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
32 // The markWord describes the header of an object.
33 //
34 // Bit-format of an object header (most significant first, big endian layout below):
35 //
36 // 32 bits:
37 // --------
38 // hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object)
39 // JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object)
40 // size:32 ------------------------------------------>| (CMS free block)
41 // PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)
42 //
43 // 64 bits:
44 // --------
45 // unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object)
46 // JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)
47 // PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)
48 // size:64 ----------------------------------------------------->| (CMS free block)
49 //
50 // unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object)
51 // JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object)
52 // narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)
53 // unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)
54 //
55 // - hash contains the identity hash value: largest value is
56 // 31 bits, see os::random(). Also, 64-bit vm's require
57 // a hash value no bigger than 32 bits because they will not
58 // properly generate a mask larger than that: see library_call.cpp
59 // and c1_CodePatterns_sparc.cpp.
60 //
61 // - the biased lock pattern is used to bias a lock toward a given
62 // thread. When this pattern is set in the low three bits, the lock
63 // is either biased toward a given thread or "anonymously" biased,
64 // indicating that it is possible for it to be biased. When the
65 // lock is biased toward a given thread, locking and unlocking can
66 // be performed by that thread without using atomic operations.
67 // When a lock's bias is revoked, it reverts back to the normal
68 // locking scheme described below.
69 //
70 // Note that we are overloading the meaning of the "unlocked" state
71 // of the header. Because we steal a bit from the age we can
72 // guarantee that the bias pattern will never be seen for a truly
73 // unlocked object.
74 //
75 // Note also that the biased state contains the age bits normally
76 // contained in the object header. Large increases in scavenge
77 // times were seen when these bits were absent and an arbitrary age
78 // assigned to all biased objects, because they tended to consume a
79 // significant fraction of the eden semispaces and were not
80 // promoted promptly, causing an increase in the amount of copying
81 // performed. The runtime system aligns all JavaThread* pointers to
82 // a very large value (currently 128 bytes (32bVM) or 256 bytes (64bVM))
83 // to make room for the age bits & the epoch bits (used in support of
84 // biased locking), and for the CMS "freeness" bit in the 64bVM (+COOPs).
85 //
86 // [JavaThread* | epoch | age | 1 | 01] lock is biased toward given thread
87 // [0 | epoch | age | 1 | 01] lock is anonymously biased
88 //
89 // - the two lock bits are used to describe three states: locked/unlocked and monitor.
90 //
91 // [ptr | 00] locked ptr points to real header on stack
92 // [header | 0 | 01] unlocked regular object header
93 // [ptr | 10] monitor inflated lock (header is wapped out)
94 // [ptr | 11] marked used by markSweep to mark an object
95 // not valid at any other time
96 //
97 // We assume that stack/thread pointers have the lowest two bits cleared.
98
99 class BasicLock;
100 class ObjectMonitor;
101 class JavaThread;
102
103 class markWord {
104 private:
105 uintptr_t _value;
106
107 public:
108 explicit markWord(uintptr_t value) : _value(value) {}
109
110 markWord() { /* uninitialized */}
111
112 // It is critical for performance that this class be trivially
113 // destructable, copyable, and assignable.
114
115 static markWord from_pointer(void* ptr) {
116 return markWord((uintptr_t)ptr);
117 }
118 void* to_pointer() const {
119 return (void*)_value;
120 }
121
122 bool operator==(const markWord& other) const {
123 return _value == other._value;
124 }
125 bool operator!=(const markWord& other) const {
126 return !operator==(other);
127 }
128
129 // Conversion
130 uintptr_t value() const { return _value; }
131
132 // Constants
133 enum { age_bits = 4,
134 lock_bits = 2,
135 biased_lock_bits = 1,
136 max_hash_bits = BitsPerWord - age_bits - lock_bits - biased_lock_bits,
137 hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits,
138 cms_bits = LP64_ONLY(1) NOT_LP64(0),
139 epoch_bits = 2
140 };
141
142 // The biased locking code currently requires that the age bits be
143 // contiguous to the lock bits.
