Re: [dpdk-dev] [PATCH v4 3/5] hash: add fallback to software CRC32 implementation

[Bruce Richardson <bruce.richardson@intel.com>]

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 пишет:
> > > > > > > On Tue, Nov 18, 2014 at 09:06:35PM +0600, Yerden Zhumabekov wrote:
> > > > > > >> 18.11.2014 20:41, Neil Horman пишет:
> > > > > > >>> 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