1 /* 2 * Copyright (c) 1997, 2020, 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 to mark an object 87 // 88 // We assume that stack/thread pointers have the lowest two bits cleared. 89 90 class BasicLock; 91 class ObjectMonitor; 92 class JavaThread; 93 94 class markWord { 95 private: 96 uintptr_t _value; 97 98 public: 99 explicit markWord(uintptr_t value) : _value(value) {} 100 101 markWord() { /* uninitialized */} 102 103 // It is critical for performance that this class be trivially 104 // destructable, copyable, and assignable. 105 106 static markWord from_pointer(void* ptr) { 107 return markWord((uintptr_t)ptr); 108 } 109 void* to_pointer() const { 110 return (void*)_value; 111 } 112 113 bool operator==(const markWord& other) const { 114 return _value == other._value; 115 } 116 bool operator!=(const markWord& other) const { 117 return !operator==(other); 118 } 119 120 // Conversion 121 uintptr_t value() const { return _value; } 122 123 // Constants 124 static const int age_bits = 4; 125 static const int lock_bits = 2; 126 static const int biased_lock_bits = 1; 127 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - biased_lock_bits; 128 static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits; 129 static const int unused_gap_bits = LP64_ONLY(1) NOT_LP64(0); 130 static const int epoch_bits = 2; 131 132 // The biased locking code currently requires that the age bits be 133 // contiguous to the lock bits. 134 static const int lock_shift = 0; 135 static const int biased_lock_shift = lock_bits; 136 static const int age_shift = lock_bits + biased_lock_bits; 137 static const int unused_gap_shift = age_shift + age_bits; 138 static const int hash_shift = unused_gap_shift + unused_gap_bits; 139 static const int epoch_shift = hash_shift; 140 141 static const uintptr_t lock_mask = right_n_bits(lock_bits); 142 static const uintptr_t lock_mask_in_place = lock_mask << lock_shift; 143 static const uintptr_t biased_lock_mask = right_n_bits(lock_bits + biased_lock_bits); 144 static const uintptr_t biased_lock_mask_in_place= biased_lock_mask << lock_shift; 145 static const uintptr_t biased_lock_bit_in_place = 1 << biased_lock_shift; 146 static const uintptr_t age_mask = right_n_bits(age_bits); 147 static const uintptr_t age_mask_in_place = age_mask << age_shift; 148 static const uintptr_t epoch_mask = right_n_bits(epoch_bits); 149 static const uintptr_t epoch_mask_in_place = epoch_mask << epoch_shift; 150 151 static const uintptr_t hash_mask = right_n_bits(hash_bits); 152 static const uintptr_t hash_mask_in_place = hash_mask << hash_shift; 153 154 // Alignment of JavaThread pointers encoded in object header required by biased locking 155 static const size_t biased_lock_alignment = 2 << (epoch_shift + epoch_bits); 156 157 static const uintptr_t locked_value = 0; 158 static const uintptr_t unlocked_value = 1; 159 static const uintptr_t monitor_value = 2; 160 static const uintptr_t marked_value = 3; 161 static const uintptr_t biased_lock_pattern = 5; 162 163 static const uintptr_t no_hash = 0 ; // no hash value assigned 164 static const uintptr_t no_hash_in_place = (address_word)no_hash << hash_shift; 165 static const uintptr_t no_lock_in_place = unlocked_value; 166 167 static const uint max_age = age_mask; 168 169 static const int max_bias_epoch = epoch_mask; 170 171 // Creates a markWord with all bits set to zero. 172 static markWord zero() { return markWord(uintptr_t(0)); } 173 174 // Biased Locking accessors. 175 // These must be checked by all code which calls into the 176 // ObjectSynchronizer and other code. The biasing is not understood 177 // by the lower-level CAS-based locking code, although the runtime 178 // fixes up biased locks to be compatible with it when a bias is 179 // revoked. 