Slow but working copyto

ext/StridedGPUArraysExt.jl

@@ -1,6 +1,6 @@
 module StridedGPUArraysExt
 
-using Strided, GPUArrays
+using Strided, GPUArrays, LinearAlgebra
 using GPUArrays: Adapt, KernelAbstractions
 using GPUArrays.KernelAbstractions: @kernel, @index
 
@@ -34,7 +34,29 @@ function Base.fill!(A::StridedView{T, N, TA, F}, x) where {T, N, TA <: AbstractG
     return A
 end
 
-function Strided.__mul!(
+# kernel-based variant for copying between wrapped GPU arrays
+@kernel function linear_copy_kernel!(dest, dstart, src, sstart, n)
+    i = @index(Global, Linear)
+    if i <= n
+        @inbounds dest[dstart+i-1] = src[sstart+i-1]
+    end
+end
+
+function Base.copyto!(dest::StridedView{TD, ND, TAD, FD}, dstart::Integer,
+                      src::StridedView{TS, NS, TAS, FS}, sstart::Integer, n::Integer) where {TD, TS, ND, NS, TAD <: AbstractGPUArray{TD}, TAS <: AbstractGPUArray{TS}, FD, FS}
+    n == 0 && return dest
+    n < 0 && throw(ArgumentError(string("tried to copy n=", n, " elements, but n should be nonnegative")))
+    destinds, srcinds = LinearIndices(dest), LinearIndices(src)
+    (checkbounds(Bool, destinds, dstart) && checkbounds(Bool, destinds, dstart+n-1)) || throw(BoundsError(dest, dstart:dstart+n-1))
+    (checkbounds(Bool, srcinds, sstart)  && checkbounds(Bool, srcinds, sstart+n-1))  || throw(BoundsError(src,  sstart:sstart+n-1))
+    kernel = linear_copy_kernel!(KernelAbstractions.get_backend(dest))
+    kernel(dest, dstart, src, sstart, n; ndrange=n)
+    return dest
+end
+Base.copyto!(dest::StridedView{TD, ND, TAD, FD}, src::StridedView{TS, NS, TAS, FS}) where {TD, TS, ND, NS, TAD <: AbstractGPUArray{TD}, TAS <: AbstractGPUArray{TS}, FD, FS} = copyto!(dest, 1, src, 1, length(src))
+
+
+function LinearAlgebra.mul!(
         C::StridedView{TC, 2, <:AnyGPUArray{TC}},
         A::StridedView{TA, 2, <:AnyGPUArray{TA}},
         B::StridedView{TB, 2, <:AnyGPUArray{TB}},