Avoid batch size of 0 for empty inputs

DifferentiationInterface/src/first_order/jacobian.jl

@@ -153,12 +153,14 @@ struct PullbackJacobianPrep{
     S<:AbstractVector{<:NTuple},
     R<:AbstractVector{<:NTuple},
     E<:PullbackPrep,
+    Y,
 } <: StandardJacobianPrep{SIG}
     _sig::Val{SIG}
     batch_size_settings::BS
     batched_seeds::S
     batched_results::R
     pullback_prep::E
+    y_example::Y
 end
 
 function prepare_jacobian_nokwarg(
@@ -212,7 +214,7 @@ function _prepare_jacobian_aux(
     ]
     batched_results = [ntuple(b -> similar(y), Val(B)) for _ in batched_seeds]
     pushforward_prep = prepare_pushforward_nokwarg(
-        strict, f_or_f!y..., backend, x, batched_seeds[1], contexts...
+        strict, f_or_f!y..., backend, x, ntuple(b -> zero(x), Val(B)), contexts...
     )
     return PushforwardJacobianPrep(
         _sig, batch_size_settings, batched_seeds, batched_results, pushforward_prep
@@ -237,10 +239,10 @@ function _prepare_jacobian_aux(
     ]
     batched_results = [ntuple(b -> similar(x), Val(B)) for _ in batched_seeds]
     pullback_prep = prepare_pullback_nokwarg(
-        strict, f_or_f!y..., backend, x, batched_seeds[1], contexts...
+        strict, f_or_f!y..., backend, x, ntuple(b -> zero(y), Val(B)), contexts...
     )
     return PullbackJacobianPrep(
-        _sig, batch_size_settings, batched_seeds, batched_results, pullback_prep
+        _sig, batch_size_settings, batched_seeds, batched_results, pullback_prep, y
     )
 end
 
@@ -367,7 +369,7 @@ function _jacobian_aux(
     (; A, B_last) = batch_size_settings
 
     pushforward_prep_same = prepare_pushforward_same_point(
-        f_or_f!y..., pushforward_prep, backend, x, batched_seeds[1], contexts...
+        f_or_f!y..., pushforward_prep, backend, x, ntuple(b -> zero(x), Val(B)), contexts...
     )
 
     jac = mapreduce(hcat, eachindex(batched_seeds)) do a
@@ -419,11 +421,16 @@ function _jacobian_aux(
     x,
     contexts::Vararg{Context,C},
 ) where {FY,SIG,B,aligned,C}
-    (; batch_size_settings, batched_seeds, pullback_prep) = prep
+    (; batch_size_settings, batched_seeds, pullback_prep, y_example) = prep
     (; A, B_last) = batch_size_settings
 
     pullback_prep_same = prepare_pullback_same_point(
-        f_or_f!y..., prep.pullback_prep, backend, x, batched_seeds[1], contexts...
+        f_or_f!y...,
+        pullback_prep,
+        backend,
+        x,
+        ntuple(b -> zero(y_example), Val(B)),
+        contexts...,
     )
 
     jac = mapreduce(vcat, eachindex(batched_seeds)) do a
@@ -487,11 +494,16 @@ function _jacobian_aux!(
     x,
     contexts::Vararg{Context,C},
 ) where {FY,SIG,B,C}
-    (; batch_size_settings, batched_seeds, batched_results, pullback_prep) = prep
+    (; batch_size_settings, batched_seeds, batched_results, pullback_prep, y_example) = prep
     (; N) = batch_size_settings
 
     pullback_prep_same = prepare_pullback_same_point(
-        f_or_f!y..., pullback_prep, backend, x, batched_seeds[1], contexts...
+        f_or_f!y...,
+        pullback_prep,
+        backend,
+        x,
+        ntuple(b -> zero(y_example), Val(B)),
+        contexts...,
     )
 
     for a in eachindex(batched_seeds, batched_results)

DifferentiationInterface/src/first_order/pullback.jl

@@ -285,7 +285,13 @@ function _prepare_pullback_aux(
     contexts::Vararg{Context,C};
 ) where {F,C}
     _sig = signature(f, backend, x, ty, contexts...; strict)
-    dx = x isa Number ? oneunit(x) : basis(x, first(CartesianIndices(x)))
+    dx = if x isa Number
+        oneunit(x)
+    elseif isempty(x)
+        zero(x)
+    else
+        basis(x, first(CartesianIndices(x)))
+    end
     pushforward_prep = prepare_pushforward_nokwarg(
         strict, f, backend, x, (dx,), contexts...
     )
@@ -303,7 +309,13 @@ function _prepare_pullback_aux(
     contexts::Vararg{Context,C};
 ) where {F,C}
     _sig = signature(f!, y, backend, x, ty, contexts...; strict)
-    dx = x isa Number ? oneunit(x) : basis(x, first(CartesianIndices(x)))
+    dx = if x isa Number
+        oneunit(x)
+    elseif isempty(x)
+        zero(x)
+    else
+        basis(x, first(CartesianIndices(x)))
+    end
     pushforward_prep = prepare_pushforward_nokwarg(
         strict, f!, y, backend, x, (dx,), contexts...
     )

