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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