Support and use broadcast with mapreduce.

src/host/mapreduce.jl

@@ -1,9 +1,14 @@
 # map-reduce
 
+const AbstractArrayOrBroadcasted = Union{AbstractArray,Broadcast.Broadcasted}
+
 # GPUArrays' mapreduce methods build on `Base.mapreducedim!`, but with an additional
 # argument `init` value to avoid eager initialization of `R` (if set to something).
-mapreducedim!(f, op, R::AbstractGPUArray, As::AbstractArray...; init=nothing) = error("Not implemented") # COV_EXCL_LINE
+mapreducedim!(f, op, R::AbstractGPUArray, A::AbstractArrayOrBroadcasted;
+              init=nothing) = error("Not implemented") # COV_EXCL_LINE
+# resolve ambiguities
 Base.mapreducedim!(f, op, R::AbstractGPUArray, A::AbstractArray) = mapreducedim!(f, op, R, A)
+Base.mapreducedim!(f, op, R::AbstractGPUArray, A::Broadcast.Broadcasted) = mapreducedim!(f, op, R, A)
 
 neutral_element(op, T) =
     error("""GPUArrays.jl needs to know the neutral element for your operator `$op`.
@@ -18,11 +23,19 @@ neutral_element(::typeof(Base.mul_prod), T) = one(T)
 neutral_element(::typeof(Base.min), T) = typemax(T)
 neutral_element(::typeof(Base.max), T) = typemin(T)
 
-function Base.mapreduce(f, op, As::AbstractGPUArray...; dims=:, init=nothing)
+# resolve ambiguities
+Base.mapreduce(f, op, A::AbstractGPUArray, As::AbstractArrayOrBroadcasted...;
+               dims=:, init=nothing) = _mapreduce(f, op, A, As...; dims=dims, init=init)
+Base.mapreduce(f, op, A::Broadcast.Broadcasted{<:AbstractGPUArrayStyle}, As::AbstractArrayOrBroadcasted...;
+               dims=:, init=nothing) = _mapreduce(f, op, A, As...; dims=dims, init=init)
+
+function _mapreduce(f, op, As...; dims, init)
+    bc = Broadcast.instantiate(Broadcast.broadcasted(f, As...))
+
     # figure out the destination container type by looking at the initializer element,
     # or by relying on inference to reason through the map and reduce functions.
     if init === nothing
-        ET = Base.promote_op(f, map(eltype, As)...)
+        ET = Broadcast.combine_eltypes(bc.f, bc.args)
         ET = Base.promote_op(op, ET, ET)
         (ET === Union{} || ET === Any) &&
             error("mapreduce cannot figure the output element type, please pass an explicit init value")
@@ -32,14 +45,10 @@ function Base.mapreduce(f, op, As::AbstractGPUArray...; dims=:, init=nothing)
         ET = typeof(init)
     end
 
-    # TODO: Broadcast-semantics after JuliaLang-julia#31020
-    A = first(As)
-    all(B -> size(A) == size(B), As) || throw(DimensionMismatch("dimensions of containers must be identical"))
-
-    sz = size(A)
-    red = ntuple(i->(dims==Colon() || i in dims) ? 1 : sz[i], ndims(A))
-    R = similar(A, ET, red)
-    mapreducedim!(f, op, R, As...; init=init)
+    sz = size(bc)
+    red = ntuple(i->(dims==Colon() || i in dims) ? 1 : sz[i], length(sz))
+    R = similar(bc, ET, red)
+    mapreducedim!(identity, op, R, bc; init=init)
 
     if dims==Colon()
         @allowscalar R[]

src/reference.jl

@@ -298,11 +298,12 @@ Adapt.adapt_storage(::Adaptor, x::JLArray{T,N}) where {T,N} =
 GPUArrays.unsafe_reinterpret(::Type{T}, A::JLArray, size::Tuple) where T =
     reshape(reinterpret(T, A.data), size)
 
-function GPUArrays.mapreducedim!(f, op, R::JLArray, As::AbstractArray...; init=nothing)
+function GPUArrays.mapreducedim!(f, op, R::JLArray, A::Union{AbstractArray,Broadcast.Broadcasted};
+                                 init=nothing)
     if init !== nothing
         fill!(R, init)
     end
-    @allowscalar Base.reducedim!(op, R.data, map(f, As...))
+    @allowscalar Base.reducedim!(op, R.data, map(f, A))
 end
 
 end

test/testsuite/mapreduce.jl

@@ -158,6 +158,15 @@ function test_mapreduce(AT)
                         ET <: Complex || @test compare(minimum, AT,rand(range, dims))
                     end
                 end
+                @testset "broadcasting behavior" begin
+                        @test compare((x,y)->mapreduce(+, +, x, y), AT,
+                                      rand(range, 1), rand(range, 2, 2))
+                        @test compare(AT, rand(range, 1), rand(range, 2, 2)) do x, y
+                            bc = Broadcast.instantiate(Broadcast.broadcasted(*, x, y))
+                            reduce(bc)
+                        end
+
+                end
             end
         end
         @testset "any all ==" begin