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

["H.J. Lu" <hjl dot tools at gmail dot com>]

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?

Thanks.


-- 
H.J.

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