Default construction of future, promise, global_ptr and persona before init()

[Amir Kamil]

The spec requires UPC++ to be initialized before any UPC++ function can be called. This prohibits any UPC++ type from having static storage duration (since initializing it would entail a call to its constructor). This includes global pointers, which should likely be exempt from this restriction, as it prevents an application from keeping track of global pointers in static storage.

I suggest that we slightly loosen this restriction to something like the following: Unless otherwise stated, it is illegal to call a UPC++ function, including a constructor, when UPC++ is in the uninitialized state. We then exempt the following functions from this restriction:

• init()
• global_ptr<T>::global_ptr(std::nullptr_t)
• global_ptr<T>::global_ptr() (This doesn't exist at the moment, but it would be nice for global_ptr to be DefaultConstructible. This would just construct a null pointer. It would likely necessitate a rank-independent representation of a null global pointer, which our implementation already does.)

[Dan Bonachea]

I agree with all of Amir's observations.

For the same usability reasons, we may also want to allow the default constructor for DefaultConstructible types upcxx::promise<...T> and upcxx:persona. However these might entail more significant implementation effort.

I'm not even sure what the semantics of a default constructed persona would be, except to serve as an empty variable that will be overwritten by a real persona after init.

[Scott Baden]

Would is be accurate to say that, anything requiring progress is off-limits? I'm not sure because of Amir's comment "a rank-independent representation of a null global pointer."

[Amir Kamil]

I don't think that's a sufficient restriction. My comment was alluding to the fact that global pointers contain a rank as part of their representation, and rank_me() would not be available before initialization. Thus, their must be a representation of null that does not depend on knowing the rank of the creator.

[Scott Baden]

Can you construct such a null without the rank?

[Amir Kamil]

My implementation uses a rank of 0 to represent a global pointer, regardless of which rank creates the pointer.

[Dan Bonachea]

For the record, we think future<T> should also be made DefaultConstructible and declarable at static scope, for the convenience of a statically-scoped container to hold futures created at runtime. The "null" future created should by the default constructor should probably just be trivially ready (or trivially never ready).

[Dan Bonachea]

At the 8-9-17 Pagoda meeting we resolved that future, promise, global_ptr and possibly persona should all be DefaultConstructible and declarable at static scope. This needs to be added to the spec, along with the semantics for a default constructed object of each type (ie when functions are called on them after init).

team and dist_object need to still prohibit default construction due to the collective initialization requirement and the lack of ordering guarantees for static initializers in general.

[Dan Bonachea]

Resolved by Amir in f89f055 Thanks!