Align SoA subarrays when using single blob

[bernhardmgruber]

This changeset will align SoA sub arrays by default when using a single blob. This can cause a potential performance loss, but improves correctness.

[codecov]

Codecov Report

Merging #648 (d21a40a) into develop (78d38bd) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #648   +/-   ##
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  Coverage    98.75%   98.75%           
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  Files           74       74           
  Lines         6818     6818           
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  Hits          6733     6733           
  Misses          85       85           

[psychocoderHPC]

Why could this result in a performance loss?

[bernhardmgruber]

Because I need to do additional arithmetic in the mapping function which depends on the array size to compute the necessary padding. If the array size is runtime, this padding can also only be computed at runtime. Furthermore, you need to repeat that computation for each struct member that comes before the one you are accessing. It is essentially this loop: https://github.com/alpaka-group/llama/blob/develop/include/llama/mapping/SoA.hpp#L119-L127. With #653, I can precompute a table and avoid the loop iff the array extents are known at compile time.