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Forward structure-preserving broadcasting to diag for Diagonal #1423

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Sep 18, 2025
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Forward structure-preserving broadcasting to diag for Diagonal #1423

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b69158d
Forward structure-preserving broadcasting to diag for `Diagonal`
jishnub Aug 24, 2025
3e4e9ca
Use views for wrappers
jishnub Sep 1, 2025
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31 changes: 24 additions & 7 deletions src/structuredbroadcast.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -230,6 +230,30 @@ end
# All structured matrices are square, and therefore they only broadcast out if they are size (1, 1)
Broadcast.newindex(D::StructuredMatrix, I::CartesianIndex{2}) = size(D) == (1,1) ? CartesianIndex(1,1) : I

# Recursively replace wrapped matrices by their parents to improve broadcasting performance
# We may do this because the indexing within `copyto!` is restricted to the stored indices
preprocess_broadcasted(::Type{T}, A) where {T} = _preprocess_broadcasted(T, A)
function preprocess_broadcasted(::Type{T}, bc::Broadcasted) where {T}
args = map(x -> preprocess_broadcasted(T, x), bc.args)
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Is there a reason to use map instead of preprocess_broadcasted.(T, bc.args)?

(I think in old versions of Julia making "enclosures" like this was bad for performance but perhaps that's ancient history.)

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map on small tuples unrolls the loop. I don't think the anonymous function would be an issue here (perhaps the issue has been resolved). The call seems type-stable and well-inferred.

Broadcast.broadcasted(bc.f, args...)
end
# fallback case that doesn't unwrap at all
_preprocess_broadcasted(::Type, x) = x

_preprocess_broadcasted(::Type{Diagonal}, d::Diagonal) = d.diag
# fallback for types that might opt into Diagonal-like structured broadcasting, e.g. wrappers
_preprocess_broadcasted(::Type{Diagonal}, d::AbstractMatrix) = diagview(d)

function copy(bc::Broadcasted{StructuredMatrixStyle{Diagonal}})
if isstructurepreserving(bc) || fzeropreserving(bc)
# forward the broadcasting operation to the diagonal
bc2 = preprocess_broadcasted(Diagonal, bc)
return Diagonal(copy(bc2))
else
@invoke copy(bc::Broadcasted)
end
end

function copyto!(dest::Diagonal, bc::Broadcasted{<:StructuredMatrixStyle})
isvalidstructbc(dest, bc) || return copyto!(dest, convert(Broadcasted{Nothing}, bc))
axs = axes(dest)
Expand Down Expand Up @@ -291,13 +315,6 @@ function copyto!(dest::Tridiagonal, bc::Broadcasted{<:StructuredMatrixStyle})
return dest
end

# Recursively replace wrapped matrices by their parents to improve broadcasting performance
# We may do this because the indexing within `copyto!` is restricted to the stored indices
preprocess_broadcasted(::Type{T}, A) where {T} = _preprocess_broadcasted(T, A)
function preprocess_broadcasted(::Type{T}, bc::Broadcasted) where {T}
args = map(x -> preprocess_broadcasted(T, x), bc.args)
Broadcast.Broadcasted(bc.f, args, bc.axes)
end
_preprocess_broadcasted(::Type{LowerTriangular}, A) = lowertridata(A)
_preprocess_broadcasted(::Type{UpperTriangular}, A) = uppertridata(A)
_preprocess_broadcasted(::Type{UpperHessenberg}, A) = upperhessenbergdata(A)
Expand Down
15 changes: 15 additions & 0 deletions test/structuredbroadcast.jl
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Expand Up @@ -410,4 +410,19 @@ end
@test UH2 isa UpperHessenberg
end

@testset "forwarding broadcast to the diag for a Diagonal" begin
D = Diagonal(1:4)
D2 = D .* 2
@test D2 isa Diagonal{Int, <:AbstractRange{Int}}

# test for wrappers that opt into Diagonal-like broadcasting
U = UpperTriangular(D)
bc = Broadcast.broadcasted(+, D, U)
bcD = Broadcast.broadcasted(+, D, D)
S = typeof(Broadcast.BroadcastStyle(typeof(bcD)))
bc2 = convert(Broadcast.Broadcasted{S}, bc)
@test copy(bc2) == copy(bc) == copy(bcD)
@test copy(bc2) isa Diagonal
end

end