This basically works now, but it's not particularly useful, because the intermediate ref allocation does still happen, so even something simple like

function -{N,T}(x::NTuple{N,T})
y = Ref{NTuple{N,T}}()
for i = 1:N
y[i] = -x[i]
end
y.x
end

ends up allocating. @JeffBezanson do we have anything in there right now that could deal with this?

i was expecting this to be implemented via the existing pointerref / pointerset intrinsics (possible with the addition of a new gep intrinsic). what's the rationale for adding this as a builtin function instead?

I was concerned about GC behavior, which is why I added a new intrinsic that atomically does this so no random pointers are flying around.

I was concerned about GC behavior, which is why I added a new intrinsic that atomically does this so no random pointers are flying around.

fair enough. a lot of the Ref code is built around late / lazy conversion to pointer under the premise that we can use that to ensure Ref ends up assigned to some local variable before having unsafe_convert called on it.

since setindex! returns the Ref, I think that ensures GC must keep it alive.

Except it doesn't. It returns the assigned value.

Except it doesn't. It returns the assigned value.

it isn't supposed to:

julia> x=[1,2,3]
3-element Array{Int64,1}:
 1
 2
 3

julia> setindex!(x, 5, 2)
3-element Array{Int64,1}:
 1
 5
 3

Oh, huh, why did I think it returned the assigned value?

Oh, yeah:

julia> a = [1,2,3]
3-element Array{Int64,1}:
 1
 2
 3

julia> a[1] = 2
2

In any case though, after inlining is the Ref still guaranteed to be live?

it doesn't really matter if it's alive or not once you've lost the reference to it, since you wouldn't be able to take the final value back out anyways

what if the base case (and new intrinsic) was offsetof, rather than mimicking GEP or the C behavior for this function.

then the implementation code could look something like:

function setindex!{T}(ref::Ref{T}, val, indices)
  ptr = unsafe_convert(::Ptr{T}, ref)
  offset = Intrinsics.offsetof(T, indices)
  unsafe_store!(ptr + offset, val)
  ref
end

(tagged 0.4.0 because underlying issue is also tagged 0.4.0)

Not enough progress has been made, kicking to 1.0.

Superseded by #21912