Improve rules for sum (#336)

* tweaks to sum rules

* Apply suggestions from code review

Co-authored-by: Lyndon White <oxinabox@ucc.asn.au>

* comma

* v0.7.62

* gone cold one more time...

Co-authored-by: David Widmann <devmotion@users.noreply.github.com>

Co-authored-by: Michael Abbott <me@escbook>
Co-authored-by: Lyndon White <oxinabox@ucc.asn.au>
Co-authored-by: David Widmann <devmotion@users.noreply.github.com>

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRules"
 uuid = "082447d4-558c-5d27-93f4-14fc19e9eca2"
-version = "0.7.62"
+version = "0.7.63"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"

src/rulesets/Base/mapreduce.jl

@@ -10,9 +10,10 @@ function rrule(::typeof(sum), x::AbstractArray{T}; dims=:) where {T<:Number}
     y = sum(x; dims=dims)
     function sum_pullback(ȳ)
         # broadcasting the two works out the size no-matter `dims`
-        x̄ = broadcast(x, ȳ) do xi, ȳi
-            ȳi
-        end
+        x̄ = InplaceableThunk(
+            @thunk(broadcast(last∘tuple, x, ȳ)),
+            x -> x .+= ȳ
+        )
         return (NO_FIELDS, x̄)
     end
     return y, sum_pullback
@@ -29,7 +30,9 @@ function frule(
     ∂y = if dims isa Colon
         2 * real(dot(x, ẋ))
     elseif VERSION ≥ v"1.2" # multi-iterator mapreduce introduced in v1.2
-        2 * mapreduce(_realconjtimes, +, x, ẋ; dims=dims)
+        mapreduce(+, x, ẋ; dims=dims) do xi, dxi
+            2 * _realconjtimes(xi, dxi)
+        end
     else
         2 * sum(_realconjtimes.(x, ẋ); dims=dims)
     end
@@ -44,7 +47,11 @@ function rrule(
 ) where {T<:Union{Real,Complex}}
     y = sum(abs2, x; dims=dims)
     function sum_abs2_pullback(ȳ)
-        return (NO_FIELDS, DoesNotExist(), 2 .* real.(ȳ) .* x)
+        x_thunk = InplaceableThunk(
+            @thunk(2 .* real.(ȳ) .* x),
+            dx -> dx .+= 2 .* real.(ȳ) .* x
+        )
+        return (NO_FIELDS, DoesNotExist(), x_thunk)
     end
     return y, sum_abs2_pullback
 end