kokkos
mdarray: Apply no_unique_address optimization to members of mdarray
@johan-overbye reports that:
mdarrays with all static extents seem to require an extra 4 bytes on top of the container. (Latest MSVC for x86-64 with/std:c++latest.)
The reference implementation of mdarray doesn't currently use any of various techniques (such as [[no_unique_address]]) for not using space for members with no data,
https://github.com/kokkos/mdspan/blob/9ccf361e13dcc3bff2791fda327c2b10e081cb12/include/experimental/__p1684_bits/mdarray.hpp#L454
but the reference implementation of mdspan uses a "compressed pair" abstraction for this purpose.
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https://github.com/kokkos/mdspan/blob/9ccf361e13dcc3bff2791fda327c2b10e081cb12/include/experimental/__p0009_bits/mdspan.hpp#L116
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https://github.com/kokkos/mdspan/blob/9ccf361e13dcc3bff2791fda327c2b10e081cb12/include/experimental/__p0009_bits/mdspan.hpp#L380
We should consider using this abstraction for mdarray as well.
