Struct wrapping NamedTuple is not `isbits` (depending on how it is defined)

General Usage
,
1

I am wrapping NamedTuple to define some new/different behaviour without committing type piracy and ran into the following problem:

struct Wrapper{SYMS, TT}
   x::NamedTuple{SYMS, TT}
end
randnt() = Wrapper((a = randn(), b = rand(), c = rand(1:5), d = randn(SVector{3, Float64})))
X = randnt()
@allocated randnt()  # 64
isbits(X)   # false
isbits(X.x)   # true

This is not good, I really need Wrapper to be non-allocating. Does this have anything to do with the fact that Symbol is not isbits?

Bizarrely, though, (for me anyhow…)

struct Wrapper1{NT} 
   x::NT
end
randnt1() = Wrapper1((a = randn(), b = rand(), c = rand(1:5), d = randn(SVector{3, Float64})))
X1 = randnt1()
@allocated randnt1() # 0 
isbits(X1) # true
isbits(X1.x) # true

I cannot grasp the fundamental different between these two.

To explain why I prefer the first implementation Wrapper: I use quite a few generated functions which need the SYMS information. With a tiny bit of extra code this can of course be done for Wrapper1 as well, so not too big a deal. (well-somewhat, I now have to fix a few 100 lines of code and not introduce bugs). But I am very curious and would like to understand what the difference here is?

3 Likes
2

Isn’t the named tuple still abstract because the length is not defined?

3

I don’t think so. The codes all seem type stable.

4

On my computer (old mac), both the first and second codes give “0 true true” with Julia 1.6.1. So the behavior may depend on versions?

2 Likes
5

Same here:

julia> VERSION, isbits(X), isbits(X.x)
(v"1.6.1", true, true)

julia> VERSION, isbits(X), isbits(X.x)
(v"1.7.0", false, true)
1 Like
6

That would explain why I never noticed it until now :slight_smile:

7

Uh oh. That might be a bug then, I’d recommend opening an issue if there’s not one already.

2 Likes
8

Will do … EDIT: done

1 Like
9

Yeah, definitely looks like a bug to me:

julia> nt(::Wrapper{S,T}) where {S,T} = NamedTuple{S,T}
nt (generic function with 1 method)

julia> nt(X)
NamedTuple{(:a, :b, :c, :d), Tuple{Float64, Float64, Int64, SVector{3, Float64}}}

julia> nt(X) === typeof(X.x)
true

julia> isbits(X)   # false
false

julia> isbits(X.x)   # true
true

The wrapper seems to be matching the NamedTuple exactly.

2 Likes