Comment by uecker

Integer arithmetic working correctly, converting a pointer to an integer, and converting an integer back to the same object is something which should work. This is what we made sure to guarantee in the provenance TS. Making this work for memory outside of any object or converting back to a different object whose address was not converted to an integer previously is difficult. It can be made to work, but then you need to give up a lot (and rewrite your compilers). Outside of an object is clear, because there might be no memory mapped. Converting back to an arbitrary object does not work because the compiler must know somehow that it can not put the object into a register. If you allow conversion back to arbitrary objects, it can not put anything into registers. This would be bad.

Fabricating pointers in an implementation-defined way is obviously ok. This would cover mmap / sbrk or mapped I/O. Note also that historically, the C standard left things UB exactly so that implementations can use it for extensions (e.g. mmap). The idea that UB == invalid program is fairly recent misinformation, but we have to react to it and make things at least implementation-defined (which also meant a bit something else before).