Re: ipv4: Simplify ARP hash function.

From: John Heffner <redacted>
Date: Fri, 8 Jul 2011 14:03:45 -0400

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

quoted

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.

Ok, there was also an unintended bug in my original patch,
I lost the bottom 8 bits in the fold, the hash function
should instead be:

+static inline u32 arp_hashfn(u32 key, const struct net_device *dev, u32 hash_rnd)
+{
+	u32 val = key ^ dev->ifindex ^ hash_rnd;
+
+	return val ^ (val >> 8) ^ (val >> 16) ^ (val >> 24);
+}