getindex allocates more than array

[WMapollo]

A simple test show that structarray allocates more than array.

using BenchmarkTools
using StaticArrays
using StructArrays

n = 1000
positions = [SVector{3, Float64}(randn(3)) for _ in 1:n]
volumes = rand(n)

struct TestPoint
    pos::SVector{3, Float64}
    vol::Float64
end

struct TestBodyStruct
    points::StructVector{TestPoint}
end

points_struct = StructVector{TestPoint}((positions, volumes))
body_struct = TestBodyStruct(points_struct)

struct TestBodyDirect
    pos::Vector{SVector{3, Float64}}
    vol::Vector{Float64}
end

body_direct = TestBodyDirect(positions, volumes)

function test_structarray_access_float(body, iterations=10000)
    for _ in 1:iterations
        for i in 1:length(body.points)
            V = body.points.vol[i]
        end
    end
    return nothing
end

function test_structarray_access_SV(body, iterations=10000)
    for _ in 1:iterations
        for i in 1:length(body.points)
            P = body.points.pos[i]
        end
    end
    return nothing
end

function test_direct_access_float(body, iterations=10000)
    for _ in 1:iterations
        for i in 1:length(body.pos)
            V = body.vol[i]
        end
    end
    return nothing
end

function test_direct_access_SV(body, iterations=10000)
    for _ in 1:iterations
        for i in 1:length(body.pos)
            P = body.pos[i]
        end
    end
    return nothing
end

println("\nStructArray getindex of Float64:")
@btime test_structarray_access_float($body_struct, 100)

println("\nArray getindex of Float64:")
@btime test_direct_access_float($body_direct, 100)


println("\nStructArray getindex of SVector:")
@btime test_structarray_access_SV($body_struct, 100)

println("\nArray getindex of SVector:")
@btime test_direct_access_SV($body_direct, 100)

And I got the results like:

StructArray getindex of Float64:
  9.265 ms (398100 allocations: 9.13 MiB)

Array getindex of Float64:
  3.058 μs (0 allocations: 0 bytes)

StructArray getindex of SVector:
  8.912 ms (398100 allocations: 10.65 MiB)

Array getindex of SVector:
  1.900 ns (0 allocations: 0 bytes)

[aplavin]

Your code is type-unstable because

struct TestBodyStruct
    points::StructVector{TestPoint}
end

doesn't fully specify the type of points. Try

struct TestBodyStruct{PT}
    points::PT
end