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

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Date:	Mon, 24 Feb 2014 14:09:48 +0100
From:	Daniel Borkmann <borkmann@...earbox.net>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Jan Beulich <JBeulich@...e.com>,
	Daniel Borkmann <dborkman@...hat.com>, 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 02:01 PM, H. Peter Anvin wrote:
> On 02/24/2014 04:51 AM, H. Peter Anvin wrote:
>> 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...
>
> Looking through the DPDK project git history, it seems that this was a bug introduced when changing from using inline assembly to using intrinsics:
>
>   static inline uint32_t
>   rte_hash_crc_4byte(uint32_t data, uint32_t init_val)
>   {
> -    asm volatile("crc32 %[data], %[init_val]"
> -                 : [init_val]"=r" (init_val)
> -                 : [data]"r" (data), "[init_val]" (init_val));
> -    return init_val;
> +    return _mm_crc32_u32(data, init_val);
>   }

Good point, I also just noticed that in the git blame.

> The operand order, of course, of the intrinsic being the opposite of AT&T-style assembly.
>
> I never expected that the CRC32 operation would be commutative.  Very fascinating.

Indeed.

>      -hpa
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