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Add AccThunk to avoid prematurely unthunking thunks #1562

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Add AccThunk to avoid prematurely unthunking thunks #1562

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1 change: 1 addition & 0 deletions src/Zygote.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@ using IRTools
using MacroTools
using MacroTools: @forward

import ChainRules
import Distributed: pmap, CachingPool, workers
export Params, withgradient, gradient, withjacobian, jacobian, hessian, diaghessian, pullback, pushforward, @code_adjoint
export rrule_via_ad
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27 changes: 16 additions & 11 deletions src/lib/lib.jl
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@@ -1,15 +1,22 @@
using Base: RefValue
using Base: ismutabletype
using Base: RefValue, ismutabletype, tail

struct AccThunk{T} <: AbstractThunk
thunks::Vector{T}
end

_maybe_thunks(x) = x
_maybe_thunks(x::AccThunk) = x.thunks

ChainRules.unthunk(acc::AccThunk) = reduce(accum, unthunk.(acc.thunks))

# Interfaces

accum() = nothing
accum(x) = x

accum(x, y) =
x === nothing ? y :
y === nothing ? x :
x + y
x === nothing ?
y : (y === nothing ? x : x + y)

accum(x, y, zs...) = accum(accum(x, y), zs...)

Expand All @@ -35,9 +42,9 @@ accum(x::ChainRulesCore.Tangent, y::NamedTuple) = accum(wrap_chainrules_output(x
accum(x::Nothing, y::AbstractThunk) = y
accum(x::AbstractThunk, y::Nothing) = x

accum(x, y::AbstractThunk) = accum(x, unthunk(y))
accum(x::AbstractThunk, y) = accum(unthunk(x), y)
accum(x::AbstractThunk, y::AbstractThunk) = accum(unthunk(x), unthunk(y))
accum(x::AbstractThunk, y::AbstractThunk) = AccThunk(vcat(_maybe_thunks(x), _maybe_thunks(y)))
accum(x::AbstractThunk, y) = AccThunk(vcat(_maybe_thunks(x), [y]))
accum(x, y::AbstractThunk) = AccThunk(vcat([x], _maybe_thunks(y)))

# Core functions
@_adjoint_keepthunks deepcopy(x) = deepcopy(x), ȳ -> (ȳ,)
Expand All @@ -55,7 +62,7 @@ accum_param(::Context{false}, _, Δ) = Δ
isbitstype(x) && return :(Δ)
quote
if haskey(cache(cx), x)
cache(cx)[x] = accum(cache(cx)[x],Δ)
cache(cx)[x] = accum(cache(cx)[x], Δ)
return
else
return Δ
Expand Down Expand Up @@ -96,8 +103,6 @@ end

# Tuples

using Base: tail

@_adjoint_keepthunks tuple(xs...) = xs, identity

@_adjoint_keepthunks function literal_getindex(xs::NTuple{N,Any}, ::Val{i}) where {N,i}
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12 changes: 12 additions & 0 deletions test/features_tests.jl
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Expand Up @@ -882,4 +882,16 @@ end
@test g1[1] ≈ g2[1] ≈ g3[1]
end

# https://github.com/FluxML/Flux.jl/issues/2585
@testset "Nested thunks" begin
W = ones(Float32, 10, 10)
x = [ones(Float32, 10) for i in 1:512]
gs = gradient(W) do W
sum((W * xi)[1] for xi in x)
end
dW = gs[1]
@test all(dW[1, :] .== 512)
@test all(dW[2:end, :] .== 0)
end

end
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