Rationale for pointer comparisons outside an array to be UB

Question

So, the standard (referring to N1570) says the following about comparing pointers:

C99 6.5.8/5 Relational operators

When two pointers are

Answer 1

I believe it's undefined so that C can be run on architectures where, in effect, "smart pointers" are implemented in hardware, with various checks to ensure that pointers never accidentally point outside of the memory regions they're defined to refer to. I've never personally used such a machine, but the way to think about them is that computing an invalid pointer is precisely as forbidden as dividing by 0; you're likely to get a run-time exception that terminates your program. Furthermore, what's forbidden is computing the pointer, you don't even have to dereference it to get the exception.

Yes, I believe the definition also ended up permitting more-efficient comparisons of offset registers in old 8086 code, but that was not the only reason.

Yes, a compiler for one of these protected pointer architectures could theoretically implement the "forbidden" comparisons by converting to unsigned or the equivalent, but (a) it would likely be significantly less efficient to do so and (b) that would be a wantonly deliberate circumvention of the architecture's intended protection, protection which at least some of the architecture's C programmers would presumably want to have enabled (not disabled).