So, modern CPUs have built-in functions for a lot of things, these days, which can offer you blisteringly fast access to some otherwise rather awkward and slow algorithms.
In my case, I wanted to access the CPU’s popcount function (count how many set bits there are in a value). The trick is that either the CPU supports it or it doesn’t, and different systems have different ways of testing if it does, and accessing it if it does. Of course if the CPU doesn’t support it, you still have to do it in software anyway.
So, I thought I’d write something extensible, and I thought I’d share.