markOop.hpp revision 196:d1605aabd0a1
1/* 2 * Copyright 1997-2008 Sun Microsystems, Inc. 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, 20 * CA 95054 USA or visit www.sun.com if you need additional information or 21 * have any 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): 31// 32// 32 bits: unused:0 hash:25 age:4 biased_lock:1 lock:2 33// 64 bits: unused:24 hash:31 cms:2 age:4 biased_lock:1 lock:2 34// unused:20 size:35 cms:2 age:4 biased_lock:1 lock:2 (if cms 35// free chunk) 36// 37// - hash contains the identity hash value: largest value is 38// 31 bits, see os::random(). Also, 64-bit vm's require 39// a hash value no bigger than 32 bits because they will not 40// properly generate a mask larger than that: see library_call.cpp 41// and c1_CodePatterns_sparc.cpp. 42// 43// - the biased lock pattern is used to bias a lock toward a given 44// thread. When this pattern is set in the low three bits, the lock 45// is either biased toward a given thread or "anonymously" biased, 46// indicating that it is possible for it to be biased. When the 47// lock is biased toward a given thread, locking and unlocking can 48// be performed by that thread without using atomic operations. 49// When a lock's bias is revoked, it reverts back to the normal 50// locking scheme described below. 51// 52// Note that we are overloading the meaning of the "unlocked" state 53// of the header. Because we steal a bit from the age we can 54// guarantee that the bias pattern will never be seen for a truly 55// unlocked object. 56// 57// Note also that the biased state contains the age bits normally 58// contained in the object header. Large increases in scavenge 59// times were seen when these bits were absent and an arbitrary age 60// assigned to all biased objects, because they tended to consume a 61// significant fraction of the eden semispaces and were not 62// promoted promptly, causing an increase in the amount of copying 63// performed. The runtime system aligns all JavaThread* pointers to 64// a very large value (currently 128 bytes) to make room for the 65// age bits when biased locking is enabled. 66// 67// [JavaThread* | epoch | age | 1 | 01] lock is biased toward given thread 68// [0 | epoch | age | 1 | 01] lock is anonymously biased 69// 70// - the two lock bits are used to describe three states: locked/unlocked and monitor. 71// 72// [ptr | 00] locked ptr points to real header on stack 73// [header | 0 | 01] unlocked regular object header 74// [ptr | 10] monitor inflated lock (header is wapped out) 75// [ptr | 11] marked used by markSweep to mark an object 76// not valid at any other time 77// 78// We assume that stack/thread pointers have the lowest two bits cleared. 79 80class BasicLock; 81class ObjectMonitor; 82class JavaThread; 83 84class markOopDesc: public oopDesc { 85 private: 86 // Conversion 87 uintptr_t value() const { return (uintptr_t) this; } 88 89 public: 90 // Constants 91 enum { age_bits = 4, 92 lock_bits = 2, 93 biased_lock_bits = 1, 94 max_hash_bits = BitsPerWord - age_bits - lock_bits - biased_lock_bits, 95 hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits, 96 cms_bits = LP64_ONLY(1) NOT_LP64(0), 97 epoch_bits = 2 98 }; 99 100 // The biased locking code currently requires that the age bits be 101 // contiguous to the lock bits. Class data sharing would prefer the 102 // hash bits to be lower down to provide more random hash codes for 103 // shared read-only symbolOop objects, because these objects' mark 104 // words are set to their own address with marked_value in the lock 105 // bit, and using lower bits would make their identity hash values 106 // more random. However, the performance decision was made in favor 107 // of the biased locking code. 108 109 enum { lock_shift = 0, 110 biased_lock_shift = lock_bits, 111 age_shift = lock_bits + biased_lock_bits, 112 cms_shift = age_shift + age_bits, 113 hash_shift = cms_shift + cms_bits, 114 epoch_shift = hash_shift 115 }; 116 117 enum { lock_mask = right_n_bits(lock_bits), 118 lock_mask_in_place = lock_mask << lock_shift, 119 biased_lock_mask = right_n_bits(lock_bits + biased_lock_bits), 120 biased_lock_mask_in_place= biased_lock_mask << lock_shift, 121 biased_lock_bit_in_place = 1 << biased_lock_shift, 122 age_mask = right_n_bits(age_bits), 123 age_mask_in_place = age_mask << age_shift, 124 epoch_mask = right_n_bits(epoch_bits), 125 epoch_mask_in_place = epoch_mask << epoch_shift, 126 cms_mask = right_n_bits(cms_bits), 127 cms_mask_in_place = cms_mask << cms_shift 128#ifndef _WIN64 129 ,hash_mask = right_n_bits(hash_bits), 130 hash_mask_in_place = (address_word)hash_mask << hash_shift 131#endif 132 }; 133 134 // Alignment of JavaThread pointers encoded in object header required by biased locking 135 enum { biased_lock_alignment = 2 << (epoch_shift + epoch_bits) 136 }; 137 138#ifdef _WIN64 139 // These values are too big for Win64 140 const static uintptr_t hash_mask = right_n_bits(hash_bits); 141 const static uintptr_t hash_mask_in_place = 142 (address_word)hash_mask << hash_shift; 143#endif 144 145 enum { locked_value = 0, 146 unlocked_value = 1, 147 monitor_value = 2, 148 marked_value = 3, 149 biased_lock_pattern = 5 150 }; 151 152 enum { no_hash = 0 }; // no hash value assigned 153 154 enum { no_hash_in_place = (address_word)no_hash << hash_shift, 155 no_lock_in_place = unlocked_value 156 }; 157 158 enum { max_age = age_mask }; 159 160 enum { max_bias_epoch = epoch_mask }; 161 162 // Biased Locking accessors. 163 // These must be checked by all code which calls into the 164 // ObjectSynchronizer and other code. The biasing is not understood 165 // by the lower-level CAS-based locking code, although the runtime 166 // fixes up biased locks to be compatible with it when a bias is 167 // revoked. 168 bool has_bias_pattern() const { 169 return (mask_bits(value(), biased_lock_mask_in_place) == biased_lock_pattern); 170 } 171 JavaThread* biased_locker() const { 172 assert(has_bias_pattern(), "should not call this otherwise"); 173 return (JavaThread*) ((intptr_t) (mask_bits(value(), ~(biased_lock_mask_in_place | age_mask_in_place | epoch_mask_in_place)))); 174 } 175 // Indicates that the mark has the bias bit set but that it has not 176 // yet been biased toward a particular thread 177 bool is_biased_anonymously() const { 178 return (has_bias_pattern() && (biased_locker() == NULL)); 179 } 180 // Indicates epoch in which this bias was acquired. If the epoch 181 // changes due to too many bias revocations occurring, the biases 182 // from the previous epochs are all considered invalid. 183 int bias_epoch() const { 184 assert(has_bias_pattern(), "should not call this otherwise"); 185 return (mask_bits(value(), epoch_mask_in_place) >> epoch_shift); 186 } 187 markOop set_bias_epoch(int epoch) { 188 assert(has_bias_pattern(), "should not call this otherwise"); 189 assert((epoch & (~epoch_mask)) == 0, "epoch overflow"); 190 return markOop(mask_bits(value(), ~epoch_mask_in_place) | (epoch << epoch_shift)); 191 } 192 markOop incr_bias_epoch() { 193 return set_bias_epoch((1 + bias_epoch()) & epoch_mask); 194 } 195 // Prototype mark for initialization 196 static markOop biased_locking_prototype() { 197 return markOop( biased_lock_pattern ); 198 } 199 200 // lock accessors (note that these assume lock_shift == 0) 201 bool is_locked() const { 202 return (mask_bits(value(), lock_mask_in_place) != unlocked_value); 203 } 204 bool is_unlocked() const { 205 return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); 206 } 207 bool is_marked() const { 208 return (mask_bits(value(), lock_mask_in_place) == marked_value); 209 } 210 bool is_neutral() const { return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); } 211 212 // Special temporary state of the markOop while being inflated. 213 // Code that looks at mark outside a lock need to take this into account. 214 bool is_being_inflated() const { return (value() == 0); } 215 216 // Distinguished markword value - used when inflating over 217 // an existing stacklock. 0 indicates the markword is "BUSY". 