TID_MASK = (first_gcflag << 7) - 1
+# The remaining unused bits (GCFLAG_HIGH_MASK) are set to a known pattern
+# in all objects (GCFLAG_HIGH). From a random address in the nursery, it
+# let us know if it points to a valid object: either "definitely not", or
+# "very likely". We can't be sure, though, so be careful.
GCFLAG_HIGH_MASK = intmask(~TID_MASK)
assert GCFLAG_HIGH_MASK < 0 and not (GCFLAG_HIGH_MASK & GCFLAG_CARDS_SET)
GCFLAG_HIGH = intmask(0x5555555555555555 & GCFLAG_HIGH_MASK)