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

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Date:	Tue, 26 Aug 2014 10:17:57 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	David Miller <davem@...emloft.net>
Cc:	dborkman@...hat.com, 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

David Miller <davem@...emloft.net> writes:

> 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.

Yes, I should, but I'm afraid I can't. I only have x86 hardware, and
even if I managed to set up some cross-compiler toolchain, I wouldn't
know what CPUs to target. So I was hoping someone else could test this
on appropriate platforms.

Thanks,
Rasmus
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