Efficient SMatrix SubArrays

[tkoolen]

Maybe this is an issue for Base, but I thought I'd get your input first.

I have an application where I know the maximum size of a matrix at compile time, but the number of columns is only known at runtime. I was hoping that combining SMatrix and view would solve this problem. Making a view of an SMatrix works fine (awesome!), but there's a performance issue.

Consider the following code:

using StaticArrays
function view_colon()
    data = rand(SMatrix{3, 3, Float64})
    view(data, :, 1:2)
end

function view_range()
    data = rand(SMatrix{3, 3, Float64})
    view(data, 1:3, 1:2)
end

Base.linearindexing(view_colon()) # Base.LinearFast()
Base.linearindexing(view_range()) # Base.LinearSlow()

isbits(view_colon()) # false
isbits(view_range()) # true

So currently, I have to choose between a LinearFast view that is not isbits (resulting in lots of allocations down the line), or a LinearSlow view that is.

The only reason the colon version is not isbits seems to be that Colon is a type, instead of being immutable. So would it be possible to change that in Base without breaking things? Or should this be handled in StaticArrays instead?

[timholy]

Please do fix this in Base. Thanks for noticing this!

[andyferris]

Oh dear... Colon is a type not an immutable. Seems like a mistake in Base to me, also.

[andyferris]

In fact, it's an easy enough mistake to make that I think the compiler should probably treat all field-less types as isbits singletons. I can't see the point of having a mutable singleton... It seems that Colon.instance is defined so maybe this is a pretty trivial fix for isbits? Or am I overlooking something?

[andyferris]

For now @tkoolen, have you considered using a Vector of SVectors, or else a full SVector and keep track of how many elements you use separately? Maybe this would be good enough for the mean-time.

[tkoolen]

Thanks. I'm not in a hurry, so I'd prefer to have this be fixed in Base rather than hack around it. I'll submit a PR to Base once I'm done running tests.

[tkoolen]

Fixed in base.

[andyferris]

Nice work! Thanks