Re: ipv4: Simplify ARP hash function.

On Fri, Jul 8, 2011 at 1:47 PM, David Miller [off-list ref] wrote:

From: Martin Mares <mj@ucw.cz>
Date: Fri, 8 Jul 2011 19:40:55 +0200

quoted

The hash function is linear, so it can be reduced to:

      a = key ^ dev->ifindex
      return (a >> 8) ^ (a >> 16) ^ (a >> 24)                         // (1)
           ^ (hash_rnd >> 8) ^ (hash_rnd >> 16) ^ (hash_rnd >> 24)    // (2)

Is this really the same?  The inclusion of a full 32-bit xor
with hash_rnd before folding was intentional, so that the
final folding occurs on a completely "random" value.

Martin's reduction looks exactly correct to me.

  -John