1 /* 2 * Copyright (c) 1997, 2008, 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 // The markOop describes the header of an object. 26 // 27 // Note that the mark is not a real oop but just a word. 28 // It is placed in the oop hierarchy for historical reasons. 29 // 30 // Bit-format of an object header (most significant first, big endian layout below): 31 // 32 // 32 bits: 33 // -------- 34 // hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object) 35 // JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object) 36 // size:32 ------------------------------------------>| (CMS free block) 37 // PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object) 38 // 39 // 64 bits: 40 // -------- 41 // unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object) 42 // JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object) 43 // PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object) 44 // size:64 ----------------------------------------------------->| (CMS free block) 45 // 46 // unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object) 47 // JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object) 48 // narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object) 49 // unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block) 50 // 51 // - hash contains the identity hash value: largest value is 52 // 31 bits, see os::random(). Also, 64-bit vm's require 53 // a hash value no bigger than 32 bits because they will not 54 // properly generate a mask larger than that: see library_call.cpp 55 // and c1_CodePatterns_sparc.cpp. 56 // 57 // - the biased lock pattern is used to bias a lock toward a given 58 // thread. When this pattern is set in the low three bits, the lock 59 // is either biased toward a given thread or "anonymously" biased, 60 // indicating that it is possible for it to be biased. When the 61 // lock is biased toward a given thread, locking and unlocking can 62 // be performed by that thread without using atomic operations. 63 // When a lock's bias is revoked, it reverts back to the normal 64 // locking scheme described below. 65 // 66 // Note that we are overloading the meaning of the "unlocked" state 67 // of the header. Because we steal a bit from the age we can 68 // guarantee that the bias pattern will never be seen for a truly 69 // unlocked object. 70 // 71 // Note also that the biased state contains the age bits normally 72 // contained in the object header. Large increases in scavenge 73 // times were seen when these bits were absent and an arbitrary age 74 // assigned to all biased objects, because they tended to consume a 75 // significant fraction of the eden semispaces and were not 76 // promoted promptly, causing an increase in the amount of copying 77 // performed. The runtime system aligns all JavaThread* pointers to 78 // a very large value (currently 128 bytes (32bVM) or 256 bytes (64bVM)) 79 // to make room for the age bits & the epoch bits (used in support of 80 // biased locking), and for the CMS "freeness" bit in the 64bVM (+COOPs). 81 // 82 // [JavaThread* | epoch | age | 1 | 01] lock is biased toward given thread 83 // [0 | epoch | age | 1 | 01] lock is anonymously biased 84 // 85 // - the two lock bits are used to describe three states: locked/unlocked and monitor. 86 // 87 // [ptr | 00] locked ptr points to real header on stack 88 // [header | 0 | 01] unlocked regular object header 89 // [ptr | 10] monitor inflated lock (header is wapped out) 90 // [ptr | 11] marked used by markSweep to mark an object 91 // not valid at any other time 92 // 93 // We assume that stack/thread pointers have the lowest two bits cleared. 