https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63303
--- Comment #20 from Joshua Green <jvg1981 at aim dot com> --- > "But if we don't know which pointer is greater, it gets more complicated: > ..." > > I'm not sure that this is true. For types that are larger than 1 byte, it > seems that one can do the subtraction after any division(s), hence only > costing an additional division (or shift): > > T * p; > T * q; > > . > . > . > > intptr_t pVal = ((uintptr_t) p)/(sizeof *p); > intptr_t qVal = ((uintptr_t) q)/(sizeof *q); > > ptrdiff_t p_q = pVal - qVal; > > This should work in well-defined cases, for if p and q are pointers into the > same array then (presumably) ((uintptr_t) p) and ((uintptr_t) q) must have > the same remainder modulo sizeof(T). > > Of course, even an additional shift may be too expensive in some cases, so > it's not entirely clear that this change should be made. It occurred to me that such contortions can be avoided in the (possibly) common case when (say) q is actually an array.
