Store the array length next to its dimensions. (JuliaGPU#1303)

src/device/array.jl

@@ -28,12 +28,13 @@ struct CuDeviceArray{T,N,A} <: DenseArray{T,N}
     maxsize::Int
 
     dims::Dims{N}
+    len::Int
 
     # inner constructors, fully parameterized, exact types (ie. Int not <:Integer)
     # TODO: deprecate; put `ptr` first like CuArray
     CuDeviceArray{T,N,A}(dims::Dims{N}, ptr::LLVMPtr{T,A},
                          maxsize::Int=prod(dims)*sizeof(T)) where {T,A,N} =
-        new(ptr, maxsize, dims)
+        new(ptr, maxsize, dims, prod(dims))
 end
 
 const CuDeviceVector = CuDeviceArray{T,1,A} where {T,A}
@@ -67,11 +68,13 @@ CuDeviceMatrix{T,A}(m::Integer, n::Integer,   p::LLVMPtr{T,A}) where {T,A}   = C
 ## array interface
 
 Base.elsize(::Type{<:CuDeviceArray{T}}) where {T} = sizeof(T)
-Base.size(g::CuDeviceArray) = g.dims
-Base.length(g::CuDeviceArray) = prod(g.dims)
 
+Base.size(g::CuDeviceArray) = g.dims
 Base.sizeof(x::CuDeviceArray) = Base.elsize(x) * length(x)
 
+# we store the array length too; computing prod(size) is expensive
+Base.length(g::CuDeviceArray) = g.len
+
 Base.pointer(x::CuDeviceArray{T,<:Any,A}) where {T,A} = Base.unsafe_convert(LLVMPtr{T,A}, x)
 @inline function Base.pointer(x::CuDeviceArray{T,<:Any,A}, i::Integer) where {T,A}
     Base.unsafe_convert(LLVMPtr{T,A}, x) + Base._memory_offset(x, i)