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

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Date:	Mon, 24 Feb 2014 05:01:16 -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 04:51 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);
  }

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.

	-hpa

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