1 /*
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 11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 12  * version 2 for more details (a copy is included in the LICENSE file that
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 24 
 25 #ifndef SHARE_OOPS_MARKWORD_HPP
 26 #define SHARE_OOPS_MARKWORD_HPP
 27 
 28 #include "metaprogramming/integralConstant.hpp"
 29 #include "metaprogramming/primitiveConversions.hpp"
 30 #include "oops/oopsHierarchy.hpp"
 31 #include "runtime/globals.hpp"
 32 
 33 // The markWord describes the header of an object.
 34 //
 35 // Bit-format of an object header (most significant first, big endian layout below):
 36 //
 37 //  32 bits:
 38 //  --------
 39 //             hash:25 ------------>| age:4    biased_lock:1 lock:2 (normal object)
 40 //             JavaThread*:23 epoch:2 age:4    biased_lock:1 lock:2 (biased object)
 41 //
 42 //  64 bits:
 43 //  --------
 44 //  nklass:32 hash:25 -->| unused_gap:1   age:4    biased_lock:1 lock:2 (normal object)
 45 //  JavaThread*:54 epoch:2 unused_gap:1   age:4    biased_lock:1 lock:2 (biased object)
 46 //
 47 //  - hash contains the identity hash value: largest value is
 48 //    31 bits, see os::random().  Also, 64-bit vm's require
 49 //    a hash value no bigger than 32 bits because they will not
 50 //    properly generate a mask larger than that: see library_call.cpp
 51 //
 52 //  - the biased lock pattern is used to bias a lock toward a given
 53 //    thread. When this pattern is set in the low three bits, the lock
 54 //    is either biased toward a given thread or "anonymously" biased,
 55 //    indicating that it is possible for it to be biased. When the
 56 //    lock is biased toward a given thread, locking and unlocking can
 57 //    be performed by that thread without using atomic operations.
 58 //    When a lock's bias is revoked, it reverts back to the normal
 59 //    locking scheme described below.
 60 //
 61 //    Note that we are overloading the meaning of the "unlocked" state
 62 //    of the header. Because we steal a bit from the age we can
 63 //    guarantee that the bias pattern will never be seen for a truly
 64 //    unlocked object.
 65 //
 66 //    Note also that the biased state contains the age bits normally
 67 //    contained in the object header. Large increases in scavenge
 68 //    times were seen when these bits were absent and an arbitrary age
 69 //    assigned to all biased objects, because they tended to consume a
 70 //    significant fraction of the eden semispaces and were not
 71 //    promoted promptly, causing an increase in the amount of copying
 72 //    performed. The runtime system aligns all JavaThread* pointers to
 73 //    a very large value (currently 128 bytes (32bVM) or 256 bytes (64bVM))
 74 //    to make room for the age bits & the epoch bits (used in support of
 75 //    biased locking).
 76 //
 77 //    [JavaThread* | epoch | age | 1 | 01]       lock is biased toward given thread
 78 //    [0           | epoch | age | 1 | 01]       lock is anonymously biased
 79 //
 80 //  - the two lock bits are used to describe three states: locked/unlocked and monitor.
 81 //
 82 //    [ptr             | 00]  locked             ptr points to real header on stack
 83 //    [header      | 0 | 01]  unlocked           regular object header
 84 //    [ptr             | 10]  monitor            inflated lock (header is wapped out)
 85 //    [ptr             | 11]  marked             used to mark an object
 86 //    [0 ............ 0| 00]  inflating          inflation in progress
 87 //
 88 //    We assume that stack/thread pointers have the lowest two bits cleared.
 89 //
 90 //  - INFLATING() is a distinguished markword value of all zeros that is
 91 //    used when inflating an existing stack-lock into an ObjectMonitor.
 92 //    See below for is_being_inflated() and INFLATING().
 93 
 94 class BasicLock;
 95 class ObjectMonitor;
 96 class JavaThread;
 97 class Klass;
 98 class outputStream;
 99 
100 class markWord {
101  private:
102   uintptr_t _value;
103 
104  public:
105   explicit markWord(uintptr_t value) : _value(value) {}
106 
107   markWord() { /* uninitialized */}
108 
109   // It is critical for performance that this class be trivially
110   // destructable, copyable, and assignable.
