1 /* 2 * Copyright (c) 1997, 2023, 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/primitiveConversions.hpp" 29 #include "oops/oopsHierarchy.hpp" 30 #include "runtime/globals.hpp" 31 32 #include <type_traits> 33 34 // The markWord describes the header of an object. 35 // 36 // Bit-format of an object header (most significant first, big endian layout below): 37 // 38 // 32 bits: 39 // -------- 40 // hash:25 ------------>| age:4 unused_gap:1 lock:2 (normal object) 41 // 42 // 64 bits: 43 // -------- 44 // unused:25 hash:31 -->| unused_gap:1 age:4 unused_gap:1 lock:2 (normal object) 45 // 46 // - hash contains the identity hash value: largest value is 47 // 31 bits, see os::random(). Also, 64-bit vm's require 48 // a hash value no bigger than 32 bits because they will not 49 // properly generate a mask larger than that: see library_call.cpp 50 // 51 // - the two lock bits are used to describe three states: locked/unlocked and monitor. 52 // 53 // [ptr | 00] locked ptr points to real header on stack (stack-locking in use) 54 // [header | 00] locked locked regular object header (fast-locking in use) 55 // [header | 01] unlocked regular object header 56 // [ptr | 10] monitor inflated lock (header is swapped out) 57 // [ptr | 11] marked used to mark an object 58 // [0 ............ 0| 00] inflating inflation in progress (stack-locking in use) 59 // 60 // We assume that stack/thread pointers have the lowest two bits cleared. 61 // 62 // 63 // - INFLATING() is a distinguished markword value of all zeros that is 64 // used when inflating an existing stack-lock into an ObjectMonitor. 65 // See below for is_being_inflated() and INFLATING(). 66 // 67 // 68 // 69 // Valhalla 70 // 71 // <CMH: merge this doc into the text above> 72 // 73 // Project Valhalla has mark word encoding requirements for the following oops: 74 // 75 // * inline types: have alternative bytecode behavior, e.g. can not be locked 76 // - "larval state": mutable state, but only during object init, observable 77 // by only by a single thread (generally do not mutate markWord) 78 // 79 // * flat arrays: load/decode of klass layout helper is expensive for aaload 80 // 81 // * "null free" arrays: load/decode of klass layout helper again for aaload 82 // 83 // EnableValhalla 84 // 85 // Formerly known as "biased lock bit", "unused_gap" is free to use: using this 86 // bit to indicate inline type, combined with "unlocked" lock bits, means we 87 // will not interfere with lock encodings (displaced, inflating, and monitor), 88 // since inline types can't be locked. 89 // 90 // Further state encoding 91 // 92 // 32 bit plaforms currently have no further room for encoding. No room for 93 // "denormalized layout helper bits", these fast mark word tests can only be made on 94 // 64 bit platforms. 32-bit platforms need to load the klass->_layout_helper. This 95 // said, the larval state bit is still required for operation, stealing from the hash 96 // code is simplest mechanism. 97 // 98 // Valhalla specific encodings 99 // 100 // Revised Bit-format of an object header (most significant first, big endian layout below): 101 // 102 // 32 bits: 103 // -------- 104 // hash:24 ------------>| larval:1 age:4 inline_type:1 lock:2 105 // 106 // 64 bits: 107 // -------- 108 // unused:1 | <-- hash:31 -->| unused:22 larval:1 age:4 flat_array:1 null_free_array:1 inline_type:1 lock:2 109 // 110 // The "fast" static type bits (flat_array, null_free_array, and inline_type) 111 // are placed lowest next to lock bits to more easily decode forwarding pointers. 112 // G1 for example, implicitly clears age bits ("G1FullGCCompactionPoint::forward()") 113 // using "oopDesc->forwardee()", so it necessary for "markWord::decode_pointer()" 114 // to return a non-nullptr for this case, but not confuse the static type bits for 115 // a pointer. 116 // 117 // Static types bits are recorded in the "klass->prototype_header()", displaced 118 // mark should simply use the prototype header as "slow path", rather chasing 119 // monitor or stack lock races. 120 // 121 // Lock patterns (note inline types can't be locked/monitor/inflating)... 