From: "H. Peter Anvin" <hpa@zytor.com>
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()
Date: Mon, 24 Feb 2014 04:51:36 -0800 [thread overview]
Message-ID: <530B4058.80705@zytor.com> (raw)
In-Reply-To: <530B4C0C020000780011EC0D@nat28.tlf.novell.com>
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
next prev parent reply other threads:[~2014-02-24 12:51 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-02-21 10:33 [PATCH 2/3] x86/hash: swap parameters of crc32_u32() Jan Beulich
2014-02-22 12:09 ` Daniel Borkmann
2014-02-24 8:03 ` Jan Beulich
2014-02-24 10:22 ` Daniel Borkmann
2014-02-24 10:53 ` Jan Beulich
2014-02-24 11:46 ` Daniel Borkmann
2014-02-24 12:16 ` Jan Beulich
2014-02-24 12:32 ` H. Peter Anvin
2014-02-24 12:41 ` Jan Beulich
2014-02-24 12:51 ` H. Peter Anvin [this message]
2014-02-24 13:01 ` H. Peter Anvin
2014-02-24 13:07 ` Jan Beulich
2014-02-24 13:09 ` Daniel Borkmann
2014-02-24 13:16 ` H. Peter Anvin
2014-02-24 12:35 ` H. Peter Anvin
2014-02-25 20:26 ` H. Peter Anvin
2014-02-25 20:34 ` Daniel Borkmann
2014-02-25 20:37 ` H. Peter Anvin
2014-02-26 9:29 ` Jan Beulich
2014-02-26 16:06 ` H. Peter Anvin
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