111 
112   static markWord from_pointer(void* ptr) {
113     return markWord((uintptr_t)ptr);
114   }
115   void* to_pointer() const {
116     return (void*)_value;
117   }
118 
119   bool operator==(const markWord& other) const {
120     return _value == other._value;
121   }
122   bool operator!=(const markWord& other) const {
123     return !operator==(other);
124   }
125 
126   // Conversion
127   uintptr_t value() const { return _value; }
128 
129   // Constants
130   static const int age_bits                       = 4;
131   static const int lock_bits                      = 2;
132   static const int biased_lock_bits               = 1;
133   static const int self_forwarded_bits            = 1;
134   static const int max_hash_bits                  = BitsPerWord - age_bits - lock_bits - self_forwarded_bits;
135   static const int hash_bits                      = max_hash_bits > 25 ? 25 : max_hash_bits;
136 #ifdef _LP64
137   static const int klass_bits                     = 32;
138 #endif
139   static const int epoch_bits                     = 2;
140 
141   // The biased locking code currently requires that the age bits be
142   // contiguous to the lock bits.
143   static const int lock_shift                     = 0;
144   static const int biased_lock_shift              = lock_bits;
145   static const int self_forwarded_shift           = lock_shift + lock_bits;
146   static const int age_shift                      = self_forwarded_shift + self_forwarded_bits;
147   static const int hash_shift                     = age_shift + age_bits;
148 #ifdef _LP64
149   static const int klass_shift                    = hash_shift + hash_bits;
150 #endif
151   static const int epoch_shift                    = hash_shift;
152 
153   static const uintptr_t lock_mask                = right_n_bits(lock_bits);
154   static const uintptr_t lock_mask_in_place       = lock_mask << lock_shift;
155   static const uintptr_t biased_lock_mask         = right_n_bits(lock_bits + biased_lock_bits);
156   static const uintptr_t biased_lock_mask_in_place= biased_lock_mask << lock_shift;
157   static const uintptr_t biased_lock_bit_in_place = 1 << biased_lock_shift;
158   static const uintptr_t self_forwarded_mask      = right_n_bits(self_forwarded_bits);
159   static const uintptr_t self_forwarded_mask_in_place = self_forwarded_mask << self_forwarded_shift;
160   static const uintptr_t age_mask                 = right_n_bits(age_bits);
161   static const uintptr_t age_mask_in_place        = age_mask << age_shift;
162   static const uintptr_t epoch_mask               = right_n_bits(epoch_bits);
163   static const uintptr_t epoch_mask_in_place      = epoch_mask << epoch_shift;
164 
165   static const uintptr_t hash_mask                = right_n_bits(hash_bits);
166   static const uintptr_t hash_mask_in_place       = hash_mask << hash_shift;
167 
168 #ifdef _LP64
169   static const uintptr_t klass_mask               = right_n_bits(klass_bits);
170   static const uintptr_t klass_mask_in_place      = klass_mask << klass_shift;
171 #endif
172 
173   // Alignment of JavaThread pointers encoded in object header required by biased locking
174   static const size_t biased_lock_alignment       = 2 << (epoch_shift + epoch_bits);
175 
176   static const uintptr_t locked_value             = 0;
177   static const uintptr_t unlocked_value           = 1;
178   static const uintptr_t monitor_value            = 2;
179   static const uintptr_t marked_value             = 3;
180   static const uintptr_t biased_lock_pattern      = 5;
181 
182   static const uintptr_t no_hash                  = 0 ;  // no hash value assigned
183   static const uintptr_t no_hash_in_place         = (address_word)no_hash << hash_shift;
184   static const uintptr_t no_lock_in_place         = unlocked_value;
185 
186   static const uint max_age                       = age_mask;
187 
188   static const int max_bias_epoch                 = epoch_mask;
189 
190   // Creates a markWord with all bits set to zero.
191   static markWord zero() { return markWord(uintptr_t(0)); }
192 
193   // Biased Locking accessors.
194   // These must be checked by all code which calls into the
195   // ObjectSynchronizer and other code. The biasing is not understood
196   // by the lower-level CAS-based locking code, although the runtime
197   // fixes up biased locks to be compatible with it when a bias is
198   // revoked.
