NeilBrown <[email protected]> wrote:
>
> I'm still bothered by the poor showing on hash_long on 64bit
> on number that differ only in the 3rd or 4th byte (as IP addresses might
> on a little-endian host).
> I hacked around the problem in net/sunrpc/svcauth_unix.c until this
> issue got resolved, but it never did. So I am pushing again.
>
> The problem is that the bit-spares prime that is close to the
> golden ratio isn't really close enough.
>
> I propose 'fixing' it by using hash_u32 on the upper and lower halfs
> of a 64bit value. This is slightly less efficient, but most code would
> probably be happy calling hash_u32 anyway.
We end up with
static inline u32 hash_u32(u32 val, unsigned int bits)
{
u32 hash = val;
/* On some cpus multiply is faster, on others gcc will do shifts */
hash *= GOLDEN_RATIO_PRIME_32;
/* High bits are more random, so use them. */
return hash >> (32 - bits);
}
static inline u32 hash_u64(u64 val, unsigned int bits)
{
u32 hi = val >> 32;
return hash_u32(hash_u32(val, 32) ^ hi, bits);
}
So if one does
hash_u64(foo, 44)
we have a negative shift distance.
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