fix: handle constant derivatives with runtime activity for Enzyme

DifferentiationInterface/CHANGELOG.md

@@ -7,8 +7,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased](https://github.com/JuliaDiff/DifferentiationInterface.jl/compare/DifferentiationInterface-v0.7.7...main)
 
+### Fixed
+
+  - Handle constant derivatives with runtime activity for Enzyme
+
 ## [0.7.7](https://github.com/JuliaDiff/DifferentiationInterface.jl/compare/DifferentiationInterface-v0.7.6...DifferentiationInterface-v0.7.7)
 
+### Fixed
+
   - Improve support for empty inputs (still not guaranteed) ([#835](https://github.com/JuliaDiff/DifferentiationInterface.jl/pull/835))
 
 ## [0.7.6](https://github.com/JuliaDiff/DifferentiationInterface.jl/compare/DifferentiationInterface-v0.7.5...DifferentiationInterface-v0.7.6)

DifferentiationInterface/ext/DifferentiationInterfaceEnzymeExt/DifferentiationInterfaceEnzymeExt.jl

@@ -1,7 +1,7 @@
 module DifferentiationInterfaceEnzymeExt
 
 using ADTypes: ADTypes, AutoEnzyme
-using Base: Fix1
+using Base: Fix1, datatype_pointerfree
 import DifferentiationInterface as DI
 using EnzymeCore:
     Active,
@@ -42,7 +42,8 @@ using Enzyme:
     jacobian,
     make_zero,
     make_zero!,
-    onehot
+    onehot,
+    runtime_activity
 
 DI.check_available(::AutoEnzyme) = true
 

DifferentiationInterface/ext/DifferentiationInterfaceEnzymeExt/forward_onearg.jl

@@ -37,6 +37,7 @@ function DI.value_and_pushforward(
     x_and_dx = Duplicated(x, dx)
     annotated_contexts = translate_prepared!(context_shadows, contexts, Val(1))
     dy, y = autodiff(mode, f_and_df, x_and_dx, annotated_contexts...)
+    dy = runtime_activity_safeguard(backend, y, dy)
     return y, (dy,)
 end
 
@@ -54,8 +55,10 @@ function DI.value_and_pushforward(
     f_and_df = get_f_and_df_prepared!(df, f, backend, Val(B))
     x_and_tx = BatchDuplicated(x, tx)
     annotated_contexts = translate_prepared!(context_shadows, contexts, Val(B))
-    ty, y = autodiff(mode, f_and_df, x_and_tx, annotated_contexts...)
-    return y, values(ty)
+    ty_nt, y = autodiff(mode, f_and_df, x_and_tx, annotated_contexts...)
+    ty = values(ty_nt)
+    ty = runtime_activity_safeguard(backend, y, ty)
+    return y, ty
 end
 
 function DI.pushforward(
@@ -66,6 +69,9 @@ function DI.pushforward(
     tx::NTuple{1},
     contexts::Vararg{DI.Context,C},
 ) where {F,C}
+    if has_runtime_activity(backend)
+        return DI.value_and_pushforward(f, prep, backend, x, tx, contexts...)[2]
+    end
     DI.check_prep(f, prep, backend, x, tx, contexts...)
     (; df, context_shadows) = prep
     mode = forward_noprimal(backend)
@@ -85,14 +91,18 @@ function DI.pushforward(
     tx::NTuple{B},
     contexts::Vararg{DI.Context,C},
 ) where {F,B,C}
+    if has_runtime_activity(backend)
+        return DI.value_and_pushforward(f, prep, backend, x, tx, contexts...)[2]
+    end
     DI.check_prep(f, prep, backend, x, tx, contexts...)
     (; df, context_shadows) = prep
     mode = forward_noprimal(backend)
     f_and_df = get_f_and_df_prepared!(df, f, backend, Val(B))
     x_and_tx = BatchDuplicated(x, tx)
     annotated_contexts = translate_prepared!(context_shadows, contexts, Val(B))
-    ty = only(autodiff(mode, f_and_df, x_and_tx, annotated_contexts...))
-    return values(ty)
+    ty_nt = only(autodiff(mode, f_and_df, x_and_tx, annotated_contexts...))
+    ty = values(ty_nt)
+    return ty
 end
 