DifferentiationInterface/src/first_order/pushforward.jl

@@ -290,7 +290,13 @@ function _prepare_pushforward_aux(
 ) where {F,C}
     _sig = signature(f, backend, x, tx, contexts...; strict)
     y = f(x, map(unwrap, contexts)...)
-    dy = y isa Number ? oneunit(y) : basis(y, first(CartesianIndices(y)))
+    dy = if y isa Number
+        oneunit(y)
+    elseif isempty(y)
+        zero(y)
+    else
+        basis(y, first(CartesianIndices(y)))
+    end
     pullback_prep = prepare_pullback_nokwarg(strict, f, backend, x, (dy,), contexts...)
     return PullbackPushforwardPrep(_sig, pullback_prep)
 end
@@ -306,7 +312,13 @@ function _prepare_pushforward_aux(
     contexts::Vararg{Context,C};
 ) where {F,C}
     _sig = signature(f!, y, backend, x, tx, contexts...; strict)
-    dy = y isa Number ? oneunit(y) : basis(y, first(CartesianIndices(y)))
+    dy = if y isa Number
+        oneunit(y)
+    elseif isempty(y)
+        zero(y)
+    else
+        basis(y, first(CartesianIndices(y)))
+    end
     pullback_prep = prepare_pullback_nokwarg(strict, f!, y, backend, x, (dy,), contexts...)
     return PullbackPushforwardPrep(_sig, pullback_prep)
 end

DifferentiationInterface/src/utils/batchsize.jl

@@ -22,22 +22,26 @@ struct BatchSizeSettings{B,singlebatch,aligned}
 end
 
 function BatchSizeSettings{B,singlebatch,aligned}(N::Integer) where {B,singlebatch,aligned}
-    B > N && throw(ArgumentError("Batch size $B larger than input size $N"))
-    A = div(N, B, RoundUp)
-    B_last = N % B
+    B > N > 0 && throw(ArgumentError("Batch size $B larger than input size $N"))
+    if B == N == 0
+        A = B_last = 0
+    else
+        A = div(N, B, RoundUp)
+        B_last = N % B
+    end
     return BatchSizeSettings{B,singlebatch,aligned}(N, A, B_last)
 end
 
 function BatchSizeSettings{B}(::Val{N}) where {B,N}
     singlebatch = B == N
-    aligned = N % B == 0
+    aligned = (B == N == 0) || (N % B == 0)
     return BatchSizeSettings{B,singlebatch,aligned}(N)
 end
 
 function BatchSizeSettings{B}(N::Integer) where {B}
     # type-unstable
     singlebatch = B == N
-    aligned = N % B == 0
+    aligned = (B == N == 0) || (N % B == 0)
     return BatchSizeSettings{B,singlebatch,aligned}(N)
 end
 

DifferentiationInterface/test/Core/Internals/batchsize.jl

@@ -25,11 +25,14 @@ BSS = BatchSizeSettings
 end
 
 @testset "SimpleFiniteDiff (adaptive)" begin
+    @test (pick_batchsize(AutoSimpleFiniteDiff(), zeros(0))) isa BSS{0,true,true}
     @test (pick_batchsize(AutoSimpleFiniteDiff(), zeros(2))) isa BSS{2,true,true}
     @test (pick_batchsize(AutoSimpleFiniteDiff(), zeros(6))) isa BSS{6,true,true}
     @test (pick_batchsize(AutoSimpleFiniteDiff(), zeros(12))) isa BSS{12,true,true}
     @test (pick_batchsize(AutoSimpleFiniteDiff(), zeros(24))) isa BSS{12,false,true}
     @test (pick_batchsize(AutoSimpleFiniteDiff(), zeros(100))) isa BSS{12,false,false}
+    @test (@inferred pick_batchsize(AutoSimpleFiniteDiff(), @SVector(zeros(0)))) isa
+        BSS{0,true,true}
     @test (@inferred pick_batchsize(AutoSimpleFiniteDiff(), @SVector(zeros(2)))) isa
         BSS{2,true,true}
     @test (@inferred pick_batchsize(AutoSimpleFiniteDiff(), @SVector(zeros(6)))) isa

DifferentiationInterface/test/Core/ZeroBackends/test.jl

@@ -1,6 +1,7 @@
 using DifferentiationInterface
 using DifferentiationInterface: AutoZeroForward, AutoZeroReverse
 using DifferentiationInterfaceTest
+using LinearAlgebra
 using ComponentArrays: ComponentArrays
 using JLArrays: JLArrays
 using SparseMatrixColorings
@@ -50,3 +51,15 @@ end
         logging=LOGGING,
     )
 end
+
+@testset "Empty arrays" begin
+    make_empty(t) = typeof(t)[]
+    make_empty!(y, t) = nothing
+    @test gradient(sum, AutoZeroForward(), Float64[]) == Float64[]
+    @test derivative(make_empty, AutoZeroReverse(), 1.0) == Float64[]
+    @test derivative(make_empty!, Float64[], AutoZeroReverse(), 1.0) == Float64[]
+    @test_broken jacobian(copy, AutoZeroForward(), Float64[]) == I(0)
+    @test_broken jacobian(copy, AutoZeroReverse(), Float64[]) == I(0)
+    @test_broken jacobian(copyto!, Float64[], AutoZeroForward(), Float64[]) == I(0)
+    @test_broken jacobian(copyto!, Float64[], AutoZeroReverse(), Float64[]) == I(0)
+end