Messages in this thread | | | Date | Mon, 24 Feb 2014 14:09:48 +0100 | From | Daniel Borkmann <> | Subject | Re: [PATCH 2/3] x86/hash: swap parameters of crc32_u32() |
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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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