218 // Lockword mutators that use a LD...CAS idiom should always 219 // check for and avoid overwriting a 0 value installed by some 220 // other thread. (They should spin or block instead. The 0 value 221 // is transient and *should* be short-lived). 222 static markOop INFLATING() { return (markOop) 0; } // inflate-in-progress 223 224 // Should this header be preserved during GC? 225 bool must_be_preserved(oop obj_containing_mark) const { 226 if (!UseBiasedLocking) 227 return (!is_unlocked() || !has_no_hash()); 228 return must_be_preserved_with_bias(obj_containing_mark); 229 } 230 inline bool must_be_preserved_with_bias(oop obj_containing_mark) const; 231 232 // Should this header (including its age bits) be preserved in the 233 // case of a promotion failure during scavenge? 234 // Note that we special case this situation. We want to avoid 235 // calling BiasedLocking::preserve_marks()/restore_marks() (which 236 // decrease the number of mark words that need to be preserved 237 // during GC) during each scavenge. During scavenges in which there 238 // is no promotion failure, we actually don't need to call the above 239 // routines at all, since we don't mutate and re-initialize the 240 // marks of promoted objects using init_mark(). However, during 241 // scavenges which result in promotion failure, we do re-initialize 242 // the mark words of objects, meaning that we should have called 243 // these mark word preservation routines. Currently there's no good 244 // place in which to call them in any of the scavengers (although 245 // guarded by appropriate locks we could make one), but the 246 // observation is that promotion failures are quite rare and 247 // reducing the number of mark words preserved during them isn't a 248 // high priority. 249 bool must_be_preserved_for_promotion_failure(oop obj_containing_mark) const { 250 if (!UseBiasedLocking) 251 return (this != prototype()); 252 return must_be_preserved_with_bias_for_promotion_failure(obj_containing_mark); 253 } 254 inline bool must_be_preserved_with_bias_for_promotion_failure(oop obj_containing_mark) const; 255 256 // Should this header be preserved during a scavenge where CMS is 257 // the old generation? 258 // (This is basically the same body as must_be_preserved_for_promotion_failure(), 259 // but takes the klassOop as argument instead) 260 bool must_be_preserved_for_cms_scavenge(klassOop klass_of_obj_containing_mark) const { 261 if (!UseBiasedLocking) 262 return (this != prototype()); 263 return must_be_preserved_with_bias_for_cms_scavenge(klass_of_obj_containing_mark); 264 } 265 inline bool must_be_preserved_with_bias_for_cms_scavenge(klassOop klass_of_obj_containing_mark) const; 266 267 // WARNING: The following routines are used EXCLUSIVELY by 268 // synchronization functions. They are not really gc safe. 269 // They must get updated if markOop layout get changed. 270 markOop set_unlocked() const { 271 return markOop(value() | unlocked_value); 272 } 273 bool has_locker() const { 274 return ((value() & lock_mask_in_place) == locked_value); 275 } 276 BasicLock* locker() const { 277 assert(has_locker(), "check"); 278 return (BasicLock*) value(); 279 } 280 bool has_monitor() const { 281 return ((value() & monitor_value) != 0); 282 } 283 ObjectMonitor* monitor() const { 284 assert(has_monitor(), "check"); 285 // Use xor instead of &~ to provide one extra tag-bit check. 286 return (ObjectMonitor*) (value() ^ monitor_value); 287 } 288 bool has_displaced_mark_helper() const { 289 return ((value() & unlocked_value) == 0); 290 } 291 markOop displaced_mark_helper() const { 292 assert(has_displaced_mark_helper(), "check"); 293 intptr_t ptr = (value() & ~monitor_value); 294 return *(markOop*)ptr; 295 } 296 void set_displaced_mark_helper(markOop m) const { 297 assert(has_displaced_mark_helper(), "check"); 298 intptr_t ptr = (value() & ~monitor_value); 299 *(markOop*)ptr = m; 300 } 301 markOop copy_set_hash(intptr_t hash) const { 302 intptr_t tmp = value() & (~hash_mask_in_place); 303 tmp |= ((hash & hash_mask) << hash_shift); 304 