https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63303
--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> --- (In reply to mikulas from comment #6) > Regarding pointer difference, the C standard says this: > > When two pointers are subtracted, both shall point to elements of the same > array object, or one past the last element of the array object; the result > is the difference of the subscripts of the two array elements. The size of > the result is implementation-defined, and its type (a signed integer type) > is ptrdiff_t defined in the <stddef.h> header. If the result is not > representable in an object of that type, the behavior is undefined. In other > words, if the expressions P and Q point to, respectively, the i-th and j-th > elements of an array object, the expression (P)-(Q) has the value i−j > provided the value fits in an object of type ptrdiff_t. > > So: p points to the beginning, q points one past the last element, so the > first condition is valid. > > The result is the difference of the subscripts of those two array elements: > 0x50000000 - 0 = 0x50000000 - this is clearly representable in the type > ptrdiff_t, so 0x50000000 result should be returned. See what I wrote, any object size bigger than half of address space really isn't supportable, because then (char *) (P) - (char *) (Q) might not fit into ptrdiff_t. There is no point slowing down all pointer subtractions (other than char/signed char/unsigned char pointers) for something that really wouldn't work reliably anyway.
