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Sparse grads for getindex #589

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Sparse grads for getindex #589

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3923244
Init sparse grads for getindex
Drvi Feb 2, 2019
8f74c99
Loosen type signatures for SparseGrad
Drvi Feb 4, 2019
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42 changes: 41 additions & 1 deletion src/tracker/back.jl
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Expand Up @@ -31,14 +31,54 @@ back_(::Call{Missing}, Δ, once) = error("`back!` was already used")

accum!(x, Δ) = x .+ Δ
accum!(x::AbstractArray, Δ) = (x .+= Δ)
struct SparseGrad{T,N,S,P,O} <: AbstractArray{T,N} where O <: AbstractArray{T,N}
Δ::P
i::S
size::NTuple{N,Int}
function SparseGrad(Δ::P, i::S, size::NTuple{N,Int}, x::AbstractArray{T,N}) where {T,N,S,P}
new{T,N,S,P,typeof(x)}(Δ, i, Base.size(x))
end
end
accum!(x::AbstractArray, Δ::SparseGrad) = (@inbounds(x[Δ.i...] += Δ.Δ); return x)
Base.size(x::SparseGrad) = x.size
Base.similar(x::SparseGrad{T,N,S,P,O}) where {T,N,S,P,O} = similar(O, size(x))

#FIXME: Very slow getindex.
function Base.getindex(x::SparseGrad, i...)
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Is it possible to just implement the scalar version and have the rest fall back? Or is this need for the GPU?

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If I remember correctly, the scalar case is worth it only for a very small queries (the indexin()s are expensive), hence I added the allocating version for the general case.

Base.checkbounds_indices(Bool, map(Base.OneTo, size(x)), i) || throw(BoundsError(x, i))

out = zero(x)
@inbounds out[x.i...] = x.Δ
@inbounds out[i...]
end
function Base.getindex(x::SparseGrad{T,N,S,P,O}, i::Int...)::T where {T,N,S,P,O}
Base.checkbounds_indices(Bool, map(Base.OneTo, size(x)), i) || throw(BoundsError(x, i))

li = LinearIndices(size(x))
@inbounds nonempty = li[x.i...]
@inbounds queryindices = li[i...]

outidx = indexin(queryindices, nonempty)[1]
isnothing(outidx) ? zero(T) : @inbounds x.Δ[outidx]::T
end
function Base.getindex(x::SparseGrad{T,N,S,P,O}, i::Int...)::T where {T,N,O,S<:NTuple{N,Int},P<:T}
Base.checkbounds_indices(Bool, map(Base.OneTo, size(x)), i) || throw(BoundsError(x, i))
x.i == i ? x.Δ : zero(T)
end


function back(x::Tracked, Δ, once)
x.isleaf && (x.grad = accum!(x.grad, Δ); return)
ref = x.ref -= 1
grad = if isdefined(x, :grad)
x.grad = accum!(x.grad, Δ)
elseif ref > 0
x.grad = Δ
if Δ isa SparseGrad
x.grad = zero(Δ)
@inbounds x.grad[Δ.i...] = Δ.Δ
else
x.grad = Δ
end
else
Δ
end
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5 changes: 2 additions & 3 deletions src/tracker/lib/array.jl
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Expand Up @@ -97,9 +97,8 @@ Base.getindex(xs::TrackedArray, i...) = track(getindex, xs, i...)

@grad function getindex(xs::AbstractArray, i...)
data(xs)[i...], function (Δ)
Δ′ = zero(xs)
Δ′[i...] = data(Δ)
(nobacksies(:getindex, Δ′), map(_->nothing, i)...)
checkbounds(xs, i...)
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Is this needed given that we already did xs[i...]?

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Good catch:)

(nobacksies(:getindex, SparseGrad(data(Δ), i, size(data(xs)), data(xs))), map(_->nothing, i)...)
end
end

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2 changes: 2 additions & 0 deletions test/tracker.jl
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Expand Up @@ -36,6 +36,8 @@ gradtest(f, dims...) = gradtest(f, rand.(Float64, dims)...)

@testset "indexing & slicing" begin
gradtest(x->view(x, 1:2, 1:2), rand(4, 4))
gradtest(x->getindex(x, 2, :, 3:4, [3,1]), rand(4, 4, 4, 4))
gradtest(x->getindex(x, 1, 2, 3, 4), rand(4, 4, 4, 4))
end

function promotiontest(f, A, B, C)
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