[Bug c/61502] == comparison on "one-past" pointer gives wrong result

[joseph at codesourcery dot com]

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.