https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61502
--- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- Except within a larger object, I'm not aware of any reason the cases of two objects following or not following each other in memory must be mutually exclusive. (If the implementation can track the origins of bit-patterns and where copies of those bit-patterns have got to, it might have a compacting garbage collector that relocates objects and changes what's adjacent to what, for example - I think such implementations are within the scope of what the C standard is intended to support. Or if you're concerned about how this changes bit-patterns of pointers, imagine that a C pointer is a (object key, offset) pair, and that comparison first converts the C pointer into a hardware address, where it's the mapping from object keys to hardware addresses that changes as a result of garbage collection rather than anything about the representation of the pointer.) So the only way within the C standard you could deduce that two objects follow each other in memory is that the address of one compares equal to one past the address of the other - but that does not mean they follow each other in memory for any other comparison. An object having a constant address (6.2.4#2) is described non-normatively in footnote 33 in terms of comparisons of pointers to that object. I don't think it should be taken to mean comparisons of pointers to different objects need to have constant results.
