Creating SArray with more that 14 entries uses allocation

[wsshin]

julia> VERSION
v"0.6.3-pre.0"

julia> using BenchmarkTools

julia> @btime SVector{14}(0,0,0,0,0,0,0,0,0,0,0,0,0,0);
  2.181 ns (0 allocations: 0 bytes)

julia> @btime SVector{15}(0,0,0,0,0,0,0,0,0,0,0,0,0,0,0);
  12.230 ns (1 allocation: 128 bytes)

julia> @btime SMatrix{14,1}(0,0,0,0,0,0,0,0,0,0,0,0,0,0);
  2.186 ns (0 allocations: 0 bytes)

julia> @btime SMatrix{15,1}(0,0,0,0,0,0,0,0,0,0,0,0,0,0,0);
  11.333 ns (1 allocation: 128 bytes)

julia> @btime SArray{Tuple{14}}(0,0,0,0,0,0,0,0,0,0,0,0,0,0);
  2.186 ns (0 allocations: 0 bytes)

julia> @btime SArray{Tuple{15}}(0,0,0,0,0,0,0,0,0,0,0,0,0,0,0);
  12.952 ns (1 allocation: 128 bytes)

Is this an expected behavior?

[tkoolen]

This is the splatting penalty for

StaticArrays.jl/src/convert.jl

Line 3 in f7bf751

@inline (::Type{SA})(x...) where {SA <: StaticArray} = SA(x)

See e.g. JuliaLang/julia#22545, JuliaLang/julia#22370. Could work around the penalty in StaticArrays with an @generated constructor, but I'm not sure that that's a good idea. What's your use case?

[wsshin]

I'm just creating a large matrix with >14 entries.

[martinholters]

If possible, just pass a tuple:

julia> @btime SVector{15}(0,0,0,0,0,0,0,0,0,0,0,0,0,0,0);
  13.507 ns (1 allocation: 128 bytes)

julia> @btime SVector{15}((0,0,0,0,0,0,0,0,0,0,0,0,0,0,0));
  3.701 ns (0 allocations: 0 bytes)

[andyferris]

Correct. In code, I recommend using tuples to construct static arrays. There's also a Tuple(::StaticArray) constructor to go the other way.

The non-tuple constructors are only mostly for convenience. They are nice for creating 3-vectors or playing at the REPL but actually don't make any sense w.r.t. all the AbstractArray constructor interface discussions that happened during Julia v0.7 development. There are also the macros which also use tuples to avoid the splatting penalty.

Sorry @wsshin - since there's not much I can do about the splatting penalty from this side, I'll close this issue.