Fastest way to initialize StaticArray of dynamic size with state

General Usage Performance
3

In some cases you can use an MVector, perform your operations, and convert to SVector. If the array doesn’t escape, everything is stack allocated and fast:

function test3(::Val{K},x::Int) where K
    y = MVector{K,Int}(undef)
    y[1] = K
    for i in 2:K
        y[i] = y[i-1] + 1
    end
    return SVector(y)
end

Comparing to the code using Setfield:

1.7.2> @btime test2(Val(4),1)
  3.200 ns (0 allocations: 0 bytes);

1.7.2> @btime test3(Val(4),1);
  2.300 ns (0 allocations: 0 bytes)

The difference grows larger for bigger arrays:

1.7.2> @btime test2(Val(8),1);
  11.011 ns (0 allocations: 0 bytes)

1.7.2> @btime test3(Val(8),1);
  2.600 ns (0 allocations: 0 bytes)

1.7.2> @btime test2(Val(16),1);
  37.538 ns (0 allocations: 0 bytes)

1.7.2> @btime test3(Val(16),1);
  3.100 ns (0 allocations: 0 bytes)

1.7.2> @btime test2(Val(32),1);
  110.645 ns (0 allocations: 0 bytes)

1.7.2> @btime test3(Val(32),1);
  21.486 ns (0 allocations: 0 bytes)

1.7.2> @btime test2(Val(64),1);
  1.300 μs (0 allocations: 0 bytes)

1.7.2> @btime test3(Val(64),1);
  38.547 ns (0 allocations: 0 bytes)
3 Likes