1 /*
2 * Copyright (c) 1997, 2019, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
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 // unused:25 hash:31 -->| 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 // and c1_CodePatterns_sparc.cpp.
52 //
53 // - the biased lock pattern is used to bias a lock toward a given
54 // thread. When this pattern is set in the low three bits, the lock
55 // is either biased toward a given thread or "anonymously" biased,
56 // indicating that it is possible for it to be biased. When the
57 // lock is biased toward a given thread, locking and unlocking can
58 // be performed by that thread without using atomic operations.
59 // When a lock's bias is revoked, it reverts back to the normal
60 // locking scheme described below.
61 //
62 // Note that we are overloading the meaning of the "unlocked" state
63 // of the header. Because we steal a bit from the age we can
64 // guarantee that the bias pattern will never be seen for a truly
65 // unlocked object.
66 //
67 // Note also that the biased state contains the age bits normally
68 // contained in the object header. Large increases in scavenge
69 // times were seen when these bits were absent and an arbitrary age
70 // assigned to all biased objects, because they tended to consume a
71 // significant fraction of the eden semispaces and were not
72 // promoted promptly, causing an increase in the amount of copying
73 // performed. The runtime system aligns all JavaThread* pointers to
74 // a very large value (currently 128 bytes (32bVM) or 256 bytes (64bVM))
75 // to make room for the age bits & the epoch bits (used in support of
76 // biased locking).
77 //
78 // [JavaThread* | epoch | age | 1 | 01] lock is biased toward given thread
79 // [0 | epoch | age | 1 | 01] lock is anonymously biased
80 //
81 // - the two lock bits are used to describe three states: locked/unlocked and monitor.
82 //
83 // [ptr | 00] locked ptr points to real header on stack
84 // [header | 0 | 01] unlocked regular object header
85 // [ptr | 10] monitor inflated lock (header is wapped out)
86 // [ptr | 11] marked used by markSweep to mark an object
87 // not valid at any other time
88 //
89 // We assume that stack/thread pointers have the lowest two bits cleared.
90
91 class BasicLock;
92 class ObjectMonitor;
93 class JavaThread;
94
95 class markWord {
96 private:
97 uintptr_t _value;
98
99 public:
100 explicit markWord(uintptr_t value) : _value(value) {}
101
102 markWord() { /* uninitialized */}
103
104 // It is critical for performance that this class be trivially
105 // destructable, copyable, and assignable.
106
107 static markWord from_pointer(void* ptr) {
108 return markWord((uintptr_t)ptr);
109 }
110 void* to_pointer() const {
111 return (void*)_value;
112 }
113
114 bool operator==(const markWord& other) const {
115 return _value == other._value;
116 }
117 bool operator!=(const markWord& other) const {
118 return !operator==(other);
119 }
120
121 // Conversion
122 uintptr_t value() const { return _value; }
123
124 // Constants
125 static const int age_bits = 4;
126 static const int lock_bits = 2;
127 static const int biased_lock_bits = 1;
128 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - biased_lock_bits;
129 static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
130 static const int unused_gap_bits = LP64_ONLY(1) NOT_LP64(0);
131 static const int epoch_bits = 2;
132
133 // The biased locking code currently requires that the age bits be
134 // contiguous to the lock bits.
135 static const int lock_shift = 0;
136 static const int biased_lock_shift = lock_bits;
137 static const int age_shift = lock_bits + biased_lock_bits;
138 static const int unused_gap_shift = age_shift + age_bits;
139 static const int hash_shift = unused_gap_shift + unused_gap_bits;
140 static const int epoch_shift = hash_shift;
141
142 static const uintptr_t lock_mask = right_n_bits(lock_bits);
143 static const uintptr_t lock_mask_in_place = lock_mask << lock_shift;
144 static const uintptr_t biased_lock_mask = right_n_bits(lock_bits + biased_lock_bits);
145 static const uintptr_t biased_lock_mask_in_place= biased_lock_mask << lock_shift;
146 static const uintptr_t biased_lock_bit_in_place = 1 << biased_lock_shift;
147 static const uintptr_t age_mask = right_n_bits(age_bits);
148 static const uintptr_t age_mask_in_place = age_mask << age_shift;
149 static const uintptr_t epoch_mask = right_n_bits(epoch_bits);
150 static const uintptr_t epoch_mask_in_place = epoch_mask << epoch_shift;
151
152 static const uintptr_t hash_mask = right_n_bits(hash_bits);
153 static const uintptr_t hash_mask_in_place = hash_mask << hash_shift;
154
155 // Alignment of JavaThread pointers encoded in object header required by biased locking
156 static const size_t biased_lock_alignment = 2 << (epoch_shift + epoch_bits);
157
158 static const uintptr_t locked_value = 0;
159 static const uintptr_t unlocked_value = 1;
160 static const uintptr_t monitor_value = 2;
161 static const uintptr_t marked_value = 3;
162 static const uintptr_t biased_lock_pattern = 5;
163
164 static const uintptr_t no_hash = 0 ; // no hash value assigned
165 static const uintptr_t no_hash_in_place = (address_word)no_hash << hash_shift;
166 static const uintptr_t no_lock_in_place = unlocked_value;
167
168 static const uint max_age = age_mask;
169
170 static const int max_bias_epoch = epoch_mask;
171
172 // Creates a markWord with all bits set to zero.
