Unfortunately, low bits vary considerably from a large number to zero. If you need a minimum number of bits to be reliably available then you have to look at the high bits of a pointer. Naturally, implementations use the low bits of alignment makes them available.

Hm, what I have seen so far is that pointers returned by system malloc are usually aligned either to 8-byte boundary (windows) or 16-byte boundary (linux). I think jemalloc interprets the C standard guarantees a bit differently and will return the 8-byte aligned pointer for allocations whose size <= 8. But even this still leaves us with 3 LBSs to use.