Handling hvcat

[3f6a]

julia> t = @SMatrix randn(2,2)
2×2 SMatrix{2, 2, Float64, 4} with indices SOneTo(2)×SOneTo(2):
 -0.427379  -0.583282
 -0.099235  -0.644796
julia> [t t]
2×4 SMatrix{2, 4, Float64, 8} with indices SOneTo(2)×SOneTo(4):
 -0.427379  -0.583282  -0.427379  -0.583282
 -0.099235  -0.644796  -0.099235  -0.644796

In this example, the concatenated [t t] correctly returns a SMatrix. However,

julia> [t t; t t]
4×4 Matrix{Float64}:
 -0.427379  -0.583282  -0.427379  -0.583282
 -0.099235  -0.644796  -0.099235  -0.644796
 -0.427379  -0.583282  -0.427379  -0.583282
 -0.099235  -0.644796  -0.099235  -0.644796

returns a raw Matrix instead. It seems hvcat is not correctly overloaded.

[david-macmahon]

Here's some more detail:

julia> @code_lowered [t t]
CodeInfo(
1 ─      nothing
│   %2 = StaticArrays.Size(a)
│   %3 = StaticArrays.Size(b)
│   %4 = StaticArrays._hcat(%2, %3, a, b)
└──      return %4
)

julia> @code_lowered [t;t]
CodeInfo(
1 ─      nothing
│   %2 = StaticArrays.Size(a)
│   %3 = StaticArrays.Size(b)
│   %4 = StaticArrays._vcat(%2, %3, a, b)
└──      return %4
)

julia> @code_lowered [t t; t t]
CodeInfo(
1 ─ %1 = Base.typed_hvcat
│   %2 = Core.tuple($(Expr(:static_parameter, 1)), rows)
│   %3 = Core._apply_iterate(Base.iterate, %1, %2, xs)
└──      return %3
)

[david-macmahon]

This can be worked around by forcing the hcat and vcat to be separate (my t had different values):

julia> [[t t];[t t]]
4×4 SMatrix{4, 4, Float64, 16} with indices SOneTo(4)×SOneTo(4):
  0.723556  -0.644347   0.723556  -0.644347
 -0.031864  -1.14712   -0.031864  -1.14712
  0.723556  -0.644347   0.723556  -0.644347
 -0.031864  -1.14712   -0.031864  -1.14712

[mateuszbaran]

Note also:

julia> f(t) = [t t; t t]
f (generic function with 1 method)

julia> @code_lowered f(t)
CodeInfo(
1 ─ %1 = Core.tuple(2, 2)
│   %2 = Base.hvcat(%1, t, t, t, t)
└──      return %2
)

We need to statically know %1 to have the type of the returned static array be type-stable, which may not be particularly reliable. We generally prefer type-stable Array over type-unstable StaticArray as a result type.