Based on recent blog posts from some Google folks regarding quantizing neural nets I'm going to guess 8-bit fixed point.

For example -> https://petewarden.com/2016/05/03/how-to-quantize-neural-net...

Probably.

In addition, there's a lot of literature on optimizing hardware implementations of fundamental arithmetic operations like addition and multiplication. I recall seeing a paper a while ago which talked about reducing the number of gates by allowing some bounded imprecision in the results - unfortunately, I don't remember the title right now, but it sounds like that's what they may be doing.

When I first read the blog post earlier in the day, it actually said they were using 8-bits. I remember it because it seem quite small to me.

Or small ints, possibly working in log space.