Do we have free bits in the info pointer itself?

[Florian Weimer]

* Ryan Newton:

> Our heap object header is one word -- an info table pointer
> <https://ghc.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage/HeapObjects>.
>
> Well, a 64 bit info table pointer leaves *at least* 16 high bits inside the
> object header for other purposes, right?

x86_64 has signed pointers, so the uppoer 16 bits are either all zero
or all ones.  Some systems use both, but it's rare on Linux.

Other 64-bit platforms use more bits.