122 // 123 // [ptr | 000] locked ptr points to real header on stack 124 // [header | ?01] unlocked regular object header 125 // [ptr | 010] monitor inflated lock (header is wapped out) 126 // [ptr | ?11] marked used to mark an object 127 // [0 ............ | 000] inflating inflation in progress 128 // 129 // 130 131 class BasicLock; 132 class ObjectMonitor; 133 class JavaThread; 134 class outputStream; 135 136 class markWord { 137 private: 138 uintptr_t _value; 139 140 public: 141 explicit markWord(uintptr_t value) : _value(value) {} 142 143 markWord() = default; // Doesn't initialize _value. 144 145 // It is critical for performance that this class be trivially 146 // destructable, copyable, and assignable. 147 ~markWord() = default; 148 markWord(const markWord&) = default; 149 markWord& operator=(const markWord&) = default; 150 151 static markWord from_pointer(void* ptr) { 152 return markWord((uintptr_t)ptr); 153 } 154 void* to_pointer() const { 155 return (void*)_value; 156 } 157 158 bool operator==(const markWord& other) const { 159 return _value == other._value; 160 } 161 bool operator!=(const markWord& other) const { 162 return !operator==(other); 163 } 164 165 // Conversion 166 uintptr_t value() const { return _value; } 167 168 // Constants, in least significant bit order 169 static const int lock_bits = 2; 170 static const int first_unused_gap_bits = 1; // When !EnableValhalla 171 // EnableValhalla: static prototype header bits (fast path instead of klass layout_helper) 172 static const int inline_type_bits = 1; 173 static const int null_free_array_bits = LP64_ONLY(1) NOT_LP64(0); 174 static const int flat_array_bits = LP64_ONLY(1) NOT_LP64(0); 175 // instance state 176 static const int age_bits = 4; 177 static const int larval_bits = 1; 178 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - inline_type_bits - larval_bits - flat_array_bits - null_free_array_bits; 179 static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits; 180 static const int second_unused_gap_bits = LP64_ONLY(1) NOT_LP64(0); // !EnableValhalla: unused 181 182 static const int lock_shift = 0; 183 static const int inline_type_shift = lock_bits; 184 static const int null_free_array_shift = inline_type_shift + inline_type_bits; 185 static const int flat_array_shift = null_free_array_shift + null_free_array_bits; 186 static const int age_shift = flat_array_shift + flat_array_bits; 187 static const int unused_gap_shift = age_shift + age_bits; // !EnableValhalla: unused 188 static const int larval_shift = age_shift + age_bits; 189 static const int hash_shift = LP64_ONLY(32) NOT_LP64(larval_shift + larval_bits); 190 191 static const uintptr_t lock_mask = right_n_bits(lock_bits); 192 static const uintptr_t lock_mask_in_place = lock_mask << lock_shift; 193 static const uintptr_t inline_type_mask = right_n_bits(lock_bits + inline_type_bits); 194 static const uintptr_t inline_type_mask_in_place = inline_type_mask << lock_shift; 195 static const uintptr_t inline_type_bit_in_place = 1 << inline_type_shift; 196 static const uintptr_t null_free_array_mask = right_n_bits(null_free_array_bits); 197 static const uintptr_t null_free_array_mask_in_place = (null_free_array_mask << null_free_array_shift) | lock_mask_in_place; 198 static const uintptr_t null_free_array_bit_in_place = (1 << null_free_array_shift); 199 static const uintptr_t flat_array_mask = right_n_bits(flat_array_bits); 200 static const uintptr_t flat_array_mask_in_place = (flat_array_mask << flat_array_shift) | null_free_array_mask_in_place | lock_mask_in_place; 201 static const uintptr_t flat_array_bit_in_place = (1 << flat_array_shift); 202 203 static const uintptr_t age_mask = right_n_bits(age_bits); 204 static const uintptr_t age_mask_in_place = age_mask << age_shift; 205 206 static const uintptr_t larval_mask = right_n_bits(larval_bits); 207 static const uintptr_t larval_mask_in_place = (larval_mask << larval_shift) | inline_type_mask_in_place; 208 static const uintptr_t larval_bit_in_place = (1 << larval_shift); 209 210 static const uintptr_t hash_mask = right_n_bits(hash_bits); 211 static const uintptr_t hash_mask_in_place = hash_mask << hash_shift; 212 213 static const uintptr_t locked_value = 0; 214 static