JuliaDiff / JuliaDiff/ReverseDiff.jl

The number of arguments limitation of the forward macro.

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Julia
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Description

The docstring of ReverseDiff.@forward says:

Currently, only length(args) <= 2 is supported.

However, in a simple function, this macro looks working well even for >3 arguments:

julia> import ReverseDiff

julia> ReverseDiff.@forward f(x, y, z, w) = x + 2y + 3z + 4w
ReverseDiff.ForwardOptimize{##hidden_f}(#hidden_f)

julia> f(1.0, 2.0, 3.0, 4.0)
30.0

julia> ∇f = ReverseDiff.compile_gradient(x -> f(x[1], x[2], x[3], x[4]), zeros(4))
(::#301) (generic function with 1 method)

julia> ∇f(zeros(4), ones(4))
4-element Array{Float64,1}:
 1.0
 2.0
 3.0
 4.0

Can I expect this works always or is there any pitfall in the case?

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Research direction

Start by reading the ReverseDiff.@forward docstring and reproducing the shown example with more than two arguments. Then inspect how @forward interacts with compile_gradient; done means establishing whether the documented limitation is real and recording the supported behavior or any pitfall.

Written by the indexing model from the issue text.

Assessment

Tech stack
julia
Domain
tooling
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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