return (markOop)tmp; 305 } 306 // it is only used to be stored into BasicLock as the 307 // indicator that the lock is using heavyweight monitor 308 static markOop unused_mark() { 309 return (markOop) marked_value; 310 } 311 // the following two functions create the markOop to be 312 // stored into object header, it encodes monitor info 313 static markOop encode(BasicLock* lock) { 314 return (markOop) lock; 315 } 316 static markOop encode(ObjectMonitor* monitor) { 317 intptr_t tmp = (intptr_t) monitor; 318 return (markOop) (tmp | monitor_value); 319 } 320 static markOop encode(JavaThread* thread, int age, int bias_epoch) { 321 intptr_t tmp = (intptr_t) thread; 322 assert(UseBiasedLocking && ((tmp & (epoch_mask_in_place | age_mask_in_place | biased_lock_mask_in_place)) == 0), "misaligned JavaThread pointer"); 323 assert(age <= max_age, "age too large"); 324 assert(bias_epoch <= max_bias_epoch, "bias epoch too large"); 325 return (markOop) (tmp | (bias_epoch << epoch_shift) | (age << age_shift) | biased_lock_pattern); 326 } 327 328 // used to encode pointers during GC 329 markOop clear_lock_bits() { return markOop(value() & ~lock_mask_in_place); } 330 331 // age operations 332 markOop set_marked() { return markOop((value() & ~lock_mask_in_place) | marked_value); } 333 334 int age() const { return mask_bits(value() >> age_shift, age_mask); } 335 markOop set_age(int v) const { 336 assert((v & ~age_mask) == 0, "shouldn't overflow age field"); 337 return markOop((value() & ~age_mask_in_place) | (((intptr_t)v & age_mask) << age_shift)); 338 } 339 markOop incr_age() const { return age() == max_age ? markOop(this) : set_age(age() + 1); } 340 341 // hash operations 342 intptr_t hash() const { 343 return mask_bits(value() >> hash_shift, hash_mask); 344 } 345 346 bool has_no_hash() const { 347 return hash() == no_hash; 348 } 349 350 // Prototype mark for initialization 351 static markOop prototype() { 352 return markOop( no_hash_in_place | no_lock_in_place ); 353 } 354 355 // Helper function for restoration of unmarked mark oops during GC 356 static inline markOop prototype_for_object(oop obj); 357 358 // Debugging 359 void print_on(outputStream* st) const; 360 361 // Prepare address of oop for placement into mark 362 inline static markOop encode_pointer_as_mark(void* p) { return markOop(p)->set_marked(); } 363 364 // Recover address of oop from encoded form used in mark 365 inline void* decode_pointer() { if (UseBiasedLocking && has_bias_pattern()) return NULL; return clear_lock_bits(); } 366 367 // see the definition in markOop.cpp for the gory details 368 bool should_not_be_cached() const; 369 370 // These markOops indicate cms free chunk blocks and not objects. 371 // In 64 bit, the markOop is set to distinguish them from oops. 372 // These are defined in 32 bit mode for vmStructs. 373 const static uintptr_t cms_free_chunk_pattern = 0x1; 374 375 // Constants for the size field. 376 enum { size_shift = cms_shift + cms_bits, 377 size_bits = 35 // need for compressed oops 32G 378 }; 379 // These values are too big for Win64 380 const static uintptr_t size_mask = LP64_ONLY(right_n_bits(size_bits)) 381 NOT_LP64(0); 382 const static uintptr_t size_mask_in_place = 383 (address_word)size_mask << size_shift; 384 385#ifdef _LP64 386 static markOop cms_free_prototype() { 387 return markOop(((intptr_t)prototype() & ~cms_mask_in_place) | 388 ((cms_free_chunk_pattern & cms_mask) << cms_shift)); 389 } 390 uintptr_t cms_encoding() const { 391 return mask_bits(value() >> cms_shift, cms_mask); 392 } 393 bool is_cms_free_chunk() const { 394 return is_neutral() && 395 (cms_encoding() & cms_free_chunk_pattern) == cms_free_chunk_pattern; 396 } 397 398 size_t get_size() const { return (size_t)(value() >> size_shift); } 399 static markOop set_size_and_free(size_t size) { 400 assert((size & ~size_mask) == 0, "shouldn't overflow size field"); 401 return markOop(((intptr_t)cms_free_prototype() & ~size_mask_in_place) | 402 (((intptr_t)size & size_mask) << size_shift)); 403 } 404#endif // _LP64 405}; 406