netdev - Re: ipv4: Simplify ARP hash function.

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Message-ID: <CABrhC0kaRCpvmKw=TdPZ+7dRFzdcrjF7hoc-VhbtAfXu=WBU_Q@mail.gmail.com>
Date:	Fri, 8 Jul 2011 14:03:45 -0400
From:	John Heffner <johnwheffner@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	mj@....cz, netdev@...r.kernel.org
Subject: Re: ipv4: Simplify ARP hash function.

On Fri, Jul 8, 2011 at 1:47 PM, David Miller <davem@...emloft.net> wrote:
> From: Martin Mares <mj@....cz>
> Date: Fri, 8 Jul 2011 19:40:55 +0200
>
>> 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
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