netdev - Re: ipv4: Simplify ARP hash function.

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Message-Id: <20110708.164751.1543109601226116469.davem@davemloft.net>
Date:	Fri, 08 Jul 2011 16:47:51 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...tta.com
Cc:	roland@...estorage.com, johnwheffner@...il.com, mj@....cz,
	netdev@...r.kernel.org
Subject: Re: ipv4: Simplify ARP hash function.

From: Stephen Hemminger <shemminger@...tta.com>
Date: Fri, 8 Jul 2011 16:41:28 -0700

> What about using murmur hash which has a four byte pass as well.
>   https://sites.google.com/site/murmurhash/

I'm trying to avoid multiplies that are not done in hardware on some
cpus.

Right now I'm looking at one of Thomas Wang's hashes, referenced on
Bob Jenkin's hash analysis page:

u32 hashint(u32 a)
{
	a += ~(a<<15);
	a ^=  (a>>10);
	a +=  (a<<3);
	a ^=  (a>>6);
	a += ~(a<<11);
	a ^=  (a>>16);

	return a;
}

It's 15 instructions, and produces better entropy in the low bits of
the result than the high bits, which is fine for how we'll use this
thing.
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