Re: [boost] Boost SIMD beta release

Subject: Re: [boost] Boost SIMD beta release
From: Joel Falcou (joel.falcou_at_[hidden])
Date: 2012-12-20 13:01:47

Le 20/12/2012 18:34, Peter Dimov a écrit :
> What is the recommended Boost.SIMD way to write a function like
>
>     void add_n( float const * s, float const * s2, float * d, size_t n );
>     // d[i] = s[i] + s2[i]
>
> where none of s, s2, d are guaranteed to be aligned?

You should align them ;)

More seriously, you can run a for using with pack and unaligned_load/store:

void add_n( float const * s, float const * s2, float * d, size_t n )
{
   size_t c = pack<float>::static_size;
   size_t vn = v / c * c;
   size_t sn = v % c;

   for(std::size_t i=0, i<vn; i+= c, d+=c,s+=c,s2+=c)
     store(unaligned_load<pack<T>>(s) + unaligned_load<pack<T>>(s2), d );

   for(std::size_t i=0, i<sn; i++,d++,s++,s2++)
     *d = *s + *s2;
}

This also takes the fact n can be not a multiple of pack cardinal into 
account.

If pointer is aligned you can actually just use simd::input_iterator and 
feed this to std::transform.

Maybe we should have a simd::unaligned_input_iterator and/or make 
simd::transform accept non aligned data. Note that on any pre-Nehalem 
CPU, the unaligned load will be horrendsously slow.