Fastest way to fill SIMD-vector with data

[Matthias Kretz]

Hi,

On Thursday 31 October 2013 10:30:37 Kulakov, Igor wrote:
> what is actually current the fastest way to fill a vector with data?

an aligned load. Meaning, the data you want to have in the vector is stored in 
memory in the order you will need it later.

> Before Vc didn't let us to assign a data entry by entry and we used Memory
> class. Like this:
> 
>  float_v::Memory tmpFloat;
>  for(int iV=0; iV < float_v::Size; iV++)
>    tmpFloat[iV] = t0[iV]->X();
>  tmpVec.load( tmpFloat );
>  t.SetX(tmpVec);
> 
> 
> But will it be optimised by compiler to be equivalent to desired:
> 
> 
>  for(int iV=0; iV < float_v::Size; iV++)
>    t.fX[iV] = t0[iV]->X();

You can do the latter with Vc now. The former should be equivalent with a good 
optimizing compiler. But, the latter is obviously a lot more readable and 
states the intent of the programmer much clearer.

> I tried to find a use case of Vc::Memory in Vc-0.7 doxygen documentation and
> failed. When exactly we should use this class?

Vc::Memory is meant as a smarter array. I.e. one that knows how to iterate 
over SIMD vectors and scalars. And it ensures padding at the end.

You could also use std::vector<float_v>, but in that case scalar iteration is 
a bit awkward (because std::vector does not provide the extra API):
void f(const std::vector<float_v> &data) {
  for (float_v entry : data)
    for (size_t i = 0; i < entry.Size; ++i)
      std::cout << entry[i] << '\n';
}


-- 
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 Dipl.-Phys. Matthias Kretz

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 Web:   http://compeng.uni-frankfurt.de/?mkretz

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