A small puzzle...

puzzle.jl

function test1(x::AbstractVector)
    dims = [10]

    a = zeros(Int, dims...)

    for i in 1:length(x)
        a[1] += 1
    end

    a
end

function test2(x::AbstractVector)
    dims = [10]

    a = zeros(Int, dims...)

    for i in 1:length(x)
        a[1] = 1
    end

    a
end

function test3(x::AbstractVector)
    dims = (10,)

    a = zeros(Int, dims)

    for i in 1:length(x)
        a[1] += 1
    end

    a
end

a = [1:10000000]

precompile(test1, (a,))
precompile(test2, (a,))
precompile(test3, (a,))

@time test1(a)
@time test2(a)
@time test3(a)

test1(a) == test3(a)

julia> precompile(test1, (typeof(a),))

julia> precompile(test2, (typeof(a),))

julia> precompile(test3, (typeof(a),))

julia> @time test1(a)
elapsed time: 1.239536913 seconds (319984040 bytes allocated)

julia> @time test2(a)
elapsed time: 0.272075941 seconds (408 bytes allocated)

julia> @time test3(a)
elapsed time: 0.025167431 seconds (248 bytes allocated)

In 1 and 2 the dimensionality of a can't be inferred from the type of dims, because array length is nor part of the type.

a[i] += 1 is an alias for setindex!(a,i,getindex(a,i)+1), and apparently that requires a temporary variable.