Unexpected allocation on ternary operation

[theogf]

I was surprised to see that the ternary operation creates an allocation in this example. Should this be considered a bug?

struct A
  val::Bool
  unrelated::Base.RefValue{Nothing}
end

f(a) = a.val ? a : nothing
g(a) = a

a = A(true, Ref(nothing))
@btime f($a);
# 6.563 ns (1 allocation: 32 bytes)
@btime g($a);
#  2.461 ns (0 allocations: 0 bytes)

The behaviour has at least these three requirements:

• A contains a non-isbitstype member type
• f has unstable output type
• f has one branch returning an A instance

If any of them is not satisfied, no allocation happens.

[bvdmitri]

f has unstable output type

Given you description it makes sense that it allocates the storage for the output?

[theogf]

Given you description it makes sense that it allocates the storage for the output?

Right but the type instability is still handled without allocations if e.g. A is defined as

struct B
  val::Bool
  unrelated::Int
end
 b = B(true, 2)

@btime f($b);
# 2.691 ns (0 allocations: 0 bytes)
@btime g($b);
# 2.065 ns (0 allocations: 0 bytes)

It is confusing to not know what will allocate or not

[bvdmitri]

Hm, I see, that's indeed confusing, just to add to the collection:

julia> mutable struct C
           val::Bool
           unrelated::Int
       end

julia> c = C(true, 2)
C(true, 2)

julia> @btime f($c)
  1.666 ns (0 allocations: 0 bytes)
C(true, 2)

julia> @btime g($c)
  1.666 ns (0 allocations: 0 bytes)
C(true, 2)

julia> mutable struct D
         val::Bool
         unrelated::Base.RefValue{Nothing}
       end

julia> d = D(true, Ref(nothing))
D(true, Base.RefValue{Nothing}(nothing))

julia> @btime f($d)
  1.666 ns (0 allocations: 0 bytes)
D(true, Base.RefValue{Nothing}(nothing))

julia> @btime g($d)
  1.666 ns (0 allocations: 0 bytes)
D(true, Base.RefValue{Nothing}(nothing))

Interestingly enough making the struct mutable also eliminates the intermediate allocation. D also produces nice and clean @code_llvm output while the @code_llvm for A is overly complex

[Rajveer100]

Julia structs aren't mutable so we are making a copy I believe in the snippet shared initially (and the unrelated pointer) compared to standard datatypes in the second case, whereas for mutable struct pass by reference is the case?