 function DI.value_and_pushforward!(
@@ -168,7 +178,9 @@ function DI.gradient(
     derivs = gradient(
         mode, f_and_df, x, annotated_contexts...; chunk=Val(B), shadows=basis_shadows
     )
-    return first(derivs)
+    deriv = first(derivs)
+    deriv = runtime_activity_safeguard(backend, x, deriv)
+    return deriv
 end
 
 function DI.value_and_gradient(
@@ -186,7 +198,9 @@ function DI.value_and_gradient(
     (; derivs, val) = gradient(
         mode, f_and_df, x, annotated_contexts...; chunk=Val(B), shadows=basis_shadows
     )
-    return val, first(derivs)
+    deriv = first(derivs)
+    deriv = runtime_activity_safeguard(backend, x, deriv)
+    return val, deriv
 end
 
 function DI.gradient!(

DifferentiationInterface/ext/DifferentiationInterfaceEnzymeExt/reverse_onearg.jl

@@ -7,6 +7,7 @@ function seeded_autodiff_thunk(
 ) where {ReturnPrimal,FA<:Annotation,RA<:Annotation,N}
     forward, reverse = autodiff_thunk(rmode, FA, RA, typeof.(args)...)
     tape, result, shadow_result = forward(f, args...)
+    shadow_result = runtime_activity_safeguard(rmode, result, shadow_result)
     if RA <: Active
         dinputs = only(reverse(f, args..., dresult, tape))
     else
@@ -30,6 +31,7 @@ function batch_seeded_autodiff_thunk(
     rmode_rightwidth = ReverseSplitWidth(rmode, Val(B))
     forward, reverse = autodiff_thunk(rmode_rightwidth, FA, RA, typeof.(args)...)
     tape, result, shadow_results = forward(f, args...)
+    shadow_results = runtime_activity_safeguard(rmode_rightwidth, result, shadow_results)
     if RA <: Active
         dinputs = only(reverse(f, args..., dresults, tape))
     else

DifferentiationInterface/ext/DifferentiationInterfaceEnzymeExt/utils.jl

@@ -193,3 +193,39 @@ end
 
 batchify_activity(::Type{Active{T}}, ::Val{B}) where {T,B} = Active{T}
 batchify_activity(::Type{Duplicated{T}}, ::Val{B}) where {T,B} = BatchDuplicated{T,B}
+
+has_runtime_activity(mode::Mode) = runtime_activity(mode)
+has_runtime_activity(::AutoEnzyme{Nothing}) = false
+has_runtime_activity(backend::AutoEnzyme{<:Mode}) = has_runtime_activity(backend.mode)
+
+function runtime_activity_safeguard(
+    backend_or_mode::Union{<:AutoEnzyme,<:Mode}, primal::T, shadow::T
+) where {T}
+    # TODO: improve datatype_pointerfree to take Ptr into account
+    if has_runtime_activity(backend_or_mode) &&
+        !datatype_pointerfree(T) &&
+        pointer(primal) === pointer(shadow)  # TODO: doesn't work beyond arrays
+        return make_zero(shadow)
+    else
+        return shadow
+    end
+end
+
+function runtime_activity_safeguard(
+    backend_or_mode::Union{<:AutoEnzyme,<:Mode},
+    primal::T,
+    shadow::Union{NTuple{N,T},NamedTuple},
+) where {T,N}
+    # TODO: improve datatype_pointerfree to take Ptr into account
+    if has_runtime_activity(backend_or_mode) &&
+        !datatype_pointerfree(T) &&
+        pointer(primal) === pointer(shadow[1])  # TODO: doesn't work beyond arrays
+        return make_zero(shadow)
+    else
+        return shadow
+    end
+end
+
+function runtime_activity_safeguard(::Union{<:AutoEnzyme,<:Mode}, primal, shadow::Nothing)
+    return nothing
+end

