exercise 9.)
+ */
+#define GOLDEN_RATIO_32 0x61C88647
+#define GOLDEN_RATIO_64 0x61C8864680B583EBull
-static inline unsigned long __hash_long(unsigned long val)
+static inline unsigned long __hash_long(uint64_t val)
{
- unsigned long hash = val;
+ uint64_t hash = val;
#if BITS_PER_LONG == 64
+ hash *= GOLDEN_RATIO_64;
+#else
/* Sigh, gcc can't optimise this alone like it does for 32 bits. */
- unsigned long n = hash;
+ uint64_t n = hash;
n <<= 18;
hash -= n;
n <<= 33;
hash += n;
n <<= 2;
hash += n;
-#else
- /* On some cpus multiply is faster, on others gcc will do shifts */
- hash *= GOLDEN_RATIO_PRIME;
#endif
return hash;
static inline uint64_t __hash_u64(uint64_t val)
{
- return val * GR_PRIME_64;
+ return val * GOLDEN_RATIO_64;
}
static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
* Bob Jenkins jhash
*/
-#define JHASH_INITVAL GOLDEN_RATIO_PRIME
+#define JHASH_INITVAL GOLDEN_RATIO_32
static inline uint32_t rol32(uint32_t word, uint32_t shift)
{
/* Last block: affect all 32 bits of (c) */
/* All the case statements fall through */
switch (length) {
- case 12: c += (uint32_t) k[11] << 24;
- case 11: c += (uint32_t) k[10] << 16;
- case 10: c += (uint32_t) k[9] << 8;
- case 9: c += k[8];
- case 8: b += (uint32_t) k[7] << 24;
- case 7: b += (uint32_t) k[6] << 16;
- case 6: b += (uint32_t) k[5] << 8;
- case 5: b += k[4];
- case 4: a += (uint32_t) k[3] << 24;
- case 3: a += (uint32_t) k[2] << 16;
- case 2: a += (uint32_t) k[1] << 8;
+ case 12: c += (uint32_t) k[11] << 24; fallthrough;
+ case 11: c += (uint32_t) k[10] << 16; fallthrough;
+ case 10: c += (uint32_t) k[9] << 8; fallthrough;
+ case 9: c += k[8]; fallthrough;
+ case 8: b += (uint32_t) k[7] << 24; fallthrough;
+ case 7: b += (uint32_t) k[6] << 16; fallthrough;
+ case 6: b += (uint32_t) k[5] << 8; fallthrough;
+ case 5: b += k[4]; fallthrough;
+ case 4: a += (uint32_t) k[3] << 24; fallthrough;
+ case 3: a += (uint32_t) k[2] << 16; fallthrough;
+ case 2: a += (uint32_t) k[1] << 8; fallthrough;
case 1: a += k[0];
__jhash_final(a, b, c);
+ fallthrough;
case 0: /* Nothing left to add */
break;
}