199   bool has_bias_pattern() const {
200     return (mask_bits(value(), biased_lock_mask_in_place) == biased_lock_pattern);
201   }
202   JavaThread* biased_locker() const {
203     assert(has_bias_pattern(), "should not call this otherwise");
204     return (JavaThread*) mask_bits(value(), ~(biased_lock_mask_in_place | age_mask_in_place | epoch_mask_in_place));
205   }
206   // Indicates that the mark has the bias bit set but that it has not
207   // yet been biased toward a particular thread
208   bool is_biased_anonymously() const {
209     return (has_bias_pattern() && (biased_locker() == NULL));
210   }
211   // Indicates epoch in which this bias was acquired. If the epoch
212   // changes due to too many bias revocations occurring, the biases
213   // from the previous epochs are all considered invalid.
214   int bias_epoch() const {
215     assert(has_bias_pattern(), "should not call this otherwise");
216     return (mask_bits(value(), epoch_mask_in_place) >> epoch_shift);
217   }
218   markWord set_bias_epoch(int epoch) {
219     assert(has_bias_pattern(), "should not call this otherwise");
220     assert((epoch & (~epoch_mask)) == 0, "epoch overflow");
221     return markWord(mask_bits(value(), ~epoch_mask_in_place) | (epoch << epoch_shift));
222   }
223   markWord incr_bias_epoch() {
224     return set_bias_epoch((1 + bias_epoch()) & epoch_mask);
225   }
226   // Prototype mark for initialization
227   static markWord biased_locking_prototype() {
228     return markWord( biased_lock_pattern );
229   }
230 
231   // lock accessors (note that these assume lock_shift == 0)
232   bool is_locked()   const {
233     return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
234   }
235   bool is_unlocked() const {
236     return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value);
237   }
238   bool is_marked()   const {
239     return (mask_bits(value(), lock_mask_in_place) == marked_value);
240   }
241   bool is_neutral()  const { return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); }
242 
243   // Special temporary state of the markWord while being inflated.
244   // Code that looks at mark outside a lock need to take this into account.
245   bool is_being_inflated() const { return (value() == 0); }
246 
247   // Distinguished markword value - used when inflating over
248   // an existing stack-lock.  0 indicates the markword is "BUSY".
249   // Lockword mutators that use a LD...CAS idiom should always
250   // check for and avoid overwriting a 0 value installed by some
251   // other thread.  (They should spin or block instead.  The 0 value
252   // is transient and *should* be short-lived).
253   static markWord INFLATING() { return zero(); }    // inflate-in-progress
254 
255   // Should this header be preserved during GC?
256   inline bool must_be_preserved(const oopDesc* obj) const;
257 
258   // Should this header (including its age bits) be preserved in the
259   // case of a promotion failure during scavenge?
260   // Note that we special case this situation. We want to avoid
261   // calling BiasedLocking::preserve_marks()/restore_marks() (which
262   // decrease the number of mark words that need to be preserved
263   // during GC) during each scavenge. During scavenges in which there
264   // is no promotion failure, we actually don't need to call the above
265   // routines at all, since we don't mutate and re-initialize the
266   // marks of promoted objects using init_mark(). However, during
267   // scavenges which result in promotion failure, we do re-initialize
268   // the mark words of objects, meaning that we should have called
269   // these mark word preservation routines. Currently there's no good
270   // place in which to call them in any of the scavengers (although
271   // guarded by appropriate locks we could make one), but the
272   // observation is that promotion failures are quite rare and
273   // reducing the number of mark words preserved during them isn't a
274   // high priority.
275   inline bool must_be_preserved_for_promotion_failure(const oopDesc* obj) const;
276 
277   // WARNING: The following routines are used EXCLUSIVELY by
278   // synchronization functions. They are not really gc safe.
279   // They must get updated if markWord layout get changed.
280   markWord set_unlocked() const {
281     return markWord(value() | unlocked_value);
282   }
283   bool has_locker() const {
284     return !UseFastLocking && ((value() & lock_mask_in_place) == locked_value);
285   }
286   BasicLock* locker() const {
287     assert(has_locker(), "check");
288     return (BasicLock*) value();
289   }
290 
291   bool is_fast_locked() const {
292     return UseFastLocking && ((value() & lock_mask_in_place) == locked_value);
293   }
294   markWord set_fast_locked() const {
295     return markWord(value() & ~lock_mask_in_place);
296   }
297 
298   bool has_monitor() const {
299     return ((value() & monitor_value) != 0);
300   }
301   ObjectMonitor* monitor() const {
302     assert(has_monitor(), "check");
303     // Use xor instead of &~ to provide one extra tag-bit check.
