How about taking the advantage of max_align_t pointer alignment guarantees, which on x86-64 Linux (glibc) is 16-bytes? This would leave you with the 4 lowest bits to be used.

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jandrewrogers 2 days ago | parent [-]

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.

	▲	menaerus a day ago \| parent [-]
		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.