vm/oops/markOop.hpp 第30行，关于markOopDesc数据结构的说明，markOop是指向class markOopDesc类型对象的指针，源码如下：
30313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301 | // The markOop describes the header of an object.//// Note that the mark is not a real oop but just a word.// It is placed in the oop hierarchy for historical reasons.//// Bit-format of an object header (most significant first, big endian layout below)://// 32 bits:// --------// hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object)// JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object)// size:32 ------------------------------------------>| (CMS free block)// PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)//// 64 bits:// --------// unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object)// JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)// PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)// size:64 ----------------------------------------------------->| (CMS free block)//// unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object)// JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object)// narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)// unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)//// - hash contains the identity hash value: largest value is// 31 bits, see os::random(). Also, 64-bit vm's require// a hash value no bigger than 32 bits because they will not// properly generate a mask larger than that: see library_call.cpp// and c1_CodePatterns_sparc.cpp.//// - the biased lock pattern is used to bias a lock toward a given// thread. When this pattern is set in the low three bits, the lock// is either biased toward a given thread or "anonymously" biased,// indicating that it is possible for it to be biased. When the// lock is biased toward a given thread, locking and unlocking can// be performed by that thread without using atomic operations.// When a lock's bias is revoked, it reverts back to the normal// locking scheme described below.//// Note that we are overloading the meaning of the "unlocked" state// of the header. Because we steal a bit from the age we can// guarantee that the bias pattern will never be seen for a truly// unlocked object.//// Note also that the biased state contains the age bits normally// contained in the object header. Large increases in scavenge// times were seen when these bits were absent and an arbitrary age// assigned to all biased objects, because they tended to consume a// significant fraction of the eden semispaces and were not// promoted promptly, causing an increase in the amount of copying// performed. The runtime system aligns all JavaThread* pointers to// a very large value (currently 128 bytes (32bVM) or 256 bytes (64bVM))// to make room for the age bits & the epoch bits (used in support of// biased locking), and for the CMS "freeness" bit in the 64bVM (+COOPs).//// [JavaThread* | epoch | age | 1 | 01] lock is biased toward given thread// [0 | epoch | age | 1 | 01] lock is anonymously biased//// - the two lock bits are used to describe three states: locked/unlocked and monitor.//// [ptr | 00] locked ptr points to real header on stack// [header | 0 | 01] unlocked regular object header// [ptr | 10] monitor inflated lock (header is wapped out)// [ptr | 11] marked used by markSweep to mark an object// not valid at any other time//// We assume that stack/thread pointers have the lowest two bits cleared.class BasicLock;class ObjectMonitor;class JavaThread;class markOopDesc: public oopDesc { private: // Conversion uintptr_t value() const { return (uintptr_t) this; } public: // Constants enum { age_bits = 4, lock_bits = 2, biased_lock_bits = 1, max_hash_bits = BitsPerWord - age_bits - lock_bits - biased_lock_bits, hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits, cms_bits = LP64_ONLY(1) NOT_LP64(0), epoch_bits = 2 }; // The biased locking code currently requires that the age bits be // contiguous to the lock bits. enum { lock_shift = 0, biased_lock_shift = lock_bits, age_shift = lock_bits + biased_lock_bits, cms_shift = age_shift + age_bits, hash_shift = cms_shift + cms_bits, epoch_shift = hash_shift }; enum { lock_mask = right_n_bits(lock_bits), lock_mask_in_place = lock_mask << lock_shift, biased_lock_mask = right_n_bits(lock_bits + biased_lock_bits), biased_lock_mask_in_place= biased_lock_mask << lock_shift, biased_lock_bit_in_place = 1 << biased_lock_shift, age_mask = right_n_bits(age_bits), age_mask_in_place = age_mask << age_shift, epoch_mask = right_n_bits(epoch_bits), epoch_mask_in_place = epoch_mask << epoch_shift, cms_mask = right_n_bits(cms_bits), cms_mask_in_place = cms_mask << cms_shift#ifndef _WIN64 ,hash_mask = right_n_bits(hash_bits), hash_mask_in_place = (address_word)hash_mask << hash_shift#endif }; // Alignment of JavaThread pointers encoded in object header required by biased locking enum { biased_lock_alignment = 2 << (epoch_shift + epoch_bits) }; // These values are too big for Win64 const static uintptr_t hash_mask = right_n_bits(hash_bits); const static uintptr_t hash_mask_in_place = (address_word)hash_mask << hash_shift; enum { locked_value = 0, unlocked_value = 1, monitor_value = 2, marked_value = 3, biased_lock_pattern = 5 }; enum { no_hash = 0 }; // no hash value assigned enum { no_hash_in_place = (address_word)no_hash << hash_shift, no_lock_in_place = unlocked_value }; enum { max_age = age_mask }; enum { max_bias_epoch = epoch_mask }; // Biased Locking accessors. // These must be checked by all code which