Messages in this thread |  | | Date | Mon, 24 Feb 2014 05:01:16 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 2/3] x86/hash: swap parameters of crc32_u32() |
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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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