144 enum { lock_shift = 0,
145 biased_lock_shift = lock_bits,
146 age_shift = lock_bits + biased_lock_bits,
147 cms_shift = age_shift + age_bits,
148 hash_shift = cms_shift + cms_bits,
149 epoch_shift = hash_shift
150 };
151
152 enum { lock_mask = right_n_bits(lock_bits),
153 lock_mask_in_place = lock_mask << lock_shift,
154 biased_lock_mask = right_n_bits(lock_bits + biased_lock_bits),
155 biased_lock_mask_in_place= biased_lock_mask << lock_shift,
156 biased_lock_bit_in_place = 1 << biased_lock_shift,
157 age_mask = right_n_bits(age_bits),
158 age_mask_in_place = age_mask << age_shift,
159 epoch_mask = right_n_bits(epoch_bits),
160 epoch_mask_in_place = epoch_mask << epoch_shift,
161 cms_mask = right_n_bits(cms_bits),
162 cms_mask_in_place = cms_mask << cms_shift
163 };
164
165 const static uintptr_t hash_mask = right_n_bits(hash_bits);
166 const static uintptr_t hash_mask_in_place = hash_mask << hash_shift;
167
168 // Alignment of JavaThread pointers encoded in object header required by biased locking
169 enum { biased_lock_alignment = 2 << (epoch_shift + epoch_bits)
170 };
171
172 enum { locked_value = 0,
173 unlocked_value = 1,
174 monitor_value = 2,
175 marked_value = 3,
176 biased_lock_pattern = 5
177 };
178
179 enum { no_hash = 0 }; // no hash value assigned
180
181 enum { no_hash_in_place = (address_word)no_hash << hash_shift,
182 no_lock_in_place = unlocked_value
183 };
184
185 enum { max_age = age_mask };
186
187 enum { max_bias_epoch = epoch_mask };
188
189 // Creates a markWord with all bits set to zero.
190 static markWord zero() { return markWord(uintptr_t(0)); }
191
192 // Biased Locking accessors.
193 // These must be checked by all code which calls into the
194 // ObjectSynchronizer and other code. The biasing is not understood
195 // by the lower-level CAS-based locking code, although the runtime
196 // fixes up biased locks to be compatible with it when a bias is
197 // revoked.
198 bool has_bias_pattern() const {
199 return (mask_bits(value(), biased_lock_mask_in_place) == biased_lock_pattern);
200 }
201 JavaThread* biased_locker() const {
202 assert(has_bias_pattern(), "should not call this otherwise");
203 return (JavaThread*) ((intptr_t) (mask_bits(value(), ~(biased_lock_mask_in_place | age_mask_in_place | epoch_mask_in_place))));
204 }
205 // Indicates that the mark has the bias bit set but that it has not
206 // yet been biased toward a particular thread
207 bool is_biased_anonymously() const {
208 return (has_bias_pattern() && (biased_locker() == NULL));
209 }
210 // Indicates epoch in which this bias was acquired. If the epoch
211 // changes due to too many bias revocations occurring, the biases
212 // from the previous epochs are all considered invalid.
213 int bias_epoch() const {
214 assert(has_bias_pattern(), "should not call this otherwise");
215 return (mask_bits(value(), epoch_mask_in_place) >> epoch_shift);
216 }
217 markWord set_bias_epoch(int epoch) {
218 assert(has_bias_pattern(), "should not call this otherwise");
219 assert((epoch & (~epoch_mask)) == 0, "epoch overflow");
220 return markWord(mask_bits(value(), ~epoch_mask_in_place) | (epoch << epoch_shift));
221 }
222 markWord incr_bias_epoch() {
223 return set_bias_epoch((1 + bias_epoch()) & epoch_mask);
224 }
225 // Prototype mark for initialization
226 static markWord biased_locking_prototype() {
227 return markWord( biased_lock_pattern );
228 }
229
230 // lock accessors (note that these assume lock_shift == 0)
231 bool is_locked() const {
232 return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
233 }
234 bool is_unlocked() const {
235 return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value);
236 }
237 bool is_marked() const {
238 return (mask_bits(value(), lock_mask_in_place) == marked_value);
239 }
240 bool is_neutral() const { return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); }
241
242 // Special temporary state of the markWord while being inflated.
