Allow `return` or `return nothing` in kernels

[eschnett]

I use JuliaFormatter to format my source code. This adds explicit return statements. This leads to

ERROR: LoadError: Return statement not permitted in a kernel function sum2_kernel!

even if the return statement is just return nothing.

At the same time, ending a kernel with an expression (which is equivalent to a return statement) is allowed.

Could this check be changed to allow only empty return or return nothing statements?

[vchuravy]

At the same time, ending a kernel with an expression (which is equivalent to a return statement) is allowed.

KernelAbstractions will politly ignore those and insert a return nothing for you.
But yes we could explicitly allow return or return nothing

[pxl-th]

Closed by #446.

[AntonReinhard]

Related to this, I ran into a problem (or at least inconvenience) when defining local anonymous functions inside kernels:

@kernel function foo(v)
    bar = x -> begin y = 2*x; return y end
    ID = @index(Global)
    v[ID] = bar(v[ID])
end

Is not allowed by KernelAbstractions and tells me that I can't use return inside a kernel, even though this return clearly doesn't return from the kernel. This problem is fixed if I use instead:

@kernel function foo(v)
    bar = x -> begin y = 2*x; y end
    ID = @index(Global)
    v[ID] = bar(v[ID])
end

Which is valid, compiles, and runs as expected.

[szabo137]

+1 on that