Behavior of heterogeneous broadcast/operations

[dinarior]

While:

arr = ones(10)
arr2 = ones(1)
CuArray(arr) .- CuArray(arr2)

Works fine, CuArray(arr) .- arr2 fails.
Is that intended?

[vchuravy]

Yes. It is unclear (at least to me) how to handle heterogenous broadcast/operations. Would we copy the GPU array to the host or the host array to the GPU? What if there are 9 CPU arrays and 1 GPU Arrays?

[MikeInnes]

Ideally I guess you'd go by size or something but it'd no longer be type inferable then.

I think just deciding on "always move to GPU" here would be perfectly reasonable. A majority vote of all internal arrays would actually be doable as well.

[ali-ramadhan]

Would it make sense to provide this for expressions such as array1 .= array2? Not sure if only specific/simple broadcast operations can be supported.

In this case it's a bit more obvious. If array1 is a CuArray and array2 is an Array then the contents of array2 are copied from the CPU to the GPU, and vice versa.

[MikeInnes]

Yes, in in-place cases you'd have to move to wherever the target array is. That might not be too hard to implement.

[vchuravy]

If you want that I would recommend explicitly using copyto! instead of implicitly broadcasting identity