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Remove allocations of nested Jacobians of StaticArrays#810

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albert-de-montserrat:adm/nested_J
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Remove allocations of nested Jacobians of StaticArrays#810
albert-de-montserrat wants to merge 5 commits into
JuliaDiff:masterfrom
albert-de-montserrat:adm/nested_J

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@albert-de-montserrat albert-de-montserrat commented May 31, 2026

Addresses and fixes the issue raised in #798.

The same MWE now yields:

using ForwardDiff, StaticArrays, Chairmarks

@inline f(x) = SVector(x[1]^2 * x[2], sin(x[1]) + x[2]^3)

function J_flat(x) 
    y = ForwardDiff.jacobian(f, x)
    SA[y[1], y[2]]
end

function nested_jacobian(x0::SVector{2, T}) where T
    ForwardDiff.jacobian(J_flat, x0)
end

x0 = SVector(1.0, 2.0)
julia> @b nested_jacobian($x0)
19.031 ns

Looks like the compiler was giving up to inline static_dual_eval(T, f, x) in this case. Manual inlining seems to solve the problem.

@albert-de-montserrat
Refactor vector_mode_gradient and vector_mode_jacobian
27ad1e7 
Refactor gradient and jacobian functions to use direct function calls instead of static_dual_eval.
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codecov Bot commented May 31, 2026 •
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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.75%. Comparing base (39916da) to head (bfc994e).

Additional details and impacted files 
@@            Coverage Diff             @@
##           master     #810      +/-   ##
==========================================
- Coverage   90.75%   87.75%   -3.01%     
==========================================
  Files          11       10       -1     
  Lines        1071      988      -83     
==========================================
- Hits          972      867     -105     
- Misses         99      121      +22

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devmotion
devmotion reviewed Jun 1, 2026
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Can you add a test that fails on master but passes with these changes?

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albert-de-montserrat commented Jun 4, 2026

Added the following test, fails on master.

devmotion
devmotion reviewed Jun 4, 2026
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Comment thread test/AllocationsTest.jl Outdated
albert-de-montserrat and others added 2 commits June 4, 2026 16:44
@albert-de-montserrat @devmotion
Update test/AllocationsTest.jl
2efc76e 
Co-authored-by: David Müller-Widmann <devmotion@users.noreply.github.com>
@albert-de-montserrat
remove unncessary line
bfc994e
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