Unexpected type instability in Julia function

Question

I think I am getting a type instability in my code, but I'm not sure why. Consider the following code.

module test
    mutable struct Data
        arr::Array{Int64, 1}
        Data(n) = new(zeros(n))
    end

    function main()
        data = Data(5)
        data.arr
    end

    println(main())
    @code_warntype main()
end

The output of @code_warntype main() starts with the following three lines

Variables:
  #self#::test.#main
  data::Any
        ^^^

Maybe I am not interpreting the output of @code_warntype correctly, but it seems that I have a type-instability for the data variable. Can someone shed some light on what is going on here? I'm using 0.6.2 if that helps.

I can't reproduce this either. My output starts with Variables: #self# <optimized out> data <optimized out>. However, the body contains references to Array{Float64,1} which I can clean out by using Data(n) = new(zeros(Int64, n)). This also reduces the length of the native code (@code_native). There are still references to svec(Any, Int64), but you'll need someone cleverer than me to tell you what that means! — Mark Birtwistle
and a more Julian way to init Data is defining an outer constructor Data(n::Integer) = Data(zeros(n)). — Gnimuc
@woggy Could you provide the entire diagnostic output of @code_warntype of your real code? If it is too long, you could pick some red marked lines. — Gnimuc
Curiously, if I rewrite the original code as a 2-dimensional array (see my answer below), all the type instabilities seem to disappear! I've probably done something hideously stupid, so I await the rush of downvotes... :-( — Mark Birtwistle

Mark Birtwistle Mark Birtwistle · Accepted Answer · 2018-04-23T12:17:53

This is not really an answer, but it may shed some light on the problem. If I rewrite your code as a 2-dimensional array, all the type instabilities seem to disappear and the native code is minimised:

module test
    mutable struct Data
        arr::Array{Int64,2}
        Data(n::Int64, m::Int64=1) = new(zeros(Int64, n, m))
    end

    function main()
        data = Data(5)
        data.arr
    end

    println(main())
    @code_warntype main()
end

The complete output in this case is as follows:

[0; 0; 0; 0; 0]
Variables:
  #self# <optimized out>
  data::test.Data

Body:
  begin 
      data::test.Data = $(Expr(:invoke, MethodInstance for test.Data(::Int64, ::Int64), :(test.Data), 5, 1)) # line 37:
      return (Core.getfield)(data::test.Data, :arr)::Array{Int64,2}
  end::Array{Int64,2}

All references to Any now seem to have miraculously disappeared from the output. I have no idea why!

Unexpected type instability in Julia function

1 Answers