linux-kernel - Re: [PATCH 2/3] x86/hash: swap parameters of crc32

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Message-ID: <530B4058.80705@zytor.com>
Date:	Mon, 24 Feb 2014 04:51:36 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jan Beulich <JBeulich@...e.com>,
	Daniel Borkmann <dborkman@...hat.com>
CC:	davem@...emloft.net, mingo@...e.hu, tglx@...utronix.de,
	ffusco@...hat.com, tgraf@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] x86/hash: swap parameters of crc32_u32()

On 02/24/2014 04:41 AM, Jan Beulich wrote:
>>
>> So I'm guessing this hash is deliberately using the CRC32 instruction
>> "backwards", which would actually make sense: an actual CRC is actually
>> a pretty poor hash due to linearity.
>>

OK, it really is even more confusing than that.

It does seem like the crc32 instruction really *is* commutative, which 
isn't something I would personally have expected at all.

Given that fact, I suspect the ordering in the DPDK is actually a bug, 
and that we should correct the ordering (which I would do at the call 
sites because it seems to make the code clearer) because it reduces the 
size of the loop by two instructions.

I guess I should find out how to file a bug report against DPDK too...

	-hpa

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