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Avoid reshape allocation in extract_jacobian! for Matrix results #797

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Merged
devmotion merged 4 commits into from
Mar 24, 2026
Merged

Avoid reshape allocation in extract_jacobian! for Matrix results #797

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84b9290
Avoid reshape allocation in extract_jacobian! for Matrix results
ChrisRackauckas Mar 17, 2026
0964e46
Use dispatch instead of conditional _maybe_reshape for type stability
ChrisRackauckas Mar 24, 2026
5a3dda9
Fold AbstractMatrix optimization into general method as one-line change
ChrisRackauckas Mar 24, 2026
ac91112
Update Project.toml
ChrisRackauckas Mar 24, 2026
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11 changes: 11 additions & 0 deletions src/jacobian.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -92,6 +92,17 @@ jacobian(f, x::Real) = throw(DimensionMismatch("jacobian(f, x) expects that x is
# result extraction #
#####################

# Specialized method for AbstractMatrix: no reshape needed, avoids ReshapedArray allocation
# that cannot be elided under --check-bounds=yes.
function extract_jacobian!(::Type{T}, result::AbstractMatrix, ydual::AbstractArray, n) where {T}
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@devmotion devmotion Mar 24, 2026

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Move to the function below to minimize the diff to a one-line change?

ydual_reshaped = vec(ydual)
# Use closure to avoid GPU broadcasting with Type
partials_wrap(ydual, nrange) = partials(T, ydual, nrange)
result .= partials_wrap.(ydual_reshaped, transpose(1:n))
return result
end

# General method for non-matrix arrays: reshape unconditionally (type-stable).
function extract_jacobian!(::Type{T}, result::AbstractArray, ydual::AbstractArray, n) where {T}
out_reshaped = reshape(result, length(ydual), n)
ydual_reshaped = vec(ydual)
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16 changes: 16 additions & 0 deletions test/AllocationsTest.jl
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Expand Up @@ -28,4 +28,20 @@ convert_test_574() = convert(ForwardDiff.Dual{Nothing,ForwardDiff.Dual{Nothing,F
@test iszero(allocs_convert_test_574())
end

@testset "Test jacobian! allocations" begin
# jacobian! should not allocate when called with a pre-allocated result Matrix.
# Previously, reshape() inside extract_jacobian! allocated a wrapper
# object that could not be elided under --check-bounds=yes.
function allocs_jacobian!()
f!(y, x) = (y .= x .^ 2)
x = [1.0, 2.0, 3.0]
y = similar(x)
result = zeros(3, 3)
cfg = ForwardDiff.JacobianConfig(f!, y, x)
ForwardDiff.jacobian!(result, f!, y, x, cfg) # warmup
return @allocated ForwardDiff.jacobian!(result, f!, y, x, cfg)
end
@test iszero(allocs_jacobian!())
end

end
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