94 95 class BasicLock; 96 class ObjectMonitor; 97 class JavaThread; 98 99 class markOopDesc: public oopDesc { 100 private: 101 // Conversion 102 uintptr_t value() const { return (uintptr_t) this; } 103 104 public: 105 // Constants 106 enum { age_bits = 4, 107 lock_bits = 2, 108 biased_lock_bits = 1, 109 max_hash_bits = BitsPerWord - age_bits - lock_bits - biased_lock_bits, 110 hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits, 111 cms_bits = LP64_ONLY(1) NOT_LP64(0), 112 epoch_bits = 2 113 }; 114 115 // The biased locking code currently requires that the age bits be 116 // contiguous to the lock bits. Class data sharing would prefer the 117 // hash bits to be lower down to provide more random hash codes for 118 // shared read-only symbolOop objects, because these objects' mark 119 // words are set to their own address with marked_value in the lock 120 // bit, and using lower bits would make their identity hash values 121 // more random. However, the performance decision was made in favor 122 // of the biased locking code. 123 124 enum { lock_shift = 0, 125 biased_lock_shift = lock_bits, 126 age_shift = lock_bits + biased_lock_bits, 127 cms_shift = age_shift + age_bits, 128 hash_shift = cms_shift + cms_bits, 129 epoch_shift = hash_shift 130 }; 131 132 enum { lock_mask = right_n_bits(lock_bits), 133 lock_mask_in_place = lock_mask << lock_shift, 134 biased_lock_mask = right_n_bits(lock_bits + biased_lock_bits), 135 biased_lock_mask_in_place= biased_lock_mask << lock_shift, 136 biased_lock_bit_in_place = 1 << biased_lock_shift, 137 age_mask = right_n_bits(age_bits), 138 age_mask_in_place = age_mask << age_shift, 139 epoch_mask = right_n_bits(epoch_bits), 140 epoch_mask_in_place = epoch_mask << epoch_shift, 141 cms_mask = right_n_bits(cms_bits), 142 cms_mask_in_place = cms_mask << cms_shift 143 #ifndef _WIN64 144 ,hash_mask = right_n_bits(hash_bits), 145 hash_mask_in_place = (address_word)hash_mask << hash_shift 146 #endif 147 }; 148 149 // Alignment of JavaThread pointers encoded in object header required by biased locking 150 enum { biased_lock_alignment = 2 << (epoch_shift + epoch_bits) 151 }; 152 153 #ifdef _WIN64 154 // These values are too big for Win64 155 const static uintptr_t hash_mask = right_n_bits(hash_bits); 156 const static uintptr_t hash_mask_in_place = 157 (address_word)hash_mask << hash_shift; 158 #endif 159 160 enum { locked_value = 0, 161 unlocked_value = 1, 162 monitor_value = 2, 163 marked_value = 3, 164 biased_lock_pattern = 5 165 }; 166 167 enum { no_hash = 0 }; // no hash value assigned 168 169 enum { no_hash_in_place = (address_word)no_hash << hash_shift, 170 no_lock_in_place = unlocked_value 171 }; 172 173 enum { max_age = age_mask }; 174 175 enum { max_bias_epoch = epoch_mask }; 176 177 // Biased Locking accessors. 178 // These must be checked by all code which calls into the 179 // ObjectSynchronizer and other code. The biasing is not understood 180 // by the lower-level CAS-based locking code, although the runtime 181 // fixes up biased locks to be compatible with it when a bias is 182 // revoked. 183 bool has_bias_pattern() const { 184 return (mask_bits(value(), biased_lock_mask_in_place) == biased_lock_pattern); 185 } 186 JavaThread* biased_locker() const { 187 assert(has_bias_pattern(), "should not call this otherwise"); 188 return (JavaThread*) ((intptr_t) (mask_bits(value(), ~(biased_lock_mask_in_place | age_mask_in_place | epoch_mask_in_place)))); 189 } 190 // Indicates that the mark has the bias bit set but that it has not 191 // yet been biased toward a particular thread 192 bool is_biased_anonymously() const { 193 return (has_bias_pattern() && (biased_locker() == NULL)); 194 } 195 // Indicates epoch in which this bias was acquired. If the epoch 196 // changes due to too many bias revocations occurring, the biases 197 // from the previous epochs are all considered invalid. 198 int bias_epoch() const { 199 assert(has_bias_pattern(), "should not call this otherwise"); 200 return (mask_bits(value(), epoch_mask_in_place) >> epoch_shift); 201 } 202 markOop set_bias_epoch(int epoch) { 203 assert(has_bias_pattern(), "should not call this otherwise"); 204 assert((epoch & (~epoch_mask)) == 0, "epoch overflow"); 205 return markOop(mask_bits(value(), ~epoch_mask_in_place) | (epoch << epoch_shift)); 206 } 207 markOop incr_bias_epoch() { 208 return set_bias_epoch((1 + bias_epoch()) & epoch_mask); 209 } 210 // Prototype mark for initialization 211 static markOop biased_locking_prototype() { 212 return markOop( biased_lock_pattern ); 213 } 214 215 // lock accessors (note that these assume lock_shift == 0) 216 bool is_locked() const { 217 return (mask_bits(value(), lock_mask_in_place) != unlocked_value); 218 } 219 bool is_unlocked() const { 220 return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); 221 } 222 bool is_marked() const { 223 return (mask_bits(value(), lock_mask_in_place) == marked_value); 224 } 225 bool is_neutral() const { return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); } 226 227 // Special temporary state of the markOop while being inflated. 