const uintptr_t unlocked_value = 1; 215 static const uintptr_t monitor_value = 2; 216 static const uintptr_t marked_value = 3; 217 218 static const uintptr_t inline_type_pattern = inline_type_bit_in_place | unlocked_value; 219 static const uintptr_t null_free_array_pattern = null_free_array_bit_in_place | unlocked_value; 220 static const uintptr_t flat_array_pattern = flat_array_bit_in_place | null_free_array_pattern; 221 // Has static klass prototype, used for decode/encode pointer 222 static const uintptr_t static_prototype_mask = LP64_ONLY(right_n_bits(inline_type_bits + flat_array_bits + null_free_array_bits)) NOT_LP64(right_n_bits(inline_type_bits)); 223 static const uintptr_t static_prototype_mask_in_place = static_prototype_mask << lock_bits; 224 static const uintptr_t static_prototype_value_max = (1 << age_shift) - 1; 225 226 static const uintptr_t larval_pattern = larval_bit_in_place | inline_type_pattern; 227 228 static const uintptr_t no_hash = 0 ; // no hash value assigned 229 static const uintptr_t no_hash_in_place = (uintptr_t)no_hash << hash_shift; 230 static const uintptr_t no_lock_in_place = unlocked_value; 231 232 static const uint max_age = age_mask; 233 234 // Creates a markWord with all bits set to zero. 235 static markWord zero() { return markWord(uintptr_t(0)); } 236 237 bool is_inline_type() const { 238 return (mask_bits(value(), inline_type_mask_in_place) == inline_type_pattern); 239 } 240 241 // lock accessors (note that these assume lock_shift == 0) 242 bool is_locked() const { 243 return (mask_bits(value(), lock_mask_in_place) != unlocked_value); 244 } 245 bool is_unlocked() const { 246 return (mask_bits(value(), lock_mask_in_place) == unlocked_value); 247 } 248 bool is_marked() const { 249 return (mask_bits(value(), lock_mask_in_place) == marked_value); 250 } 251 252 // is unlocked and not an inline type (which cannot be involved in locking, displacement or inflation) 253 // i.e. test both lock bits and the inline type bit together 254 bool is_neutral() const { 255 return (mask_bits(value(), inline_type_mask_in_place) == unlocked_value); 256 } 257 258 // Special temporary state of the markWord while being inflated. 259 // Code that looks at mark outside a lock need to take this into account. 260 bool is_being_inflated() const { return (value() == 0); } 261 262 // Distinguished markword value - used when inflating over 263 // an existing stack-lock. 0 indicates the markword is "BUSY". 264 // Lockword mutators that use a LD...CAS idiom should always 265 // check for and avoid overwriting a 0 value installed by some 266 // other thread. (They should spin or block instead. The 0 value 267 // is transient and *should* be short-lived). 268 // Fast-locking does not use INFLATING. 269 static markWord INFLATING() { return zero(); } // inflate-in-progress 270 271 // Should this header be preserved during GC? 272 bool must_be_preserved(const oopDesc* obj) const { 273 return (!is_unlocked() || !has_no_hash() || (EnableValhalla && is_larval_state())); 274 } 275 276 // WARNING: The following routines are used EXCLUSIVELY by 277 // synchronization functions. They are not really gc safe. 278 // They must get updated if markWord layout get changed. 279 markWord set_unlocked() const { 280 return markWord(value() | unlocked_value); 281 } 282 bool has_locker() const { 283 assert(LockingMode == LM_LEGACY, "should only be called with legacy stack locking"); 284 return (value() & lock_mask_in_place) == locked_value; 285 } 286 BasicLock* locker() const { 287 assert(has_locker(), "check"); 288 return (BasicLock*) value(); 289 } 290 291 bool is_fast_locked() const { 292 assert(LockingMode == LM_LIGHTWEIGHT, "should only be called with new lightweight locking"); 293 return (value() & lock_mask_in_place) == locked_value; 294 } 295 markWord set_fast_locked() const { 296 // Clear the lock_mask_in_place bits to set locked_value: 297 return markWord(value() & ~lock_mask_in_place); 298 } 299 300 bool has_monitor() const { 301 return ((value() & lock_mask_in_place) == monitor_value); 302 } 303 ObjectMonitor* monitor() const { 304 assert(has_monitor(), "check"); 305 // Use xor instead of &~ to provide one extra tag-bit check. 