Allow init argument for sum (#306)

src/dispatch.jl

@@ -13,8 +13,8 @@
 
 abstract type AbstractMutable end
 
-function Base.sum(a::AbstractArray{<:AbstractMutable})
-    return operate(sum, a)
+function Base.sum(a::AbstractArray{<:AbstractMutable}; kwargs...)
+    return operate(sum, a; kwargs...)
 end
 
 # When doing `x'y` where the elements of `x` and/or `y` are arrays, redirecting

src/reduce.jl

@@ -50,11 +50,10 @@ function fused_map_reduce(op::F, args::Vararg{Any,N}) where {F<:Function,N}
     return accumulator
 end
 
-function operate(::typeof(sum), a::AbstractArray)
-    return mapreduce(
-        identity,
-        add!!,
-        a;
-        init = zero(promote_operation(+, eltype(a), eltype(a))),
-    )
+function operate(
+    ::typeof(sum),
+    a::AbstractArray;
+    init = zero(promote_operation(+, eltype(a), eltype(a))),
+)
+    return mapreduce(identity, add!!, a; init)
 end

test/dispatch.jl

@@ -106,3 +106,28 @@ end
         end
     end
 end
+
+function non_mutable_sum_pr306(x)
+    y = zero(eltype(x))
+    for xi in x
+        y += xi
+    end
+    return y
+end
+
+@testset "sum_with_init" begin
+    x = convert(Vector{DummyBigInt}, 1:100)
+    # compilation
+    @allocated sum(x)
+    @allocated sum(x; init = DummyBigInt(0))
+    @allocated non_mutable_sum_pr306(x)
+    # now test actual allocations
+    no_init = @allocated sum(x)
+    with_init = @allocated sum(x; init = DummyBigInt(0))
+    no_ma = @allocated non_mutable_sum_pr306(x)
+    # There's an additional 16 bytes for kwarg version. Upper bound by 40 to be
+    # safe between Julia versions
+    @test with_init <= no_init + 40
+    # MA is at least 10-times better than no MA for this example
+    @test 10 * with_init < no_ma
+end