1 /*
2 * Copyright (c) 1997, 2025, 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 "layoutKind.hpp"
29 #include "metaprogramming/primitiveConversions.hpp"
30 #include "oops/compressedKlass.hpp"
31 #include "oops/oopsHierarchy.hpp"
32 #include "runtime/globals.hpp"
33 #include "utilities/vmEnums.hpp"
34
35 #include <type_traits>
36
37 // The markWord describes the header of an object.
38 //
39 // Bit-format of an object header (most significant first, big endian layout below):
40 //
41 // 32 bits:
42 // --------
43 // hash:25 ------------>| age:4 self-fwd:1 lock:2 (normal object)
44 //
45 // 64 bits:
46 // --------
47 // unused:22 hash:31 -->| unused_gap:4 age:4 self-fwd:1 lock:2 (normal object)
48 //
49 // 64 bits (with compact headers):
50 // -------------------------------
51 // klass:22 hash:31 -->| unused_gap:4 age:4 self-fwd:1 lock:2 (normal object)
52 //
53 // - hash contains the identity hash value: largest value is
54 // 31 bits, see os::random(). Also, 64-bit vm's require
55 // a hash value no bigger than 32 bits because they will not
56 // properly generate a mask larger than that: see library_call.cpp
57 //
58 // - the two lock bits are used to describe three states: locked/unlocked and monitor.
59 //
60 // [ptr | 00] locked ptr points to real header on stack (stack-locking in use)
61 // [header | 00] locked locked regular object header (fast-locking in use)
62 // [header | 01] unlocked regular object header
63 // [ptr | 10] monitor inflated lock (header is swapped out, UseObjectMonitorTable == false)
64 // [header | 10] monitor inflated lock (UseObjectMonitorTable == true)
65 // [ptr | 11] marked used to mark an object
66 // [0 ............ 0| 00] inflating inflation in progress (stack-locking in use)
67 //
68 // We assume that stack/thread pointers have the lowest two bits cleared.
69 //
70 //
71 // - INFLATING() is a distinguished markword value of all zeros that is
72 // used when inflating an existing stack-lock into an ObjectMonitor.
73 // See below for is_being_inflated() and INFLATING().
74 //
75 //
76 //
77 // Valhalla
78 //
79 // <CMH: merge this doc into the text above>
80 //
81 // Project Valhalla has mark word encoding requirements for the following oops:
82 //
83 // * inline types: have alternative bytecode behavior, e.g. can not be locked
84 // - "larval state": mutable state, but only during object init, observable
85 // by only by a single thread (generally do not mutate markWord)
86 //
87 // * flat arrays: load/decode of klass layout helper is expensive for aaload
88 //
89 // * "null free" arrays: load/decode of klass layout helper again for aaload
90 //
91 // EnableValhalla
92 //
93 // Formerly known as "biased lock bit", "unused_gap" is free to use: using this
94 // bit to indicate inline type, combined with "unlocked" lock bits, means we
95 // will not interfere with lock encodings (displaced, inflating, and monitor),
96 // since inline types can't be locked.
97 //
98 // Further state encoding
99 //
100 // 32 bit plaforms currently have no further room for encoding. No room for
101 // "denormalized layout helper bits", these fast mark word tests can only be made on
102 // 64 bit platforms. 32-bit platforms need to load the klass->_layout_helper. This
103 // said, the larval state bit is still required for operation, stealing from the hash
104 // code is simplest mechanism.
105 //
106 // Valhalla specific encodings
107 //
108 // Revised Bit-format of an object header (most significant first, big endian layout below):
109 //
110 // 32 bits:
111 // --------
112 // hash:24 ------------>| larval:1 age:4 inline_type:1 lock:2
113 //
114 // 64 bits:
115 // --------
116 // unused:1 | <-- hash:31 -->| unused:22 larval:1 age:4 flat_array:1 null_free_array:1 inline_type:1 lock:2
117 // klass:22 hash:31 -->| larval:1 age:4 flat_array:1 null_free_array:1 inline_type:1 self-fwd:1 lock:2 (normal object)
118 //
119 // The "fast" static type bits (flat_array, null_free_array, and inline_type)
120 // are placed lowest next to lock bits to more easily decode forwarding pointers.
