Wrapping an Array in a struct allocates

[oxinabox]

Consider:

using BenchmarkTools

struct Foo{T}
    x::T
end

const m = [1,2,3]
const k = 1:3

@btime Foo(k);
@btime Foo(m);

Output:

julia> @btime Foo(k);

  0.029 ns (0 allocations: 0 bytes)

julia> @btime Foo(m);
  4.952 ns (1 allocation: 16 bytes)

It doesn't seem like Foo(m) should allocate when Foo(k) can avoid it.

---

[oxinabox]

But @allocated does say 0

julia> @allocated Foo(k)
0

julia> @allocated Foo(m)
0

[KristofferC]

Just seems like bad benchmarking?

julia> g(m) = (Foo(m); nothing)
g (generic function with 1 method)

julia> @btime g($m)
  0.035 ns (0 allocations: 0 bytes)

If you return the object it needs to be allocated?

[oxinabox]

Ah, I see.

But still why the difference betweek Foo(k) and Foo(m) ?

[KristofferC]

Because one is isbits and not the other?

[vtjnash]

Will be closed by #34126

[JeffBezanson]

Duplicate of #14955