[$] Pointer tagging for x86 systems

Pointers are a fact of life for developers working in numerous languages.It is often convenient to be able to associate a small amount - a few bits at most - of ancillary information with a pointer.This can often be done within the pointer value itself with some carefulmasking and shifting. CPU manufacturers have been adding ways to supportthe addition of this sort of "tag" to pointers; the most recent may beAMD's "upper address ignore" (UAI) feature, support for which wasrecently postedby Bharata B Rao. This feature has an uncertain future in Linux, though,as the result of a fundamental design decision.