[PATCH] s_sinf.c: Replace floor with FLOOR_DOUBLE_TO_INT/FLOOR_INT_TO_DOUBLE_HALF

[Rajalakshmi Srinivasaraghavan]

On 12/05/2017 07:29 PM, H.J. Lu wrote:
> On Tue, Dec 5, 2017 at 5:32 AM, Joseph Myers<joseph@codesourcery.com>  wrote:
>> On Tue, 5 Dec 2017, H.J. Lu wrote:
>>
>>> diff --git a/sysdeps/generic/math_private.h b/sysdeps/generic/math_private.h
>>> index f29898c19c..a2cdce5b6a 100644
>>> --- a/sysdeps/generic/math_private.h
>>> +++ b/sysdeps/generic/math_private.h
>>> @@ -184,6 +184,14 @@ do {                                                             \
>>>   } while (0)
>>>   #endif
>>>
>>> +#ifndef FLOOR_DOUBLE_TO_INT
>>> +# define FLOOR_DOUBLE_TO_INT(x) ((int) __floor (x))
>>> +#endif
>>> +
>>> +#ifndef FLOOR_INT_TO_DOUBLE_HALF
>>> +# define FLOOR_INT_TO_DOUBLE_HALF(x) __floor ((x) / 2.0)
>>> +#endif
>> These need comments defining their semantics (argument types and ranges,
>> return values and types).
>>
>> The existing code is using unsigned int.  You're using int here for the
>> return type of FLOOR_DOUBLE_TO_INT but still converting to unsigned int in
>> the caller.  (The semantics in the caller do in fact include that the
>> argument is positive but in the range of int.)
>>
>> I'd be surprised if the FLOOR_INT_TO_DOUBLE_HALF definition dividing by
>> 2.0 and calling floor, as opposed to dividing by integer 2 and converting
>> the result to double (which is just a right shift, given the argument is
>> unsigned) is optimal anywhere, unless the compiler optimizes it into an
>> integer shift anyway.  Thus, it would make sense just to use "n / 2"
>> unconditionally in place of "__floor (n / 2.0)".
>>
>> Similarly, I'd expect a direct conversion to int (without a call to
>> __floor) to be at least as good as the present code for
>> FLOOR_DOUBLE_TO_INT everywhere, given the range constraints that make it
>> OK to do so.
>>
>> So I think the natural change would not need to add any new macros, but
>> would change s_sinf.c unconditionally to do essentially what your patch
>> does in the x86_64 case.  As the original patch of Rajalakshmi's was
>> benchmarked on s390, you could ask her to confirm such a change makes
>> performance no worse there (and possibly improves it).
> Here is the patch to replace floor with simple casts.  It improves performance
> of max and mean by 30% on x86-64.  Rajalakshmi, can you try it on s390?

Mean value reduced from 22.0 to 20.0 with this patch on s390x.

-- 
Thanks
Rajalakshmi S