Emacs internals: Deconstructing Lisp_Object in C (Part 2)
thecloudlet.github.ioSBCL uses a single zero bit to tag integers. This trick means the representation of n is just 2n, so you can add the values directly without any decoding.
It obviously also means that all the other tag values have to use 1 as the last bit.
That’s so cool. I did not know that.