121 // G1 for example, implicitly clears age bits ("G1FullGCCompactionPoint::forward()")
122 // using "oopDesc->forwardee()", so it necessary for "markWord::decode_pointer()"
123 // to return a non-nullptr for this case, but not confuse the static type bits for
124 // a pointer.
125 //
126 // Note the position of 'self-fwd' is not by accident. When forwarding an
127 // object to a new heap position, HeapWord alignment guarantees the lower
128 // bits, including 'self-fwd' are 0. "is_self_forwarded()" will be correctly
129 // set to false. Otherwise encode_pointer_as_mark() may have 'self-fwd' set.
130 //
131 //
132 // Static types bits are recorded in the "klass->prototype_header()", displaced
133 // mark should simply use the prototype header as "slow path", rather chasing
134 // monitor or stack lock races.
135 //
136 // Lock patterns (note inline types can't be locked/monitor/inflating)...
137 //
138 // [ptr | 000] locked ptr points to real header on stack
139 // [header | ?01] unlocked regular object header
140 // [ptr | 010] monitor inflated lock (header is wapped out)
141 // [ptr | ?11] marked used to mark an object
142 // [0 ............ | 000] inflating inflation in progress
143 //
144 //
145
146 class BasicLock;
147 class ObjectMonitor;
148 class JavaThread;
149 class outputStream;
150
151 class markWord {
152 private:
153 uintptr_t _value;
154
155 public:
156 explicit markWord(uintptr_t value) : _value(value) {}
157
158 markWord() = default; // Doesn't initialize _value.
159
160 // It is critical for performance that this class be trivially
161 // destructable, copyable, and assignable.
162 ~markWord() = default;
163 markWord(const markWord&) = default;
164 markWord& operator=(const markWord&) = default;
165
166 static markWord from_pointer(void* ptr) {
167 return markWord((uintptr_t)ptr);
168 }
169 void* to_pointer() const {
170 return (void*)_value;
171 }
172
173 bool operator==(const markWord& other) const {
174 return _value == other._value;
175 }
176 bool operator!=(const markWord& other) const {
177 return !operator==(other);
178 }
179
180 // Conversion
181 uintptr_t value() const { return _value; }
182
183 // Constants, in least significant bit order
184 static const int lock_bits = 2;
185 static const int self_fwd_bits = 1;
186 // EnableValhalla: static prototype header bits (fast path instead of klass layout_helper)
187 static const int inline_type_bits = 1;
188 static const int null_free_array_bits = LP64_ONLY(1) NOT_LP64(0);
189 static const int flat_array_bits = LP64_ONLY(1) NOT_LP64(0);
190 // instance state
191 static const int age_bits = 4;
192 static const int larval_bits = 1;
193 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - inline_type_bits - larval_bits - flat_array_bits - null_free_array_bits - self_fwd_bits;
194 static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
195
196 static const int lock_shift = 0;
197 static const int self_fwd_shift = lock_bits ;
198 static const int inline_type_shift = self_fwd_shift + self_fwd_bits;
199 static const int null_free_array_shift = inline_type_shift + inline_type_bits;
200 static const int flat_array_shift = null_free_array_shift + null_free_array_bits;
201 static const int age_shift = flat_array_shift + flat_array_bits;
202 static const int larval_shift = age_shift + age_bits;
203 static const int hash_shift = larval_shift + larval_bits;
204
205 static const uintptr_t lock_mask = right_n_bits(lock_bits);
206 static const uintptr_t lock_mask_in_place = lock_mask << lock_shift;
207 static const uintptr_t self_fwd_mask = right_n_bits(self_fwd_bits);
208 static const uintptr_t self_fwd_mask_in_place = self_fwd_mask << self_fwd_shift;
209 static const uintptr_t inline_type_bit_in_place = 1 << inline_type_shift;
210 static const uintptr_t inline_type_mask = inline_type_bit_in_place + lock_mask;
211 static const uintptr_t inline_type_mask_in_place = inline_type_mask << lock_shift;
212 static const uintptr_t null_free_array_mask = right_n_bits(null_free_array_bits);
213 static const uintptr_t null_free_array_mask_in_place = (null_free_array_mask << null_free_array_shift) | lock_mask_in_place;
214 static const uintptr_t null_free_array_bit_in_place = (1 << null_free_array_shift);
215 static const uintptr_t flat_array_mask = right_n_bits(flat_array_bits);
216 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;
217 static const uintptr_t flat_array_bit_in_place = (1 << flat_array_shift);
218 static const uintptr_t age_mask = right_n_bits(age_bits);
219 static const uintptr_t age_mask_in_place = age_mask << age_shift;
220
221 static const uintptr_t larval_mask = right_n_bits(larval_bits);
222 static const uintptr_t larval_mask_in_place = (larval_mask << larval_shift) | inline_type_mask_in_place;
223 static const uintptr_t larval_bit_in_place = (1 << larval_shift);
224
225 static const uintptr_t hash_mask = right_n_bits(hash_bits);
226 static const uintptr_t hash_mask_in_place = hash_mask << hash_shift;
227
228 #ifdef _LP64
229 // Used only with compact headers:
230 // We store the (narrow) Klass* in the bits 43 to 64.