DifferentiationInterface/test/Back/Enzyme/test.jl

@@ -196,3 +196,19 @@ end
         excluded=[:jacobian],
     )
 end;
+
+@testset "Runtime activity" begin
+    # TODO: higher-level operators not tested
+    test_differentiation(
+        AutoEnzyme(; mode=Enzyme.set_runtime_activity(Enzyme.Forward)),
+        DIT.unknown_activity(default_scenarios());
+        excluded=vcat(SECOND_ORDER, :jacobian, :gradient, :derivative, :pullback),
+        logging=LOGGING,
+    )
+    test_differentiation(
+        AutoEnzyme(; mode=Enzyme.set_runtime_activity(Enzyme.Reverse)),
+        DIT.unknown_activity(default_scenarios());
+        excluded=vcat(SECOND_ORDER, :jacobian, :gradient, :derivative, :pushforward),
+        logging=LOGGING,
+    )
+end

DifferentiationInterfaceTest/src/scenarios/modify.jl

@@ -163,6 +163,54 @@ function closurify(scen::Scenario{op,pl_op,pl_fun}) where {op,pl_op,pl_fun}
     )
 end
 
+struct UnknownActivityReturn{pl_fun,F}
+    f::F
+end
+
+function Base.show(io::IO, f::UnknownActivityReturn)
+    return print(io, "UnknownActivityReturn($(f.f))")
+end
+
+function (f::UnknownActivityReturn{:out})(x, yc, return_constant::Bool)
+    if return_constant
+        return copy(yc)
+    else
+        return f.f(x)
+    end
+end
+
+function (f::UnknownActivityReturn{:in})(y, x, yc, return_constant::Bool)
+    if return_constant
+        copyto!(y, copy(yc))
+    else
+        f.f(y, x)
+    end
+    return nothing
+end
+
+"""
+    unknown_activity(scen::Scenario)
+
+Return a new scenario identical to `scen` except that the function now takes an additional constant argument which is the theoretical output, and a constant boolean condition stating whether or not that output should be recomputed.
+"""
+function unknown_activity(scen::Scenario{op,pl_op,pl_fun}) where {op,pl_op,pl_fun}
+    (; f) = deepcopy(scen)
+    zero_scen = deepcopy(zero(scen))
+    @assert isempty(scen.contexts)
+    unknown_f = UnknownActivityReturn{pl_fun,typeof(f)}(f)
+    return Scenario{op,pl_op,pl_fun}(;
+        f=unknown_f,
+        x=scen.x,
+        y=scen.y,
+        t=scen.t,
+        contexts=(Constant(scen.y), Constant(true)),
+        res1=zero_scen.res1,
+        res2=zero_scen.res2,
+        prep_args=(; scen.prep_args..., contexts=(Constant(scen.y), Constant(true))),
+        name=isnothing(scen.name) ? nothing : scen.name * " [unknown activity]",
+    )
+end
+
 struct MultiplyByConstant{pl_fun,F} <: FunctionModifier
     f::F
 end
@@ -366,6 +414,7 @@ closurify(scens::AbstractVector{<:Scenario}) = closurify.(scens)
 constantify(scens::AbstractVector{<:Scenario}) = constantify.(scens)
 cachify(scens::AbstractVector{<:Scenario}; use_tuples) = cachify.(scens; use_tuples)
 constantorcachify(scens::AbstractVector{<:Scenario}) = constantorcachify.(scens)
+unknown_activity(scens::AbstractVector{<:Scenario}) = unknown_activity.(scens)
 
 ## Compute results with backend
 

DifferentiationInterfaceTest/test/weird.jl

@@ -65,6 +65,13 @@ test_differentiation(
     logging=LOGGING,
 );
 
+test_differentiation(
+    AutoFiniteDiff(),
+    unknown_activity(default_scenarios);
+    excluded=SECOND_ORDER,
+    logging=LOGGING,
+);
+
 ## Neural nets
 
 test_differentiation(