ManyPencilArraysRFFT!: avoid inefficient view + reshape

[fbignonnet]

Hi,

I propose a minor update of multiarrays_r2c.jl to fix two issues for the use of the view to the first (real valued) PencilArray of the ManyPencilArraysRFFT!.

Since the input real valued array of a RFFT uses less memory than the complex output, the inplace RFFT! requires that the view to the real valued array is reshaped from a vector SubArray vec of some larger vector data. This was previously done using vec = view(data, Base.OneTo(n)).

However reshape(vec, dims) outputs a Base.ReshapedArray{T, N, SubArray{T, 1, Vector{T}, Tuple{UnitRange{Int64}}, true}, Tuple{}}, for which the compiler may produce inefficient code.

The proposed alternative is to use vec = unsafe_wrap(typeof(data), pointer(data), n), after which reshape(vec, dims) outputs a simple Array{T,N}.

This fixes:

1. Some performance issues for the use of the real valued PencilArray view of the ManyPencilArraysRFFT!
2. Error while saving the real valued PencilArray view using PencilArrays.PencilIO

The same idea could be used in function _make_arrays of multiarrays.jl of PencilArrays, line 137: vec = view(data, Base.OneTo(n)).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.97%. Comparing base (0429e73) to head (fb0f12a).
Report is 4 commits behind head on master.

Additional details and impacted files

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##           master      #75      +/-   ##
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+ Coverage   96.81%   96.97%   +0.15%     
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  Files          11       11              
  Lines         534      529       -5     
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- Hits          517      513       -4     
+ Misses         17       16       -1     

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[jipolanco]

Looks good, thanks!

Note that we already use the same trick in PencilArrays transpositions (here.

And I agree, I'll do the same for ManyPencilArray in PencilArrays.

[fbignonnet]

Great, thanks!