228 // Code that looks at mark outside a lock need to take this into account. 229 bool is_being_inflated() const { return (value() == 0); } 230 231 // Distinguished markword value - used when inflating over 232 // an existing stacklock. 0 indicates the markword is "BUSY". 233 // Lockword mutators that use a LD...CAS idiom should always 234 // check for and avoid overwriting a 0 value installed by some 235 // other thread. (They should spin or block instead. The 0 value 236 // is transient and *should* be short-lived). 237 static markOop INFLATING() { return (markOop) 0; } // inflate-in-progress 238 239 // Should this header be preserved during GC? 240 inline bool must_be_preserved(oop obj_containing_mark) const; 241 inline bool must_be_preserved_with_bias(oop obj_containing_mark) const; 242 243 // Should this header (including its age bits) be preserved in the 244 // case of a promotion failure during scavenge? 245 // Note that we special case this situation. We want to avoid 246 // calling BiasedLocking::preserve_marks()/restore_marks() (which 247 // decrease the number of mark words that need to be preserved 248 // during GC) during each scavenge. During scavenges in which there 249 // is no promotion failure, we actually don't need to call the above 250 // routines at all, since we don't mutate and re-initialize the 251 // marks of promoted objects using init_mark(). However, during 252 // scavenges which result in promotion failure, we do re-initialize 253 // the mark words of objects, meaning that we should have called 254 // these mark word preservation routines. Currently there's no good 255 // place in which to call them in any of the scavengers (although 256 // guarded by appropriate locks we could make one), but the 257 // observation is that promotion failures are quite rare and 258 // reducing the number of mark words preserved during them isn't a 259 // high priority. 260 inline bool must_be_preserved_for_promotion_failure(oop obj_containing_mark) const; 261 inline bool must_be_preserved_with_bias_for_promotion_failure(oop obj_containing_mark) const; 262 263 // Should this header be preserved during a scavenge where CMS is 264 // the old generation? 265 // (This is basically the same body as must_be_preserved_for_promotion_failure(), 266 // but takes the klassOop as argument instead) 267 inline bool must_be_preserved_for_cms_scavenge(klassOop klass_of_obj_containing_mark) const; 268 inline bool must_be_preserved_with_bias_for_cms_scavenge(klassOop klass_of_obj_containing_mark) const; 269 270 // WARNING: The following routines are used EXCLUSIVELY by 271 // synchronization functions. They are not really gc safe. 272 // They must get updated if markOop layout get changed. 273 markOop set_unlocked() const { 274 return markOop(value() | unlocked_value); 275 } 276 bool has_locker() const { 277 return ((value() & lock_mask_in_place) == locked_value); 278 } 279 BasicLock* locker() const { 280 assert(has_locker(), "check"); 281 return (BasicLock*) value(); 282 } 283 bool has_monitor() const { 284 return ((value() & monitor_value) != 0); 285 } 286 ObjectMonitor* monitor() const { 287 assert(has_monitor(), "check"); 288 // Use xor instead of &~ to provide one extra tag-bit check. 