ReverseDiff.value(output) depends on input of tape for mutating functions

[frankschae]

When using ReverseDiff to compute the gradients of an inplace function f!, we found that the output value of ReverseDiff depends on the input value of the tape (y*2 in the MWE). For the non-mutating input function f, one obtains in

(ReverseDiff.deriv(tu), ReverseDiff.deriv(tp), ReverseDiff.value(output)) = ([-8.0], [-16.0], [8.0])

However, for the mutating function, one gets

(ReverseDiff.deriv(tu), ReverseDiff.deriv(tp), ReverseDiff.value(output)) = ([-8.0], [-16.0], 16.0])

i.e., vec(du1) != ReverseDiff.value(output)) in the last line of the MWE.
This happens for both a compiled and a non-compiled tape.

using ReverseDiff
p2 = [2.0]

f!(du,u,p,t) = du.=p[1]*u

f(u,p,t) = p[1]*u

y = [4.0]
λ = -y
t = 1.0
tape = ReverseDiff.GradientTape((y*2, p2, [t])) do u,p,t 
  du1 = similar(u, size(u))
  f!(du1,u,p,first(t))
  return vec(du1)
end

config = ReverseDiff.compile(tape)

# tape = ReverseDiff.GradientTape((y*2, p2, [t])) do u,p,t
#   vec(f(u,p,first(t)))
# end

tu, tp, tt = ReverseDiff.input_hook(config)
output = ReverseDiff.output_hook(config)
ReverseDiff.unseed!(tu) # clear any "leftover" derivatives from previous calls
ReverseDiff.unseed!(tp)
ReverseDiff.value!(tu, y)
ReverseDiff.value!(tp, p2)
ReverseDiff.value!(tt, [t])
ReverseDiff.forward_pass!(config)
ReverseDiff.increment_deriv!(output, λ)
ReverseDiff.reverse_pass!(config)
@show ReverseDiff.deriv(tu), ReverseDiff.deriv(tp), ReverseDiff.value(output)
### ([-8.0], [-16.0], [16.0])
du1 = similar(y, size(y))
f!(du1,y,p2,first(t))
vec(du1) == vec(f(y,p2,first(t)))
vec(du1) == ReverseDiff.value(output)

[ChrisRackauckas]

The behavior of this was correctly changing the input on v1.2.1 but not on v1.4, so unless @mohamed82008 has an idea what could've caused the change, the best thing to do might be to start with a git bisection to find the commit that changed the result here.

[mohdibntarek]

I will take a look.

[ChrisRackauckas]

@frankschae did you ever bisect this?

[frankschae]

no, I just checked that it doesn't occur at the v1.2.0 tag

[frankschae]

v1.3.0 is still fine, on v1.4.0 the error occurs.

(More precisely, If I checkout

commit: d1b9f22
Merge pull request #125 from JuliaDiff/mt/array_funcs
fill, vcat, hcat, cat, reshape, any, and all custom adjoints

it looks ok but with the subsequent commit:

commit: a78ae74
broadcasting perf fix
I obtain the wrong output value)

[ChrisRackauckas]

That commit looks like it only changes broadcast behavior, so I wonder, does it still fail if you change the test to: f!(du,u,p,t) = du[1]=p[1]*u[1] ?

[frankschae]

no, it's fine

f!(du,u,p,t) = du.=p[1]*u -> fails
f!(du,u,p,t) = du.=p[1]*u[1] -> works
f!(du,u,p,t) = du[1]=p[1]*u[1] -> works

[ChrisRackauckas]

boom, that narrows it down.

[ChrisRackauckas]

frankschae 5:57 PM
doesn’t show up for oop. Both f(u,p,t) = @. p[1]*u and f(u,p,t) = p[1]*u work

Looks like it's only in the in-place definition.

[mohdibntarek]

The test above still fails after replacing the functions with:

f!(du,u,p,t) = du .= u
f(u,p,t) = u

and removing the entire broadcast.jl file introduced in the commit linked. This shows that the "test" script above does something funny. In fact, calling pull_value!(output) before running the test makes the test pass using the latest ReverseDiff. I think output_hook is just not for "user" use. TrackedReals in ReverseDiff are a bit funny in that they don't necessarily hold their actual value if the value comes from a parent TrackedArray, e.g. if the TrackedReal is the result of calling getindex on the parent TrackedArray. This is why you need to call pull_value! first before asking for the value of a TrackedReal. This will bring in their value from their parent TrackedArray if one exists.

Given the above analysis, I feel this is not a real issue. If you have a real use case, e.g. computing the gradient or jacobian of a function that fails compared to finite difference or ForwardDiff, please post it here. Otherwise, please close this issue if you find this answer satisfactory.