173 static markWord zero() { return markWord(uintptr_t(0)); }
174
175 // Biased Locking accessors.
176 // These must be checked by all code which calls into the
177 // ObjectSynchronizer and other code. The biasing is not understood
178 // by the lower-level CAS-based locking code, although the runtime
179 // fixes up biased locks to be compatible with it when a bias is
180 // revoked.
181 bool has_bias_pattern() const {
182 return (mask_bits(value(), biased_lock_mask_in_place) == biased_lock_pattern);
183 }
184 JavaThread* biased_locker() const {
185 assert(has_bias_pattern(), "should not call this otherwise");
186 return (JavaThread*) mask_bits(value(), ~(biased_lock_mask_in_place | age_mask_in_place | epoch_mask_in_place));
187 }
188 // Indicates that the mark has the bias bit set but that it has not
189 // yet been biased toward a particular thread
190 bool is_biased_anonymously() const {
191 return (has_bias_pattern() && (biased_locker() == NULL));
192 }
193 // Indicates epoch in which this bias was acquired. If the epoch
194 // changes due to too many bias revocations occurring, the biases
195 // from the previous epochs are all considered invalid.
196 int bias_epoch() const {
197 assert(has_bias_pattern(), "should not call this otherwise");
198 return (mask_bits(value(), epoch_mask_in_place) >> epoch_shift);
199 }
200 markWord set_bias_epoch(int epoch) {
201 assert(has_bias_pattern(), "should not call this otherwise");
202 assert((epoch & (~epoch_mask)) == 0, "epoch overflow");
203 return markWord(mask_bits(value(), ~epoch_mask_in_place) | (epoch << epoch_shift));
204 }
205 markWord incr_bias_epoch() {
206 return set_bias_epoch((1 + bias_epoch()) & epoch_mask);
207 }
208 // Prototype mark for initialization
209 static markWord biased_locking_prototype() {
210 return markWord( biased_lock_pattern );
211 }
212
213 // lock accessors (note that these assume lock_shift == 0)
214 bool is_locked() const {
215 return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
216 }
217 bool is_unlocked() const {
218 return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value);
219 }
220 bool is_marked() const {
221 return (mask_bits(value(), lock_mask_in_place) == marked_value);
222 }
223 bool is_neutral() const { return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); }
224
225 // Special temporary state of the markWord while being inflated.
226 // Code that looks at mark outside a lock need to take this into account.
227 bool is_being_inflated() const { return (value() == 0); }
228
229 // Distinguished markword value - used when inflating over
230 // an existing stacklock. 0 indicates the markword is "BUSY".
231 // Lockword mutators that use a LD...CAS idiom should always
232 // check for and avoid overwriting a 0 value installed by some
233 // other thread. (They should spin or block instead. The 0 value
234 // is transient and *should* be short-lived).
235 static markWord INFLATING() { return zero(); } // inflate-in-progress
236
237 // Should this header be preserved during GC?
238 template <typename KlassProxy>
239 inline bool must_be_preserved(KlassProxy klass) const;
240
241 // Should this header (including its age bits) be preserved in the
242 // case of a promotion failure during scavenge?
243 // Note that we special case this situation. We want to avoid
244 // calling BiasedLocking::preserve_marks()/restore_marks() (which
245 // decrease the number of mark words that need to be preserved
246 // during GC) during each scavenge. During scavenges in which there
247 // is no promotion failure, we actually don't need to call the above
248 // routines at all, since we don't mutate and re-initialize the
249 // marks of promoted objects using init_mark(). However, during
250 // scavenges which result in promotion failure, we do re-initialize
251 // the mark words of objects, meaning that we should have called
252 // these mark word preservation routines. Currently there's no good
253 // place in which to call them in any of the scavengers (although
254 // guarded by appropriate locks we could make one), but the
255 // observation is that promotion failures are quite rare and
256 // reducing the number of mark words preserved during them isn't a
257 // high priority.
