What is `Core.Box()`?

New to Julia
2

This has to do with ‘boxing’ data, which is e.g. required when the concrete type of some variable in a function body cannot be determined from the types of the inputs of a function (type instability). See https://docs.julialang.org/en/stable/manual/performance-tips/#Avoid-changing-the-type-of-a-variable-1 for an example (although Core.Box doesn’t actually show up in the lowered code here). In this case, the actual type of the data is only known at runtime, and so Julia has to store a type tag identifying the runtime type along with the actual data. Boxed variables are heap-allocated and tracked by the garbage collector.

Seeing Core.Box in the output of code_warntype is usually a sign that inference can’t determine the concrete type of some variable, and can be a source of performance issues.

Here’s a case where Core.Box shows up in lowered code in Julia 0.6.1 (from https://github.com/JuliaLang/julia/issues/19668#issuecomment-268523270):

julia> struct KWArg
           x::Int
           KWArg(;x::Int=1) = new(x)
       end

julia> kwarg(;x::Int=1) = x
kwarg (generic function with 1 method)

julia> @code_lowered KWArg()

CodeInfo(:(begin 
        nothing
        return ((Core.getfield)($(QuoteNode(Core.Box(#call#3))), :contents))(1, #self#)
    end))

(Note: I actually found it hard to find cases where an explicit Core.Box shows up in lowered code for functions on 0.6.1.)

This is where Core.Box is defined on current master: https://github.com/JuliaLang/julia/blob/56b9593d8c25b839511e33e1fb06b58f95ce15df/base/boot.jl#L301-L305. Note the Any type of the contents, and the @nospecialize in the constructor.

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