Using side to move as a separate bit in hash key

[sje]

Another idea is to split the table into two equal size tables; one for WTM, the other for BTM. I've done this and it has worked well. In a program with a variable search depth, both tables see about the same usage regardless of the root STM.

Pre-determining which half of the hash table will be used for white and which for black will decrease the efficiency of the hash table. "about" the same usage is not enough.

Actually, it is; at least in my tests. And it will be true in any program with a variable depth search regardless of the origin(s) of the variability.

How did you test?

By activating the various metrics counters for utilization, inserts, probes, matches, etc.

So no tests of the effect on playing strength.

It is intuitively obvious that splitting the table in two and dedicating one half to white-to-move and the other to black-to-move is suboptimal, because white-to-move hash pressure and black-to-move hash pressure will not always be equal. Testing is not really needed to confirm this. Only if you could get a speed up by splitting the table might it be different, but a speed up you won't get.

Statistics suggesting that things tend to average out somewhat don't really convince me, I'm afraid.

Gosh, I don't give a crap if you're convinced or not. I'm merely reporting what I've done and what I've seen. I never claimed that each half of STM would see absolutely equal use, but in practice they effectively will in any variable depth search.

[syzygy]

Statistics suggesting that things tend to average out somewhat don't really convince me, I'm afraid.

Gosh, I don't give a crap

That's fine with me. But why go through the trouble of posting that info?