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Armin Rigo  committed c5c715b

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File pypy/rpython/memory/gc/minimark.py

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 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)