Specialize adding/subtracting mixed Upper/LowerTriangular (#56149)

stdlib/LinearAlgebra/src/triangular.jl

@@ -152,6 +152,7 @@ UnitUpperTriangular
 const UpperOrUnitUpperTriangular{T,S} = Union{UpperTriangular{T,S}, UnitUpperTriangular{T,S}}
 const LowerOrUnitLowerTriangular{T,S} = Union{LowerTriangular{T,S}, UnitLowerTriangular{T,S}}
 const UpperOrLowerTriangular{T,S} = Union{UpperOrUnitUpperTriangular{T,S}, LowerOrUnitLowerTriangular{T,S}}
+const UnitUpperOrUnitLowerTriangular{T,S} = Union{UnitUpperTriangular{T,S}, UnitLowerTriangular{T,S}}
 
 uppertriangular(M) = UpperTriangular(M)
 lowertriangular(M) = LowerTriangular(M)
@@ -221,6 +222,16 @@ function Matrix{T}(A::UnitUpperTriangular) where T
     B
 end
 
+function full(A::Union{UpperTriangular,LowerTriangular})
+    return _triangularize(A)(parent(A))
+end
+function full(A::UnitUpperOrUnitLowerTriangular)
+    isupper = A isa UnitUpperTriangular
+    Ap = _triangularize(A)(parent(A), isupper ? 1 : -1)
+    Ap[diagind(Ap, IndexStyle(Ap))] = @view A[diagind(A, IndexStyle(A))]
+    return Ap
+end
+
 function full!(A::LowerTriangular)
     B = A.data
     tril!(B)
@@ -553,6 +564,9 @@ function copyto!(A::T, B::T) where {T<:Union{LowerTriangular,UnitLowerTriangular
     return A
 end
 
+_triangularize(::UpperOrUnitUpperTriangular) = triu
+_triangularize(::LowerOrUnitLowerTriangular) = tril
+
 @inline _rscale_add!(A::AbstractTriangular, B::AbstractTriangular, C::Number, alpha::Number, beta::Number) =
     _triscale!(A, B, C, MulAddMul(alpha, beta))
 @inline _lscale_add!(A::AbstractTriangular, B::Number, C::AbstractTriangular, alpha::Number, beta::Number) =
@@ -812,7 +826,8 @@ function +(A::UnitLowerTriangular, B::UnitLowerTriangular)
     (parent(A) isa StridedMatrix || parent(B) isa StridedMatrix) && return A .+ B
     LowerTriangular(tril(A.data, -1) + tril(B.data, -1) + 2I)
 end
-+(A::AbstractTriangular, B::AbstractTriangular) = copyto!(similar(parent(A)), A) + copyto!(similar(parent(B)), B)
++(A::UpperOrLowerTriangular, B::UpperOrLowerTriangular) = full(A) + full(B)
++(A::AbstractTriangular, B::AbstractTriangular) = copyto!(similar(parent(A), size(A)), A) + copyto!(similar(parent(B), size(B)), B)
 
 function -(A::UpperTriangular, B::UpperTriangular)
     (parent(A) isa StridedMatrix || parent(B) isa StridedMatrix) && return A .- B
@@ -846,7 +861,8 @@ function -(A::UnitLowerTriangular, B::UnitLowerTriangular)
     (parent(A) isa StridedMatrix || parent(B) isa StridedMatrix) && return A .- B
     LowerTriangular(tril(A.data, -1) - tril(B.data, -1))
 end
--(A::AbstractTriangular, B::AbstractTriangular) = copyto!(similar(parent(A)), A) - copyto!(similar(parent(B)), B)
+-(A::UpperOrLowerTriangular, B::UpperOrLowerTriangular) = full(A) - full(B)
+-(A::AbstractTriangular, B::AbstractTriangular) = copyto!(similar(parent(A), size(A)), A) - copyto!(similar(parent(B), size(B)), B)
 
 # use broadcasting if the parents are strided, where we loop only over the triangular part
 for op in (:+, :-)

stdlib/LinearAlgebra/test/triangular.jl

@@ -25,6 +25,12 @@ debug && println("Test basic type functionality")
 @test_throws DimensionMismatch LowerTriangular(randn(5, 4))
 @test LowerTriangular(randn(3, 3)) |> t -> [size(t, i) for i = 1:3] == [size(Matrix(t), i) for i = 1:3]
 
+struct MyTriangular{T, A<:LinearAlgebra.AbstractTriangular{T}} <: LinearAlgebra.AbstractTriangular{T}
+    data :: A
+end
+Base.size(A::MyTriangular) = size(A.data)
+Base.getindex(A::MyTriangular, i::Int, j::Int) = A.data[i,j]
+
 # The following test block tries to call all methods in base/linalg/triangular.jl in order for a combination of input element types. Keep the ordering when adding code.
 @testset for elty1 in (Float32, Float64, BigFloat, ComplexF32, ComplexF64, Complex{BigFloat}, Int)
     # Begin loop for first Triangular matrix
@@ -1078,4 +1084,50 @@ end
     end
 end
 
+@testset "addition/subtraction of mixed triangular" begin
+    for A in (Hermitian(rand(4, 4)), Diagonal(rand(5)))
+        for T in (UpperTriangular, LowerTriangular,
+                UnitUpperTriangular, UnitLowerTriangular)
+            B = T(A)
+            M = Matrix(B)
+            R = B - B'
+            if A isa Diagonal
+                @test R isa Diagonal
+            end
+            @test R == M - M'
+            R = B + B'
+            if A isa Diagonal
+                @test R isa Diagonal
+            end
+            @test R == M + M'
+            C = MyTriangular(B)
+            @test C - C' == M - M'
+            @test C + C' == M + M'
+        end
+    end
+    @testset "unfilled parent" begin
+        @testset for T in (UpperTriangular, LowerTriangular,
+                UnitUpperTriangular, UnitLowerTriangular)
+            F = Matrix{BigFloat}(undef, 2, 2)
+            B = T(F)
+            isupper = B isa Union{UpperTriangular, UnitUpperTriangular}
+            B[1+!isupper, 1+isupper] = 2
+            if !(B isa Union{UnitUpperTriangular, UnitLowerTriangular})
+                B[1,1] = B[2,2] = 3
+            end
+            M = Matrix(B)
+            # These are broken, as triu/tril don't work with
+            # unfilled adjoint matrices
+            # See https://github.com/JuliaLang/julia/pull/55312
+            @test_broken B - B' == M - M'
+            @test_broken B + B' == M + M'
+            @test B - copy(B') == M - M'
+            @test B + copy(B') == M + M'
+            C = MyTriangular(B)
+            @test C - C' == M - M'
+            @test C + C' == M + M'
+        end
+    end
+end
+
 end # module TestTriangular