Legion types are not hashable

[Jacobfaib]

Legion::PhysicalInstance has no specialization for std::hash, but it does have a operator<(). This means it can be used in std::map but not std::unordered_map. This is surprising.

There are other such classes which appear to have operator<() defined (presumably to allow them to be put in std::map) but no std::hash specializations:

1. IndexSpace
2. IndexPartition
3. FieldSpace
4. LogicalRegion
5. LogicalPartition
6. FieldAllocator
7. UntypedBuffer
8. ArgumentMap
9. Predicate
10. Lock
11. Grant
12. PhaseBarrier
13. RegionRequirement
14. OutputRequirement
15. IndexSpaceRequirement
16. FieldSpaceRequirement
17. Future
18. FutureMap
19. PhysicalRegion

[lightsighter]

Want to make me a merge request for that if it is important?

[Jacobfaib]

Yeah, that would be useful. Off the top of my head, the most critical ones would be:

1. PhysicalInstance
2. IndexPartition
3. LogicalRegion
4. FieldSpace
5. Future
6. FutureMap

But they would all benefit from hash overloads if applicable.

[magnatelee]

I think practically the last two are the only problematic ones, as the first four reveal their unique identifiers in one way or another that we can hash off of.

[lightsighter]

Yeah, that would be useful.

Looking forward to it. Be careful about what guarantees you want to make about hash values of data structures that represent the same thing from different nodes.

[Jacobfaib]

https://gitlab.com/StanfordLegion/legion/-/merge_requests/1372

[lightsighter]

Merged