Fixing Tensor Extents

[amitsingh19975]

There is an issue with the current implementation of extents where we cannot discern the difference between static extents and fixed rank extents when the order of extents is one.

auto e = extents<4>{}; // fixed rank extents or static extents

Solution 1

Introduce an invariant where the order of extents is always greater than one. It makes the extents distinguishable.

auto e1 = extents<4>{}; // fixed rank extents
auto e2 = extents<1,4>{}; // static extents

// Valid or Invalid extents after the fix
auto e0 = extents<1>{}; // [static rank extents] invalid because of 1st order static rank extents
auto e1 = extents<1,2>{}; // [static extents] valid because of 2st order static extents
auto e2 = extents<2>{}; // [static rank extents] valid because of 2st order static rank extents
auto e3 = extents<>{4}; // [dynamic extents] invalid because of 1st order dynamic extents
auto e4 = extents<>{4,4}; // [dynamic extents] valid because of 2st order dynamic extents

But one problem arises with this approach: we lose the ability to alias our tensor into vector. This problem only plagues fixed rank tensor because when we create a vector through an alias, we always have to provide one and the size of the vector.

auto t = vector<float>{1,4}; // vector of length 4

// With no invariant
auto t = vector<float>{4}; // vector of length 4

And I don't know which solution is better:

• should we put an invariant on static rank extents where the minimum order should be greater than one and lose the ability to alias the tensor into a vector?
• should we make an exception for static rank extents and keep the aliasing ability.

Solution 2

We change the static extents' first element into the size of the extents and keep everything the same.

auto e1 = extents<4>{}; // fixed rank extents
auto e2 = extents</*Order*/1, 4>{}; // static extents with order 1

Solution 3

Use C++ 20 feature, "template non-type arguments," where the first template argument for fixed rank extents could be some other type than "std::size_t." And if we get "std::size_t," we can be hundred per cent sure that it's static extents.

auto e1 = extents<Size(4)>{}; // fixed rank extents
auto e2 = extents<4>{}; // static extents with order 1

Personal comment

I'm more inclined with solution one.