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.
/Bruce