180 bool has_bias_pattern() const { 181 return (mask_bits(value(), biased_lock_mask_in_place) == biased_lock_pattern); 182 } 183 JavaThread* biased_locker() const { 184 assert(has_bias_pattern(), "should not call this otherwise"); 185 return (JavaThread*) mask_bits(value(), ~(biased_lock_mask_in_place | age_mask_in_place | epoch_mask_in_place)); 186 } 187 // Indicates that the mark has the bias bit set but that it has not 188 // yet been biased toward a particular thread 189 bool is_biased_anonymously() const { 190 return (has_bias_pattern() && (biased_locker() == NULL)); 191 } 192 // Indicates epoch in which this bias was acquired. If the epoch 193 // changes due to too many bias revocations occurring, the biases 194 // from the previous epochs are all considered invalid. 195 int bias_epoch() const { 196 assert(has_bias_pattern(), "should not call this otherwise"); 197 return (mask_bits(value(), epoch_mask_in_place) >> epoch_shift); 198 } 199 markWord set_bias_epoch(int epoch) { 200 assert(has_bias_pattern(), "should not call this otherwise"); 201 assert((epoch & (~epoch_mask)) == 0, "epoch overflow"); 202 return markWord(mask_bits(value(), ~epoch_mask_in_place) | (epoch << epoch_shift)); 203 } 204 markWord incr_bias_epoch() { 205 return set_bias_epoch((1 + bias_epoch()) & epoch_mask); 206 } 207 // Prototype mark for initialization 208 static markWord biased_locking_prototype() { 209 return markWord( biased_lock_pattern ); 210 } 211 212 // lock accessors (note that these assume lock_shift == 0) 213 bool is_locked() const { 214 return (mask_bits(value(), lock_mask_in_place) != unlocked_value); 215 } 216 bool is_unlocked() const { 217 return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); 218 } 219 // ObjectMonitor::install_displaced_markword_in_object() uses 220 // is_marked() on ObjectMonitor::_header as part of the restoration 221 // protocol for an object's header. In this usage, the mark bits are 222 // only ever set (and cleared) on the ObjectMonitor::_header field. 223 bool is_marked() const { 224 return (mask_bits(value(), lock_mask_in_place) == marked_value); 225 } 226 bool is_neutral() const { return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); } 227 228 // Special temporary state of the markWord while being inflated. 229 // Code that looks at mark outside a lock need to take this into account. 230 bool is_being_inflated() const { return (value() == 0); } 231 232 // Distinguished markword value - used when inflating over 233 // an existing stacklock. 0 indicates the markword is "BUSY". 234 // Lockword mutators that use a LD...CAS idiom should always 235 // check for and avoid overwriting a 0 value installed by some 236 // other thread. (They should spin or block instead. The 0 value 237 // is transient and *should* be short-lived). 238 static markWord INFLATING() { return zero(); } // inflate-in-progress 239 240 // Should this header be preserved during GC? 241 template <typename KlassProxy> 242 inline bool must_be_preserved(KlassProxy klass) const; 243 244 // Should this header (including its age bits) be preserved in the 245 // case of a promotion failure during scavenge? 246 // Note that we special case this situation. We want to avoid 247 // calling BiasedLocking::preserve_marks()/restore_marks() (which 248 // decrease the number of mark words that need to be preserved 249 // during GC) during each scavenge. During scavenges in which there 250 // is no promotion failure, we actually don't need to call the above 251 // routines at all, since we don't mutate and re-initialize the 252 // marks of promoted objects using init_mark(). However, during 253 // scavenges which result in promotion failure, we do re-initialize 254 // the mark words of objects, meaning that we should have called 255 // these mark word preservation routines. Currently there's no good 256 // place in which to call them in any of the scavengers (although 257 // guarded by appropriate locks we could make one), but the 258 // observation is that promotion failures are quite rare and 259 // reducing the number of mark words preserved during them isn't a 260 // high priority. 261 template <typename KlassProxy> 262 inline bool must_be_preserved_for_promotion_failure(KlassProxy klass) const; 263 264 // WARNING: The following routines are used EXCLUSIVELY by 265 // synchronization functions. They are not really gc safe. 266 // They must get updated if markWord layout get changed. 