The utility counts the number of occurrences of each index in an array.
It's used to build offsets for mesh topology maps, or to count the
number of connected elements. Some users are geometry nodes,
the subdivision draw cache, and mesh to curve conversion.
This PR parallelizes the counting to take advantage of multiple
threads. On a Ryzen 7950x, when counting connected edges to
vertices, I observed an improvement from 10.2 to 3.0 ms.
This most likely makes the counting less efficient, but it is
quite a nice performance improvement.
The new code was much slower for me at less than four threads,
so I added a check so that counting remains single threaded in
that case.
Pull Request: https://projects.blender.org/blender/blender/pulls/109628
53416281bd