[StopDisinfo910]

> We chose 4B because that was the maximum number of bytes that would be unused otherwise

I'm sure you did but there is something funny reading this phrase while at the same time considering you have robbed two bits from your pointer to represent class - admittedly the only thing I find questionable in your design.

If that's the case it's a happy accident because having a full code point here is quite nice.

[msichert]

That's a good point. We just use pointer tagging in many different places (e.g. for pointer swizzling in our buffer pages), so including a few bits of information in a pointer just seemed obvious.