243 // Code that looks at mark outside a lock need to take this into account.
244 bool is_being_inflated() const { return (value() == 0); }
245
246 // Distinguished markword value - used when inflating over
247 // an existing stacklock. 0 indicates the markword is "BUSY".
248 // Lockword mutators that use a LD...CAS idiom should always
249 // check for and avoid overwriting a 0 value installed by some
250 // other thread. (They should spin or block instead. The 0 value
251 // is transient and *should* be short-lived).
252 static markWord INFLATING() { return zero(); } // inflate-in-progress
253
254 // Should this header be preserved during GC?
255 template <typename KlassProxy>
256 inline bool must_be_preserved(KlassProxy klass) 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 template <typename KlassProxy>
276 inline bool must_be_preserved_for_promotion_failure(KlassProxy klass) const;
277
278 // Should this header be preserved during a scavenge where CMS is
279 // the old generation?
280 // (This is basically the same body as must_be_preserved_for_promotion_failure(),
281 // but takes the Klass* as argument instead)
282 inline bool must_be_preserved_for_cms_scavenge(Klass* klass_of_obj_containing_mark) const;
283
284 // WARNING: The following routines are used EXCLUSIVELY by
285 // synchronization functions. They are not really gc safe.
286 // They must get updated if markWord layout get changed.
287 markWord set_unlocked() const {
288 return markWord(value() | unlocked_value);
289 }
290 bool has_locker() const {
291 return ((value() & lock_mask_in_place) == locked_value);
292 }
293 BasicLock* locker() const {
294 assert(has_locker(), "check");
295 return (BasicLock*) value();
296 }
297 bool has_monitor() const {
298 return ((value() & monitor_value) != 0);
299 }
300 ObjectMonitor* monitor() const {
301 assert(has_monitor(), "check");
302 // Use xor instead of &~ to provide one extra tag-bit check.
303 return (ObjectMonitor*) (value() ^ monitor_value);
304 }
305 bool has_displaced_mark_helper() const {
306 return ((value() & unlocked_value) == 0);
307 }
308 markWord displaced_mark_helper() const {
309 assert(has_displaced_mark_helper(), "check");
310 intptr_t ptr = (value() & ~monitor_value);
311 return *(markWord*)ptr;
312 }
313 void set_displaced_mark_helper(markWord m) const {
314 assert(has_displaced_mark_helper(), "check");
315 intptr_t ptr = (value() & ~monitor_value);
316 ((markWord*)ptr)->_value = m._value;
317 }
318 markWord copy_set_hash(intptr_t hash) const {
319 intptr_t tmp = value() & (~hash_mask_in_place);
320 tmp |= ((hash & hash_mask) << hash_shift);
321 return markWord(tmp);
322 }
323 // it is only used to be stored into BasicLock as the
324 // indicator that the lock is using heavyweight monitor
325 static markWord unused_mark() {
326 return markWord(marked_value);
327 }
328 // the following two functions create the markWord to be
329 // stored into object header, it encodes monitor info
330 static markWord encode(BasicLock* lock) {
331 return from_pointer(lock);
332 }
333 static markWord encode(ObjectMonitor* monitor) {
334 intptr_t tmp = (intptr_t) monitor;
335 return markWord(tmp | monitor_value);
336 }
337 static markWord encode(JavaThread* thread, uint age, int bias_epoch) {
338 intptr_t tmp = (intptr_t) thread;
339 assert(UseBiasedLocking && ((tmp & (epoch_mask_in_place | age_mask_in_place | biased_lock_mask_in_place)) == 0), "misaligned JavaThread pointer");
