Commit ef703f49 authored by George Spelvin's avatar George Spelvin
Browse files

Eliminate bad hash multipliers from hash_32() and hash_64()

The "simplified" prime multipliers made very bad hash functions, so get rid
of them.  This completes the work of 689de1d6

.

To avoid the inefficiency which was the motivation for the "simplified"
multipliers, hash_64() on 32-bit systems is changed to use a different
algorithm.  It makes two calls to hash_32() instead.

drivers/media/usb/dvb-usb-v2/af9015.c uses the old GOLDEN_RATIO_PRIME_32
for some horrible reason, so it inherits a copy of the old definition.
Signed-off-by: default avatarGeorge Spelvin <linux@sciencehorizons.net>
Cc: Antti Palosaari <crope@iki.fi>
Cc: Mauro Carvalho Chehab <m.chehab@samsung.com>
parent 92d56774
......@@ -398,6 +398,8 @@ error:
}
#define AF9015_EEPROM_SIZE 256
/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
#define GOLDEN_RATIO_PRIME_32 0x9e370001UL
/* hash (and dump) eeprom */
static int af9015_eeprom_hash(struct dvb_usb_device *d)
......
......@@ -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;
......
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