267 markWord set_unlocked() const { 268 return markWord(value() | unlocked_value); 269 } 270 bool has_locker() const { 271 return ((value() & lock_mask_in_place) == locked_value); 272 } 273 BasicLock* locker() const { 274 assert(has_locker(), "check"); 275 return (BasicLock*) value(); 276 } 277 bool has_monitor() const { 278 return ((value() & monitor_value) != 0); 279 } 280 ObjectMonitor* monitor() const { 281 assert(has_monitor(), "check"); 282 // Use xor instead of &~ to provide one extra tag-bit check. 283 return (ObjectMonitor*) (value() ^ monitor_value); 284 } 285 bool has_displaced_mark_helper() const { 286 return ((value() & unlocked_value) == 0); 287 } 288 markWord displaced_mark_helper() const { 289 assert(has_displaced_mark_helper(), "check"); 290 uintptr_t ptr = (value() & ~monitor_value); 291 return *(markWord*)ptr; 292 } 293 void set_displaced_mark_helper(markWord m) const { 294 assert(has_displaced_mark_helper(), "check"); 295 uintptr_t ptr = (value() & ~monitor_value); 296 ((markWord*)ptr)->_value = m._value; 297 } 298 markWord copy_set_hash(intptr_t hash) const { 299 uintptr_t tmp = value() & (~hash_mask_in_place); 300 tmp |= ((hash & hash_mask) << hash_shift); 301 return markWord(tmp); 302 } 303 // it is only used to be stored into BasicLock as the 304 // indicator that the lock is using heavyweight monitor 305 static markWord unused_mark() { 306 return markWord(marked_value); 307 } 308 // the following two functions create the markWord to be 309 // stored into object header, it encodes monitor info 310 static markWord encode(BasicLock* lock) { 311 return from_pointer(lock); 312 } 313 static markWord encode(ObjectMonitor* monitor) { 314 uintptr_t tmp = (uintptr_t) monitor; 315 return markWord(tmp | monitor_value); 316 } 317 static markWord encode(JavaThread* thread, uint age, int bias_epoch) { 318 uintptr_t tmp = (uintptr_t) thread; 319 assert(UseBiasedLocking && ((tmp & (epoch_mask_in_place | age_mask_in_place | biased_lock_mask_in_place)) == 0), "misaligned JavaThread pointer"); 320 assert(age <= max_age, "age too large"); 321 assert(bias_epoch <= max_bias_epoch, "bias epoch too large"); 322 return markWord(tmp | (bias_epoch << epoch_shift) | (age << age_shift) | biased_lock_pattern); 323 } 324 325 // used to encode pointers during GC 326 markWord clear_lock_bits() { return markWord(value() & ~lock_mask_in_place); } 327 328 // age operations 329 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); } 330 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); } 331 332 uint age() const { return mask_bits(value() >> age_shift, age_mask); } 333 markWord set_age(uint v) const { 334 assert((v & ~age_mask) == 0, "shouldn't overflow age field"); 335 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift)); 336 } 337 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); } 338 339 // hash operations 340 intptr_t hash() const { 341 return mask_bits(value() >> hash_shift, hash_mask); 342 } 343 344 bool has_no_hash() const { 345 return hash() == no_hash; 346 } 347 348 // Prototype mark for initialization 349 static markWord prototype() { 350 return markWord( no_hash_in_place | no_lock_in_place ); 351 } 352 353 // Helper function for restoration of unmarked mark oops during GC 354 static inline markWord prototype_for_klass(const Klass* klass); 355 356 // Debugging 357 void print_on(outputStream* st) const; 358 359 // Prepare address of oop for placement into mark 360 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); } 361 362 // Recover address of oop from encoded form used in mark 363 inline void* decode_pointer() { if (UseBiasedLocking && has_bias_pattern()) return NULL; return (void*)clear_lock_bits().value(); } 364 }; 365 366 // Support atomic operations. 367 template<> 368 struct PrimitiveConversions::Translate<markWord> : public TrueType { 369 typedef markWord Value; 370 typedef uintptr_t Decayed; 371 372 static Decayed decay(const Value& x) { return x.value(); } 373 static Value recover(Decayed x) { return Value(x); } 374 }; 375 376 #endif // SHARE_OOPS_MARKWORD_HPP