linux-kernel - Re: [PATCH/RFC] hash: Let gcc decide how to multiply

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Message-Id: <20140825.114754.1052522152538122040.davem@davemloft.net>
Date:	Mon, 25 Aug 2014 11:47:54 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dborkman@...hat.com
Cc:	linux@...musvillemoes.dk, ffusco@...hat.com, tgraf@...g.ch,
	linux-kernel@...r.kernel.org, matthew@....cx, wli@...omorphy.com,
	akpm@...ux-foundation.org, nyc@...omorphy.com
Subject: Re: [PATCH/RFC] hash: Let gcc decide how to multiply

From: Daniel Borkmann <dborkman@...hat.com>
Date: Mon, 25 Aug 2014 14:58:37 +0200

> On 08/25/2014 02:13 PM, Rasmus Villemoes wrote:
>> A 9+ years old comment in hash_64 says that gcc can't optimize
>> multiplication by GOLDEN_RATIO_PRIME_64. Well, compilers get smarter
>> and CPUs get faster all the time, so it is perhaps about time to
>> revisit that assumption.
> 
> Seems fine by me, but Cc'ing a couple of others (as those you have
> Cc'ed
> haven't written that code :)). You might want to let your changes go
> via
> Andrew's tree, too, perhaps ...

You need to test the code generation for a cpu where the multiply
instruction is more expensive than the shifts and adds.
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