Fix AABB tree performance.

[joaander]

Description

Use the fast AABB-AABB overlap check when traversing AABB trees.

I implemented this by moving the overlap(AABB, AABB) function to a member function of AABB: aabb.overlaps(other_aabb). Explicitly calling the intended overlap test ensures that the correct function is called.

Motivation and context

Increase performance.

For reasons I don't fully understand, the compiler chose to automatically convert AABBs to OBBs, then call overlap(OBB, OBB) - which is many times slower than overlap(AABB, AABB).

How has this been tested?

This pull request increases performance by ~40-50% in HPMC benchmarks (glotzerlab/hoomd-benchmarks#68). It should also improve MD performance with nlist.Tree, though I haven't tested that.

Change log

Fixed:

* Increase HPMC performance a further ~50% on the CPU.

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the HOOMD-blue Contributor Agreement.
• My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.

[joaander]

This change ensures that no future automatic conversions from AABB to OBB are allowed.

Callers must opt into the conversion explicitly: OBB obb(aabb) or OBB(aabb).

[tommy-waltmann]

Have you tested to see if it was necessary to move the function call to a class member of AABB, or if just adding explicit to the OBB constructor was enough to do the trick?

[tommy-waltmann]

Approving because I don't want to hold up the next release.

[joaander]

Have you tested to see if it was necessary to move the function call to a class member of AABB, or if just adding explicit to the OBB constructor was enough to do the trick?

When I only added explicit, I got an error compile error that there was no overlap(AABB, AABB) to call. I don't know why because it is clearly in the header file - so I moved it to a member instead.

[tommy-waltmann]

When I only added explicit, I got an error compile error that there was no overlap(AABB, AABB) to call. I don't know why because it is clearly in the header file - so I moved it to a member instead.

That seems like the reason the compiler was automatically converting the AABBs to OBBs, it couldn't find the overlap(AABB, AABB) declaration.