Comments.
amd64 currently uses only the bottom 48 bits of addresses. The high 16 bits are all ones or all zeroes, extended from bit 47. And this will be the case till 256TB is not enough VM space. So we can cram a counted/raw flag into bit 63 -- if it's different from bit 62 then it's a counted pointer that needs bit 63 to be flipped before using it.