[Feature Request] Align::[Adaptive/Element/Vector] mode and use std::array as FixedArray

[jopadan]

Description

Add SIMD compatible alignment support to FixedArray based types and use std::array.

#include <bit>
#include <array>
#include <cstdlib>
#include <cstdio>
#include <cstdint>
#include <cstddef>

enum class Align
{
Adaptive = 0,
Element  = 1,
Vector   = 2,
}

template<typename T, unsigned int VLength = 3, enum Align VAlign = Align::Adaptive, unsigned int VLengthPow2 = std::bit_ceil<unsigned int>(VLength)>
struct alignas(((VLength == VLengthPow2) && (VAlign != Align::Element)) || (VAlign == Align::Vector) ? VLengthPow2 * sizeof(T) : sizeof(T)) FixedArray : std::array<T, VLength>
{
};

FixedArray<float, 3>                 a = {1,2,3};
FixedArray<float, 3, Align::Vector>  b = {1,2,3};
FixedArray<float, 4, Align::Element> c = {1,2,3,4};
FixedArray<float, 4>                 d = {1,2,3,4};

printf("%02zu/%zu\n", alignof(a), sizeof(a));
printf("%02zu/%zu\n", alignof(b), sizeof(b));
printf("%02zu/%zu\n", alignof(c), sizeof(c));
printf("%02zu/%zu\n", alignof(d), sizeof(d));

Expected Behavior

 4/12
16/16
 4/16
16/16

Current behavior

Align::Element = alignas(sizeof(T))/(sizeof(T)*VLength);

Additional Information

• mlr
• TrenchBroom/vm - detailed explanation of the proposed code changes
• vnl issue of these changes
• std::md[span/array] fixed static extents support for arithmetic vector based types adding splice/slice

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[thewtex]

@jopadan great idea! 💡

Backwards-compatible patches are welcome for ITK 6