304     return (ObjectMonitor*) (value() ^ monitor_value);
305   }
306   bool has_displaced_mark_helper() const {
307     intptr_t lockbits = value() & lock_mask_in_place;
308     return UseFastLocking ? lockbits == monitor_value   // monitor?
309                           : (lockbits & unlocked_value) == 0; // monitor | stack-locked?
310   }
311   markWord displaced_mark_helper() const;
312   void set_displaced_mark_helper(markWord m) const;
313   markWord copy_set_hash(intptr_t hash) const {
314     uintptr_t tmp = value() & (~hash_mask_in_place);
315     tmp |= ((hash & hash_mask) << hash_shift);
316     return markWord(tmp);
317   }
318   // it is only used to be stored into BasicLock as the
319   // indicator that the lock is using heavyweight monitor
320   static markWord unused_mark() {
321     return markWord(marked_value);
322   }
323   // the following two functions create the markWord to be
324   // stored into object header, it encodes monitor info
325   static markWord encode(BasicLock* lock) {
326     return from_pointer(lock);
327   }
328   static markWord encode(ObjectMonitor* monitor) {
329     uintptr_t tmp = (uintptr_t) monitor;
330     return markWord(tmp | monitor_value);
331   }
332   static markWord encode(JavaThread* thread, uint age, int bias_epoch) {
333     uintptr_t tmp = (uintptr_t) thread;
334     assert(UseBiasedLocking && ((tmp & (epoch_mask_in_place | age_mask_in_place | biased_lock_mask_in_place)) == 0), "misaligned JavaThread pointer");
335     assert(age <= max_age, "age too large");
336     assert(bias_epoch <= max_bias_epoch, "bias epoch too large");
337     return markWord(tmp | (bias_epoch << epoch_shift) | (age << age_shift) | biased_lock_pattern);
338   }
339 
340   // used to encode pointers during GC
341   markWord clear_lock_bits() { return markWord(value() & ~lock_mask_in_place); }
342 
343   // age operations
344   markWord set_marked()   { return markWord((value() & ~lock_mask_in_place) | marked_value); }
345   markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
346 
347   uint     age()           const { return mask_bits(value() >> age_shift, age_mask); }
348   markWord set_age(uint v) const {
349     assert((v & ~age_mask) == 0, "shouldn't overflow age field");
350     return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
351   }
352   markWord incr_age()      const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
353 
354   // hash operations
355   intptr_t hash() const {
356     return mask_bits(value() >> hash_shift, hash_mask);
357   }
358 
359   bool has_no_hash() const {
360     return hash() == no_hash;
361   }
362 
363 #ifdef _LP64
364   inline Klass* klass() const;
365   inline Klass* klass_or_null() const;
366   inline Klass* safe_klass() const;
367   inline markWord set_klass(const Klass* klass) const;
368   inline narrowKlass narrow_klass() const;
369   inline markWord set_narrow_klass(const narrowKlass klass) const;
370 #endif
371 
372   // Prototype mark for initialization
373   static markWord prototype() {
374     return markWord( no_hash_in_place | no_lock_in_place );
375   }
376 
377   // Helper function for restoration of unmarked mark oops during GC
378   static inline markWord prototype_for_klass(const Klass* klass);
379 
380   // Debugging
381   void print_on(outputStream* st, bool print_monitor_info = true) const;
382 
383   // Prepare address of oop for placement into mark
384   inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
385 
386   // Recover address of oop from encoded form used in mark
387   inline void* decode_pointer() { if (UseBiasedLocking && has_bias_pattern()) return NULL; return (void*)clear_lock_bits().value(); }
388 
389   inline bool self_forwarded() const {
390     return mask_bits(value(), self_forwarded_mask_in_place) != 0;
391   }
392 
393   inline markWord set_self_forwarded() const {
394     return markWord(value() | self_forwarded_mask_in_place | marked_value);
395   }
396 };
397 
398 // Support atomic operations.
399 template<>
400 struct PrimitiveConversions::Translate<markWord> : public TrueType {
401   typedef markWord Value;
402   typedef uintptr_t Decayed;
403 
404   static Decayed decay(const Value& x) { return x.value(); }
405   static Value recover(Decayed x) { return Value(x); }
406 };
407 
408 #endif // SHARE_OOPS_MARKWORD_HPP