> -----Original Message----- > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Bruce Richardson > Sent: Wednesday, November 19, 2014 11:38 AM > To: Neil Horman > Cc: dev at dpdk.org > Subject: Re: [dpdk-dev] [PATCH v4 3/5] hash: add fallback to software CRC32 > implementation > > On Wed, Nov 19, 2014 at 06:34:08AM -0500, Neil Horman wrote: > > On Wed, Nov 19, 2014 at 10:16:14AM +0000, Bruce Richardson wrote: > > > On Tue, Nov 18, 2014 at 04:36:24PM -0500, Neil Horman wrote: > > > > On Tue, Nov 18, 2014 at 05:52:27PM +0000, Bruce Richardson wrote: > > > > > On Tue, Nov 18, 2014 at 12:46:19PM -0500, Neil Horman wrote: > > > > > > On Tue, Nov 18, 2014 at 11:13:17PM +0600, Yerden Zhumabekov wrote: > > > > > > > > > > > > > > 18.11.2014 22:00, Neil Horman ?????: > > > > > > > > On Tue, Nov 18, 2014 at 09:06:35PM +0600, Yerden Zhumabekov > > > > > > > > wrote: > > > > > > > >> 18.11.2014 20:41, Neil Horman ?????: > > > > > > > >>> On Tue, Nov 18, 2014 at 08:03:40PM +0600, Yerden Zhumabekov > > > > > > > >>> wrote: > > > > > > > >>>> /** > > > > > > > >>>> * Use single crc32 instruction to perform a hash on a 4 > > > > > > > >>>> byte value. > > > > > > > >>>> + * Fall back to software crc32 implementation in case > > > > > > > >>>> SSE4.2 is > > > > > > > >>>> + * not supported > > > > > > > >>>> * > > > > > > > >>>> * @param data > > > > > > > >>>> * Data to perform hash on. > > > > > > > >>>> @@ -376,11 +413,18 @@ crc32c_2words(uint64_t data, uint32_t > > > > > > > >>>> init_val) > > > > > > > >>>> static inline uint32_t > > > > > > > >>>> rte_hash_crc_4byte(uint32_t data, uint32_t init_val) > > > > > > > >>>> { > > > > > > > >>>> - return _mm_crc32_u32(init_val, data); > > > > > > > >>>> +#ifdef RTE_MACHINE_CPUFLAG_SSE4_2 > > > > > > > >>>> + if (likely(crc32_alg == CRC32_SSE42)) > > > > > > > >>>> + return _mm_crc32_u32(init_val, data); > > > > > > > >>>> +#endif > > > > > > > >>> you don't really need these ifdefs here anymore given that > > > > > > > >>> you have a > > > > > > > >>> constructor to do the algorithm selection. In fact you need > > > > > > > >>> to remove them, in > > > > > > > >>> the event you build on a system that doesn't support SSE42, > > > > > > > >>> but run on a system > > > > > > > >>> that does. > > > > > > > >> Originally, I thought so as well. I wrote the code without > > > > > > > >> these ifdefs, > > > > > > > >> but it didn't compile on my machine which doesn't support > > > > > > > >> SSE4.2. Error > > > > > > > >> was triggered by nmmintrin.h which has a check for respective > > > > > > > >> GCC > > > > > > > >> extension. So I think these ifdefs are indeed required. > > > > > > > >> > > > > > > > > You need to edit the makefile so that the compiler gets passed > > > > > > > > the option > > > > > > > > -msse42. That way it will know to emit sse42 instructions. It > > > > > > > > will also allow > > > > > > > > you to remove the ifdef from the include file > > > > > > > > > > > > > > In this case, I guess there are two options: > > > > > > > 1) modify all makefiles which use librte_hash > > > > > > > 2) move all function bodies from rte_hash_crc.h to separate > > > > > > > module, > > > > > > > leaving prototype definitions there only. > > > > > > > > > > > > > > Everybody's up for the second option? :) > > > > > > > > > > > > > Crud, you're right, I didn't think about the header inclusion > > > > > > issue. Is it > > > > > > worth adding the jump to enable the dynamic hash selection? > > > > > > Neil > > > > > > > > > > Maybe for cases where SSE4.2 is not currently available, i.e. for > > > > > generic builds. > > > > > For builds where we have hardware support confirmed at compile time, > > > > > just use > > > > > the function from the header file. > > > > > Does that make sense? > > > > > > > > > I'm not certain of that, as I don't think anything can be 'confirmed' > > > > at compile > > > > time. I.e. just because you have sse42 at compile time doesn't > > > > guarantee you > > > > have it at run time with a DSO. If you have these as macros, you need > > > > to enable > > > > sse42 whereever you include the file so that the intrinsic works > > > > properly. > > > > > > Well, if you compile with sse42 at compile time, the compiler is free to > > > insert > > > sse4 instructions at any place it feels like, irrespective of whether or > > > not you > > > use SSE4 intrinsics, so I would never expect such a DSO to work on a > > > system > > > without SSE42 support. > > > > > > > > > > > an alternate option would be to not use the intrinsic, and craft some > > > > explicit > > > > __asm__ statement that executes the right sse42 instructions. That way > > > > the asm > > > > is directly emitted, without requiring the -msse42 flag at all, and it > > > > will just > > > > work in all the files that call it. > > > > > > > > > > I really don't like that approach. I think using intrinsics is much more > > > maintainable. > > > > > I grant you that using an intrinsic is easier to read, but if the code > > doesn't > > compile when using the intrinsic unless you have sse42 turned on, I'm not > > sure > > what choice we have. and inline asm isn't that hard to maintain. We're > > talking > > about three lines of code: > > asm( > > "mov %[1],%eax > > mov %[2],%edx > > crc32l %edx,%eax": > > [edx] "r" (crc) /*output*/ > > : > > [1] "r" (crc), /* input */ > > [2] "r" (val) > > : > > [eax] "r" /* clobber */ > > ) > > > > I don't have the syntax quite right, but its pretty easy to read the intent. > > Its not like we dont have precidence for this, the atomic interface and > > several > > pmds do this frequently. > > > > Neil > > Fair point. If everyone else is happy enough with it, I'm ok too.
As I remember with gcc & icc it is possible to specify tht you'd like to compile that particular function for different target.