306 return (ObjectMonitor*) (value() ^ monitor_value); 307 } 308 bool has_displaced_mark_helper() const { 309 intptr_t lockbits = value() & lock_mask_in_place; 310 return LockingMode == LM_LIGHTWEIGHT ? lockbits == monitor_value // monitor? 311 : (lockbits & unlocked_value) == 0; // monitor | stack-locked? 312 } 313 markWord displaced_mark_helper() const; 314 void set_displaced_mark_helper(markWord m) const; 315 markWord copy_set_hash(intptr_t hash) const { 316 uintptr_t tmp = value() & (~hash_mask_in_place); 317 tmp |= ((hash & hash_mask) << hash_shift); 318 return markWord(tmp); 319 } 320 // it is only used to be stored into BasicLock as the 321 // indicator that the lock is using heavyweight monitor 322 static markWord unused_mark() { 323 return markWord(marked_value); 324 } 325 // the following two functions create the markWord to be 326 // stored into object header, it encodes monitor info 327 static markWord encode(BasicLock* lock) { 328 return from_pointer(lock); 329 } 330 static markWord encode(ObjectMonitor* monitor) { 331 uintptr_t tmp = (uintptr_t) monitor; 332 return markWord(tmp | monitor_value); 333 } 334 335 // used to encode pointers during GC 336 markWord clear_lock_bits() { return markWord(value() & ~lock_mask_in_place); } 337 338 // age operations 339 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); } 340 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); } 341 342 uint age() const { return (uint) mask_bits(value() >> age_shift, age_mask); } 343 markWord set_age(uint v) const { 344 assert((v & ~age_mask) == 0, "shouldn't overflow age field"); 345 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift)); 346 } 347 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); } 348 349 // hash operations 350 intptr_t hash() const { 351 return mask_bits(value() >> hash_shift, hash_mask); 352 } 353 354 bool has_no_hash() const { 355 return hash() == no_hash; 356 } 357 358 // private buffered value operations 359 markWord enter_larval_state() const { 360 return markWord(value() | larval_bit_in_place); 361 } 362 markWord exit_larval_state() const { 363 return markWord(value() & ~larval_bit_in_place); 364 } 365 bool is_larval_state() const { 366 return (mask_bits(value(), larval_mask_in_place) == larval_pattern); 367 } 368 369 #ifdef _LP64 // 64 bit encodings only 370 bool is_flat_array() const { 371 return (mask_bits(value(), flat_array_mask_in_place) == flat_array_pattern); 372 } 373 374 bool is_null_free_array() const { 375 return (mask_bits(value(), null_free_array_mask_in_place) == null_free_array_pattern); 376 } 377 #else 378 bool is_flat_array() const { 379 fatal("Should not ask this for mark word, ask oopDesc"); 380 return false; 381 } 382 383 bool is_null_free_array() const { 384 fatal("Should not ask this for mark word, ask oopDesc"); 385 return false; 386 } 387 #endif 388 // Prototype mark for initialization 389 static markWord prototype() { 390 return markWord( no_hash_in_place | no_lock_in_place ); 391 } 392 393 static markWord inline_type_prototype() { 394 return markWord(inline_type_pattern); 395 } 396 397 #ifdef _LP64 // 64 bit encodings only 398 static markWord flat_array_prototype() { 399 return markWord(flat_array_pattern); 400 } 401 402 static markWord null_free_array_prototype() { 403 return markWord(null_free_array_pattern); 404 } 405 #endif 406 407 // Debugging 408 void print_on(outputStream* st, bool print_monitor_info = true) const; 409 410 // Prepare address of oop for placement into mark 411 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); } 412 413 // Recover address of oop from encoded form used in mark 414 inline void* decode_pointer() { 415 return (EnableValhalla && _value < static_prototype_value_max) ? nullptr : 416 (void*) (clear_lock_bits().value()); 417 } 418 }; 419 420 // Support atomic operations. 421 template<> 422 struct PrimitiveConversions::Translate<markWord> : public std::true_type { 423 typedef markWord Value; 424 typedef uintptr_t Decayed; 425 426 static Decayed decay(const Value& x) { return x.value(); } 427 static Value recover(Decayed x) { return Value(x); } 428 }; 429 430 #endif // SHARE_OOPS_MARKWORD_HPP