JDK-8211238 added a means to tag a method with a meta-bit (TRANSIENT_BIT) that counts towards a Method* considered "in-use".

With JDK-8328610, we'll introduce yet another meta-bit (STICKY_BIT) that will also count towards a Method* considered "in-use"

This results in the following "in-use" predicate for Method*'s:

#define METHOD_FLAG_USED_PREVIOUS_EPOCH(method) (METHOD_FLAG_PREDICATE(method, (STICKY_BIT | TRANSIENT_BIT | PREVIOUS_EPOCH_BIT)))

STICKY_BIT and TRANSIENT_BIT are in the higher-order meta-bits, while PREVIOUS_EPOCH_BIT is the low-order "user bit".

The clearing of tag bits looks like this for a Method*:

template <>
class ClearArtifact<const Method*> {
 public:
 bool operator()(const Method* method) {
 assert(METHOD_FLAG_USED_PREVIOUS_EPOCH(method), "invariant");
 CLEAR_SERIALIZED_METHOD(method);
 assert(METHOD_IS_NOT_SERIALIZED(method), "invariant");
 assert(METHOD_IS_NOT_LEAKP(method), "invariant");
 assert(METHOD_IS_NOT_TRANSIENT(method), "invariant");
 SET_PREVIOUS_EPOCH_METHOD_CLEARED_BIT(method);
 CLEAR_PREVIOUS_EPOCH_METHOD_FLAG(method);
 assert(IS_THIS_EPOCH_METHOD_CLEARED_BIT_SET(method), "invariant");
 assert(IS_PREVIOUS_EPOCH_METHOD_CLEARED_BIT_SET(method), "invariant");
 return true;
 }

// CLEAR_SERIALIZED_METHOD uses an AND mask to clear the following meta-bits: SERIALIZED_BIT | TRANSIENT_BIT | LEAKP_BIT

// CLEAR_PREVIOUS_EPOCH_METHOD_FLAG(Method) instead uses an XOR cas to clear the PREVIOUS_EPOCH_BIT.

The problem here is with this last macro, especially as we move towards letting meta-bits also denote "in-use".

This is because CLEAR_PREVIOUS_EPOCH_METHOD_FLAG(Method) assumes PREVIOUS_EPOCH_BIT to be set.

Let's say the Method* is considered in use only because its TRANSIENT_BIT meta-bit is set.

What happens when an XOR is used to clear something that is 0? 1 XOR 0 == 1.

I.e., CLEAR_PREVIOUS_EPOCH_METHOD_FLAG(Method) sets the PREVIOUS_EPOCH_BIT, rather than clearing it, in cases where the PREVIOUS_EPOCH_BIT is not set.