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Unroll _transform_tuple to fix Enzyme.autodiff on tuples of length ≥ 33 #170

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Unroll _transform_tuple to fix Enzyme.autodiff on tuples of length ≥ 33 #170

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26 changes: 18 additions & 8 deletions src/aggregation.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -392,15 +392,25 @@ $(SIGNATURES)

Helper function for transforming tuples. Used internally, to help type inference. Use via
`transfom_tuple`.
"""
_transform_tuple(flag::LogJacFlag, x::AbstractVector, index, ::Tuple{}) =
(), logjac_zero(flag, _ensure_float(eltype(x))), index

function _transform_tuple(flag::LogJacFlag, x::AbstractVector, index, ts)
tfirst = first(ts)
yfirst, ℓfirst, index′ = transform_with(flag, tfirst, x, index)
yrest, ℓrest, index′′ = _transform_tuple(flag, x, index′, Base.tail(ts))
(yfirst, yrest...), ℓfirst + ℓrest, index′′
Implemented as a `@generated` straight-line unroll over the static tuple length.
Equivalent to the natural `Base.tail` recursion, but emits non-recursive code
so that `Enzyme.autodiff` does not hit `AssertionError("conv == 37")` on
tuples of length ≥ 33 (EnzymeAD/Enzyme.jl#3104).
Comment on lines +396 to +399

@devmotion devmotion Jun 15, 2026

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This seems very internal for being part of a docstrings? It also might change again in case of upstream compiler or Enzyme changes.

Suggested change
Implemented as a `@generated` straight-line unroll over the static tuple length.
Equivalent to the natural `Base.tail` recursion, but emits non-recursive code
so that `Enzyme.autodiff` does not hit `AssertionError("conv == 37")` on
tuples of length ≥ 33 (EnzymeAD/Enzyme.jl#3104).

@tpapp tpapp Jun 16, 2026

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This is an internal helper function anyway and not part of the API. (I like to document my internal functions too, I know this is not common to do so). As far as I am concerned this is fine.

"""
@generated function _transform_tuple(flag::LogJacFlag, x::AbstractVector, index,
ts::Tuple{Vararg{AbstractTransform,N}}) where {N}
N == 0 && return :(((), logjac_zero(flag, _ensure_float(eltype(x))), index))
ys = [Symbol(:y_, i) for i in 1:N]
ℓs = [Symbol(:ℓ_, i) for i in 1:N]
calls = [:(($(ys[i]), $(ℓs[i]), idx) = transform_with(flag, ts[$i], x, idx))
for i in 1:N]
ℓ_sum = foldl((a, b) -> :($a + $b), ℓs)
return quote
idx = index

@devmotion devmotion Jun 15, 2026

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Why is a separate idx variable needed? Couldn't we just operate with index?

$(calls...)
(($(ys...),), $ℓ_sum, idx)
end
end

"""
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