340 assert(age <= max_age, "age too large");
341 assert(bias_epoch <= max_bias_epoch, "bias epoch too large");
342 return markWord(tmp | (bias_epoch << epoch_shift) | (age << age_shift) | biased_lock_pattern);
343 }
344
345 // used to encode pointers during GC
346 markWord clear_lock_bits() { return markWord(value() & ~lock_mask_in_place); }
347
348 // age operations
349 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
350 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
351
352 uint age() const { return mask_bits(value() >> age_shift, age_mask); }
353 markWord set_age(uint v) const {
354 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
355 return markWord((value() & ~age_mask_in_place) | (((uintptr_t)v & age_mask) << age_shift));
356 }
357 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
358
359 // hash operations
360 intptr_t hash() const {
361 return mask_bits(value() >> hash_shift, hash_mask);
362 }
363
364 bool has_no_hash() const {
365 return hash() == no_hash;
366 }
367
368 // Prototype mark for initialization
369 static markWord prototype() {
370 return markWord( no_hash_in_place | no_lock_in_place );
371 }
372
373 // Helper function for restoration of unmarked mark oops during GC
374 static inline markWord prototype_for_klass(const Klass* klass);
375
376 // Debugging
377 void print_on(outputStream* st) const;
378
379 // Prepare address of oop for placement into mark
380 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
381
382 // Recover address of oop from encoded form used in mark
383 inline void* decode_pointer() { if (UseBiasedLocking && has_bias_pattern()) return NULL; return (void*)clear_lock_bits().value(); }
384
385 // These markWords indicate cms free chunk blocks and not objects.
386 // In 64 bit, the markWord is set to distinguish them from oops.
387 // These are defined in 32 bit mode for vmStructs.
388 const static uintptr_t cms_free_chunk_pattern = 0x1;
389
390 // Constants for the size field.
391 enum { size_shift = cms_shift + cms_bits,
392 size_bits = 35 // need for compressed oops 32G
393 };
394 // These values are too big for Win64
395 const static uintptr_t size_mask = LP64_ONLY(right_n_bits(size_bits))
396 NOT_LP64(0);
397 const static uintptr_t size_mask_in_place =
398 (address_word)size_mask << size_shift;
399
400 #ifdef _LP64
401 static markWord cms_free_prototype() {
402 return markWord(((intptr_t)prototype().value() & ~cms_mask_in_place) |
403 ((cms_free_chunk_pattern & cms_mask) << cms_shift));
404 }
405 uintptr_t cms_encoding() const {
406 return mask_bits(value() >> cms_shift, cms_mask);
407 }
408 bool is_cms_free_chunk() const {
409 return is_neutral() &&
410 (cms_encoding() & cms_free_chunk_pattern) == cms_free_chunk_pattern;
411 }
412
413 size_t get_size() const { return (size_t)(value() >> size_shift); }
414 static markWord set_size_and_free(size_t size) {
415 assert((size & ~size_mask) == 0, "shouldn't overflow size field");
416 return markWord(((intptr_t)cms_free_prototype().value() & ~size_mask_in_place) |
417 (((intptr_t)size & size_mask) << size_shift));
418 }
419 #endif // _LP64
420 };
421
422 // Support atomic operations.
423 template<>
424 struct PrimitiveConversions::Translate<markWord> : public TrueType {
425 typedef markWord Value;
426 typedef uintptr_t Decayed;
427
428 static Decayed decay(const Value& x) { return x.value(); }
429 static Value recover(Decayed x) { return Value(x); }
430 };
431
432 #endif // SHARE_OOPS_MARKWORD_HPP