289 return (ObjectMonitor*) (value() ^ monitor_value); 290 } 291 bool has_displaced_mark_helper() const { 292 return ((value() & unlocked_value) == 0); 293 } 294 markOop displaced_mark_helper() const { 295 assert(has_displaced_mark_helper(), "check"); 296 intptr_t ptr = (value() & ~monitor_value); 297 return *(markOop*)ptr; 298 } 299 void set_displaced_mark_helper(markOop m) const { 300 assert(has_displaced_mark_helper(), "check"); 301 intptr_t ptr = (value() & ~monitor_value); 302 *(markOop*)ptr = m; 303 } 304 markOop copy_set_hash(intptr_t hash) const { 305 intptr_t tmp = value() & (~hash_mask_in_place); 306 tmp |= ((hash & hash_mask) << hash_shift); 307 return (markOop)tmp; 308 } 309 // it is only used to be stored into BasicLock as the 310 // indicator that the lock is using heavyweight monitor 311 static markOop unused_mark() { 312 return (markOop) marked_value; 313 } 314 // the following two functions create the markOop to be 315 // stored into object header, it encodes monitor info 316 static markOop encode(BasicLock* lock) { 317 return (markOop) lock; 318 } 319 static markOop encode(ObjectMonitor* monitor) { 320 intptr_t tmp = (intptr_t) monitor; 321 return (markOop) (tmp | monitor_value); 322 } 323 static markOop encode(JavaThread* thread, int age, int bias_epoch) { 324 intptr_t tmp = (intptr_t) thread; 325 assert(UseBiasedLocking && ((tmp & (epoch_mask_in_place | age_mask_in_place | biased_lock_mask_in_place)) == 0), "misaligned JavaThread pointer"); 326 assert(age <= max_age, "age too large"); 327 assert(bias_epoch <= max_bias_epoch, "bias epoch too large"); 328 return (markOop) (tmp | (bias_epoch << epoch_shift) | (age << age_shift) | biased_lock_pattern); 329 } 330 331 // used to encode pointers during GC 332 markOop clear_lock_bits() { return markOop(value() & ~lock_mask_in_place); } 333 334 // age operations 335 markOop set_marked() { return markOop((value() & ~lock_mask_in_place) | marked_value); } 336 337 int age() const { return mask_bits(value() >> age_shift, age_mask); } 338 markOop set_age(int v) const { 339 assert((v & ~age_mask) == 0, "shouldn't overflow age field"); 340 return markOop((value() & ~age_mask_in_place) | (((intptr_t)v & age_mask) << age_shift)); 341 } 342 markOop incr_age() const { return age() == max_age ? markOop(this) : set_age(age() + 1); } 343 344 // hash operations 345 intptr_t hash() const { 346 return mask_bits(value() >> hash_shift, hash_mask); 347 } 348 349 bool has_no_hash() const { 350 return hash() == no_hash; 351 } 352 353 // Prototype mark for initialization 354 static markOop prototype() { 355 return markOop( no_hash_in_place | no_lock_in_place ); 356 } 357 358 // Helper function for restoration of unmarked mark oops during GC 359 static inline markOop prototype_for_object(oop obj); 360 361 // Debugging 362 void print_on(outputStream* st) const; 363 364 // Prepare address of oop for placement into mark 365 inline static markOop encode_pointer_as_mark(void* p) { return markOop(p)->set_marked(); } 366 367 // Recover address of oop from encoded form used in mark 368 inline void* decode_pointer() { if (UseBiasedLocking && has_bias_pattern()) return NULL; return clear_lock_bits(); } 369 370 // see the definition in markOop.cpp for the gory details 371 bool should_not_be_cached() const; 372 373 // These markOops indicate cms free chunk blocks and not objects. 374 // In 64 bit, the markOop is set to distinguish them from oops. 375 // These are defined in 32 bit mode for vmStructs. 376 const static uintptr_t cms_free_chunk_pattern = 0x1; 377 378 // Constants for the size field. 379 enum { size_shift = cms_shift + cms_bits, 380 size_bits = 35 // need for compressed oops 32G 381 }; 382 // These values are too big for Win64 383 const static uintptr_t size_mask = LP64_ONLY(right_n_bits(size_bits)) 384 NOT_LP64(0); 385 const static uintptr_t size_mask_in_place = 386 (address_word)size_mask << size_shift; 387 388 #ifdef _LP64 389 static markOop cms_free_prototype() { 390 return markOop(((intptr_t)prototype() & ~cms_mask_in_place) | 391 ((cms_free_chunk_pattern & cms_mask) << cms_shift)); 392 } 393 uintptr_t cms_encoding() const { 394 return mask_bits(value() >> cms_shift, cms_mask); 395 } 396 bool is_cms_free_chunk() const { 397 return is_neutral() && 398 (cms_encoding() & cms_free_chunk_pattern) == cms_free_chunk_pattern; 399 } 400 401 size_t get_size() const { return (size_t)(value() >> size_shift); } 402 static markOop set_size_and_free(size_t size) { 403 assert((size & ~size_mask) == 0, "shouldn't overflow size field"); 404 return markOop(((intptr_t)cms_free_prototype() & ~size_mask_in_place) | 405 (((intptr_t)size & size_mask) << size_shift)); 406 } 407 #endif // _LP64 408 };