Page tables can optionally consume a very large number of bits on x86 (57?). Not every platform enables it but your code may run on a platform that uses it. There are a bunch of proposals from Intel, AMD, ARM, et al about officially recognizing some set of high bits in 64-bit pointers as tags in user space, with implementations. Unfortunately, these “standards” don’t agree on which high bits can be safely reserved for tagging.

IIRC, the 6 high bits I mentioned was the intersection of every tag reservation implementation and/or proposal. In other words, it was the set of bits that Intel, AMD, and ARM agreed would be safe for tagging for the foreseeable future.

Fewer bits than I would like and can probably exploit, but nonetheless the number I can reasonably rely on. If a consistent standard is ever agreed upon, the number of bits may increase.