Message-ID: <530B4058.80705@zytor.com>
Date: Mon, 24 Feb 2014 04:51:36 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Jan Beulich <JBeulich@suse.com>, Daniel Borkmann <dborkman@redhat.com>
CC: davem@davemloft.net, mingo@elte.hu, tglx@linutronix.de, ffusco@redhat.com,
        tgraf@redhat.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/3] x86/hash: swap parameters of crc32_u32()
References: <53073986020000780011E2E1@nat28.tlf.novell.com> <5308938A.8070506@redhat.com> <530B0AF2020000780011E97B@nat28.tlf.novell.com> <530B1D60.8010602@redhat.com> <530B32A4020000780011EACB@nat28.tlf.novell.com> <530B3102.4050102@redhat.com> <530B3BDB.3010908@zytor.com> <530B4C0C020000780011EC0D@nat28.tlf.novell.com>
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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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