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Add fast path to stack(::AbstractVectorOfArray) for numeric inner arrays #671

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Add fast path to stack(::AbstractVectorOfArray) for numeric inner arrays #671

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7 changes: 7 additions & 0 deletions src/vector_of_array.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -1045,6 +1045,13 @@ function Base.append!(
end

function Base.stack(VA::AbstractVectorOfArray; dims = :)
# `stack.(VA.u)` exists to collapse any nested `AbstractVectorOfArray`
# (or other non-numeric-array) elements of `VA.u` into plain arrays before
# the outer `stack`. When the elements are already plain numeric arrays,
# that inner `stack.()` call is a redundant elementwise copy.
if eltype(VA.u) <: AbstractArray{<:Number}
return stack(VA.u; dims)
end
return stack(stack.(VA.u); dims)
end

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8 changes: 8 additions & 0 deletions test/Core/interface_tests.jl
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Expand Up @@ -208,6 +208,14 @@ testva = VA[VA[ones(2, 2), 2ones(2, 2)], 3ones(2, 2, 2)]
@test stack(testva) ==
[1.0 1.0; 1.0 1.0;;; 2.0 2.0; 2.0 2.0;;;; 3.0 3.0; 3.0 3.0;;; 3.0 3.0; 3.0 3.0]

# stack on plain-numeric inner arrays skips `stack.(VA.u)` (an elementwise
# copy of every inner array before the outer stack): only the final output
# array is allocated, instead of also allocating one copy per inner array.
numeric_stack = VectorOfArray([rand(2) for _ in 1:1000])
stack(numeric_stack) # compile away type instability before measuring
@test (@allocated stack(numeric_stack)) < 20_000 # final 2x1000 Float64 array alone is 16000 bytes
@test stack(numeric_stack) == reduce(hcat, numeric_stack.u)

# convert array from VectorOfArray/DiffEqArray
t = 1:8
recs = [rand(10, 7) for i in 1:8]
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