258 template <typename KlassProxy>
259 inline bool must_be_preserved_for_promotion_failure(KlassProxy klass) const;
260
261 // WARNING: The following routines are used EXCLUSIVELY by
262 // synchronization functions. They are not really gc safe.
263 // They must get updated if markWord layout get changed.
264 markWord set_unlocked() const {
265 return markWord(value() | unlocked_value);
266 }
267 bool has_locker() const {
268 return ((value() & lock_mask_in_place) == locked_value);
269 }
270 BasicLock* locker() const {
271 assert(has_locker(), "check");
272 return (BasicLock*) value();
273 }
274 bool has_monitor() const {
275 return ((value() & monitor_value) != 0);
276 }
277 ObjectMonitor* monitor() const {
278 assert(has_monitor(), "check");
279 // Use xor instead of &~ to provide one extra tag-bit check.
280 return (ObjectMonitor*) (value() ^ monitor_value);
281 }
282 bool has_displaced_mark_helper() const {
283 return ((value() & unlocked_value) == 0);
284 }
285 markWord displaced_mark_helper() const {
286 assert(has_displaced_mark_helper(), "check");
287 uintptr_t ptr = (value() & ~monitor_value);
288 return *(markWord*)ptr;
289 }
290 void set_displaced_mark_helper(markWord m) const {
291 assert(has_displaced_mark_helper(), "check");
292 uintptr_t ptr = (value() & ~monitor_value);
293 ((markWord*)ptr)->_value = m._value;
294 }
295 markWord copy_set_hash(intptr_t hash) const {
296 uintptr_t tmp = value() & (~hash_mask_in_place);
297 tmp |= ((hash & hash_mask) << hash_shift);
298 return markWord(tmp);
299 }
300 // it is only used to be stored into BasicLock as the
301 // indicator that the lock is using heavyweight monitor
302 static markWord unused_mark() {
303 return markWord(marked_value);
304 }
305 // the following two functions create the markWord to be
306 // stored into object header, it encodes monitor info
307 static markWord encode(BasicLock* lock) {
308 return from_pointer(lock);
309 }
310 static markWord encode(ObjectMonitor* monitor) {
311 uintptr_t tmp = (uintptr_t) monitor;
312 return markWord(tmp | monitor_value);
313 }
314 static markWord encode(JavaThread* thread, uint age, int bias_epoch) {
315 uintptr_t tmp = (uintptr_t) thread;
316 assert(UseBiasedLocking && ((tmp & (epoch_mask_in_place | age_mask_in_place | biased_lock_mask_in_place)) == 0), "misaligned JavaThread pointer");
317 assert(age <= max_age, "age too large");
318 assert(bias_epoch <= max_bias_epoch, "bias epoch too large");
319 return markWord(tmp | (bias_epoch << epoch_shift) | (age << age_shift) | biased_lock_pattern);
320 }
321
322 // used to encode pointers during GC
323 markWord clear_lock_bits() { return markWord(value() & ~lock_mask_in_place); }
324
325 // age operations
326 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
327 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
328
329 uint age() const { return mask_bits(value() >> age_shift, age_mask); }
330 markWord set_age(uint v) const {
331 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
332 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
333 }
334 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
335
336 // hash operations
337 intptr_t hash() const {
338 return mask_bits(value() >> hash_shift, hash_mask);
339 }
340
341 bool has_no_hash() const {
342 return hash() == no_hash;
343 }
344
345 // Prototype mark for initialization
346 static markWord prototype() {
347 return markWord( no_hash_in_place | no_lock_in_place );
348 }
349
350 // Helper function for restoration of unmarked mark oops during GC
351 static inline markWord prototype_for_klass(const Klass* klass);
352
353 // Debugging
354 void print_on(outputStream* st) const;
355
356 // Prepare address of oop for placement into mark
357 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
358
359 // Recover address of oop from encoded form used in mark
360 inline void* decode_pointer() { if (UseBiasedLocking && has_bias_pattern()) return NULL; return (void*)clear_lock_bits().value(); }
361 };
362
363 // Support atomic operations.
364 template<>
365 struct PrimitiveConversions::Translate<markWord> : public TrueType {
366 typedef markWord Value;
367 typedef uintptr_t Decayed;
368
369 static Decayed decay(const Value& x) { return x.value(); }
370 static Value recover(Decayed x) { return Value(x); }
371 };
372
373 #endif // SHARE_OOPS_MARKWORD_HPP