> stuff like "t += t >> 16" is a many-to-one function, not a permutation

Yes, I am aware of that. However, the number of collisions here is really quite small. It's very unlikely to hit one by accident.

I also chose >> over << for two reasons:

1. It brings the high-order in play: https://bugs.python.org/msg326117

2. It avoids collisions on the low-order bits: when you do t ^= t << 7, then you are not changing the lower 7 bits at all. So applications using hash(x) % 128 will still see all the problems that we are trying to fix.