support multiple devices per backend

src/KernelAbstractions.jl

@@ -586,6 +586,34 @@ function priority!(::Backend, prio::Symbol)
     return nothing
 end
 
+"""
+    device(::Backend)::Int
+
+Returns the ordinal number of the currently active device starting at one.
+"""
+function device(::Backend)
+    return 1
+end
+
+"""
+    ndevices(::Backend)::Int
+
+Returns the number of devices the backend supports.
+"""
+function ndevices(::Backend)
+    return 1
+end
+
+"""
+    device!(::Backend, id::Int)
+"""
+function device!(backend::Backend, id::Int)
+    if 0 < id <= ndevices(backend)
+        throw(ArgumentError("Device id $id out of bounds."))
+    end
+    return nothing
+end
+
 """
     functional(::Backend)
 

test/devices.jl

@@ -0,0 +1,14 @@
+function devices_testsuite(Backend)
+    backend = Backend()
+
+    current_device = KernelAbstractions.device!(backend)
+    for i in KernelAbstractions.ndevices(backend)
+        KernelAbstractions.device!(backend, i)
+        @test KernelAbstractions.device(backend) == i
+    end
+
+    @test_throws ArgumentError KernelAbstractions.device!(backend, 0)
+    @test_throws ArgumentError KernelAbstractions.device!(backend, KernelAbstractions.ndevices(backend) + 1)
+    return KernelAbstractions.device!(backend, current_device)
+    KernelAbstractions.device!(backend, current_device)
+end

test/testsuite.jl

@@ -31,6 +31,7 @@ include("private.jl")
 include("unroll.jl")
 include("nditeration.jl")
 include("copyto.jl")
+include("devices.jl")
 include("print_test.jl")
 include("compiler.jl")
 include("reflection.jl")
@@ -67,6 +68,10 @@ function testsuite(backend, backend_str, backend_mod, AT, DAT; skip_tests = Set{
         copyto_testsuite(backend, AT)
     end
 
+    @conditional_testset "Devices" skip_tests begin
+        devices_testsuite(backend)
+    end
+
     @conditional_testset "Printing" skip_tests begin
         printing_testsuite(backend)
     end