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Less clever sum rrule for the default case #493

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Less clever sum rrule for the default case #493

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34 changes: 33 additions & 1 deletion src/rulesets/Base/mapreduce.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,35 @@ function frule((_, ẋ), ::typeof(sum), x; dims=:)
return sum(x; dims=dims), sum(ẋ; dims=dims)
end

function rrule(::typeof(sum), x::AbstractArray{T}; dims=:) where {T<:Number}
# Internal helper for filling while maintaining array type.
# TODO: Ideallty we'd only need typeof(x) here, but Base doesn't have the
# interfaces for that.
function _typed_fill(x, ȳ, axes)
fill!(similar(x, typeof(ȳ), axes), ȳ)
end

function rrule(::typeof(_typed_fill), x, ȳ, axes)
function _typed_fill_pullback(Ȳ)
return (NoTangent(), NoTangent(), sum(Ȳ), NoTangent())
end
return _typed_fill(x, ȳ, axes), _typed_fill_pullback
end

function sum_rrule(x, dims::Colon)
y = sum(x; dims=dims)
let xdims=size(x)
function sum_pullback(ȳ)
x̄ = InplaceableThunk(
x -> x .+= ȳ,
@thunk(_typed_fill(x, ȳ, xdims...)),
)
return (NoTangent(), x̄)
end
y, sum_pullback
end
end

function sum_rrule(x, dims)
y = sum(x; dims=dims)
function sum_pullback(ȳ)
# broadcasting the two works out the size no-matter `dims`
Expand All @@ -19,6 +47,10 @@ function rrule(::typeof(sum), x::AbstractArray{T}; dims=:) where {T<:Number}
return y, sum_pullback
end

function rrule(::typeof(sum), x::AbstractArray{T}; dims=:) where {T<:Number}
return sum_rrule(x, dims)
end

# Can't map over Adjoint/Transpose Vector
function rrule(
config::RuleConfig{>:HasReverseMode},
Expand Down