calls into the // ObjectSynchronizer and other code. The biasing is not understood // by the lower-level CAS-based locking code, although the runtime // fixes up biased locks to be compatible with it when a bias is // revoked. bool has_bias_pattern() const { return (mask_bits(value(), biased_lock_mask_in_place) == biased_lock_pattern); } JavaThread* biased_locker() const { assert(has_bias_pattern(), "should not call this otherwise"); return (JavaThread*) ((intptr_t) (mask_bits(value(), ~(biased_lock_mask_in_place | age_mask_in_place | epoch_mask_in_place)))); } // Indicates that the mark has the bias bit set but that it has not // yet been biased toward a particular thread bool is_biased_anonymously() const { return (has_bias_pattern() && (biased_locker() == NULL)); } // Indicates epoch in which this bias was acquired. If the epoch // changes due to too many bias revocations occurring, the biases // from the previous epochs are all considered invalid. int bias_epoch() const { assert(has_bias_pattern(), "should not call this otherwise"); return (mask_bits(value(), epoch_mask_in_place) >> epoch_shift); } markOop set_bias_epoch(int epoch) { assert(has_bias_pattern(), "should not call this otherwise"); assert((epoch & (~epoch_mask)) == 0, "epoch overflow"); return markOop(mask_bits(value(), ~epoch_mask_in_place) | (epoch << epoch_shift)); } markOop incr_bias_epoch() { return set_bias_epoch((1 + bias_epoch()) & epoch_mask); } // Prototype mark for initialization static markOop biased_locking_prototype() { return markOop( biased_lock_pattern ); } // lock accessors (note that these assume lock_shift == 0) bool is_locked() const { return (mask_bits(value(), lock_mask_in_place) != unlocked_value); } bool is_unlocked() const { return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); } bool is_marked() const { return (mask_bits(value(), lock_mask_in_place) == marked_value); } bool is_neutral() const { return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); } // Special temporary state of the markOop while being inflated. // Code that looks at mark outside a lock need to take this into account. bool is_being_inflated() const { return (value() == 0); } // Distinguished markword value - used when inflating over // an existing stacklock. 0 indicates the markword is "BUSY". // Lockword mutators that use a LD...CAS idiom should always // check for and avoid overwriting a 0 value installed by some // other thread. (They should spin or block instead. The 0 value // is transient and *should* be short-lived). static markOop INFLATING() { return (markOop) 0; } // inflate-in-progress // Should this header be preserved during GC? inline bool must_be_preserved(oop obj_containing_mark) const; inline bool must_be_preserved_with_bias(oop obj_containing_mark) const; // Should this header (including its age bits) be preserved in the // case of a promotion failure during scavenge? // Note that we special case this situation. We want to avoid // calling BiasedLocking::preserve_marks()/restore_marks() (which // decrease the number of mark words that need to be preserved // during GC) during each scavenge. During scavenges in which there // is no promotion failure, we actually don't need to call the above // routines at all, since we don't mutate and re-initialize the // marks of promoted objects using init_mark(). However, during // scavenges which result in promotion failure, we do re-initialize // the mark words of objects, meaning that we should have called // these mark word preservation routines. Currently there's no good // place in which to call them in any of the scavengers (although // guarded by appropriate locks we could make one), but the // observation is that promotion failures are quite rare and // reducing the number of mark words preserved during them isn't a // high priority. inline bool must_be_preserved_for_promotion_failure(oop obj_containing_mark) const; inline bool must_be_preserved_with_bias_for_promotion_failure(oop obj_containing_mark) const; // Should this header be preserved during a scavenge where CMS is // the old generation? // (This is basically the same body as must_be_preserved_for_promotion_failure(), // but takes the Klass* as argument instead) inline bool must_be_preserved_for_cms_scavenge(Klass* klass_of_obj_containing_mark) const; inline bool must_be_preserved_with_bias_for_cms_scavenge(Klass* klass_of_obj_containing_mark) const; // WARNING: The following routines are used EXCLUSIVELY by // synchronization functions. They are not really gc safe. // They must get updated if markOop layout get changed. markOop set_unlocked() const { return markOop(value() | unlocked_value); } bool has_locker() const { return ((value() & lock_mask_in_place) == locked_value); } BasicLock* locker() const { assert(has_locker(), "check"); return (BasicLock*) value(); } bool has_monitor() const { return ((value() & monitor_value) != 0); } ObjectMonitor* monitor() const { assert(has_monitor(), "check"); // Use xor instead of &~ to provide one extra tag-bit check. return (ObjectMonitor*) (value() ^ monitor_value); } bool has_displaced_mark_helper() const { return ((value() & unlocked_value) == 0); } markOop displaced_mark_helper() const { assert(has_displaced_mark_helper(), "check"); intptr_t ptr = (value() & ~monitor_value); return *(markOop*)ptr; } void set_displaced_mark_helper(markOop m) const { assert(has_displaced_mark_helper(), "check"); intptr_t ptr = (value() & ~monitor_value); *(markOop*)ptr = m; } |