Use Pointers to store CuckooSet

[crichez]

Objectives

This pull request improves the per-insert performance of CuckooSet by changing its backing storage from Array to UnsafeMutablePointer.

Implementation

When initializing a set, every pointer in the buffer is initialized to nil to indicate an empty bucket. This operation still takes linear time as the initializer must iterate through every pointer in the buffer. The buffer's header is empty. Instead of storing capacity and count in the header, they are stored inline at the class definition. This may change to simplify copy-on-write semantics, but for now should provide meager performance benefits.

Performance

Benchmark output on the CuckooSet<Int> Insert task outputs the following comparison statistics:

Tasks with difference scores larger than 1.05:
  Score   Sum     Improvements Regressions  Name
  1.341   1.341   1.341(#76)   1.000(#0)    CuckooSet<Int> Insert (*)

The CuckooSet<Int> Contains task showed no significant improvement.

Most of this improvement is associated with the removed bounds checks associated with unsafe pointers. This significantly reduces memory safety, but our tests guarantee this implementation is safe.

[crichez]

I am dealing with the same issue locally and remotely where tests pass individually but fail when ran as a batch. This does suggest some memory safety issues. I need to look into ManagedBuffer more. I believe I am meant to subclass with a deinit override to deallocate everything before the program exits.

[crichez]

Last commit uses a CuckooStorage class that references capacity, count and the base pointer to the elements. These deinitialized and deallocated when the storage instance is deallocated.