231
232 // These are for bit-precise extraction of the narrow Klass* from the 64-bit Markword
233 static constexpr int klass_offset_in_bytes = 4;
234 static constexpr int klass_shift = hash_shift + hash_bits;
235 static constexpr int klass_shift_at_offset = klass_shift - klass_offset_in_bytes * BitsPerByte;
236 static constexpr int klass_bits = 22;
237 static constexpr uintptr_t klass_mask = right_n_bits(klass_bits);
238 static constexpr uintptr_t klass_mask_in_place = klass_mask << klass_shift;
239 #endif
240
241
242 static const uintptr_t locked_value = 0;
243 static const uintptr_t unlocked_value = 1;
244 static const uintptr_t monitor_value = 2;
245 static const uintptr_t marked_value = 3;
246
247 static const uintptr_t inline_type_pattern = inline_type_bit_in_place | unlocked_value;
248 static const uintptr_t null_free_array_pattern = null_free_array_bit_in_place | unlocked_value;
249 static const uintptr_t null_free_flat_array_pattern = flat_array_bit_in_place | null_free_array_pattern;
250 static const uintptr_t nullable_flat_array_pattern = flat_array_bit_in_place | unlocked_value;
251
252 // Has static klass prototype, used for decode/encode pointer
253 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));
254 static const uintptr_t static_prototype_mask_in_place = static_prototype_mask << lock_bits;
255 static const uintptr_t static_prototype_value_max = (1 << age_shift) - 1;
256
257 static const uintptr_t larval_pattern = larval_bit_in_place | inline_type_pattern;
258
259 static const uintptr_t no_hash = 0 ; // no hash value assigned
260 static const uintptr_t no_hash_in_place = (uintptr_t)no_hash << hash_shift;
261 static const uintptr_t no_lock_in_place = unlocked_value;
262
263 static const uint max_age = age_mask;
264
265 // Creates a markWord with all bits set to zero.
266 static markWord zero() { return markWord(uintptr_t(0)); }
267
268 bool is_inline_type() const {
269 return (mask_bits(value(), inline_type_mask_in_place) == inline_type_pattern);
270 }
271
272 // lock accessors (note that these assume lock_shift == 0)
273 bool is_locked() const {
274 return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
275 }
276 bool is_unlocked() const {
277 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
278 }
279 bool is_marked() const {
280 return (mask_bits(value(), lock_mask_in_place) == marked_value);
281 }
282
283 // is unlocked and not an inline type (which cannot be involved in locking, displacement or inflation)
284 // i.e. test both lock bits and the inline type bit together
285 bool is_neutral() const { // Not locked, or marked - a "clean" neutral state
286 return (mask_bits(value(), inline_type_mask_in_place) == unlocked_value);
287 }
288
289 bool is_forwarded() const {
290 // Returns true for normal forwarded (0b011) and self-forwarded (0b1xx).
291 return mask_bits(value(), lock_mask_in_place | self_fwd_mask_in_place) >= static_cast<intptr_t>(marked_value);
292 }
293
294 // Special temporary state of the markWord while being inflated.
295 // Code that looks at mark outside a lock need to take this into account.
296 bool is_being_inflated() const { return (value() == 0); }
297
298 // Distinguished markword value - used when inflating over
299 // an existing stack-lock. 0 indicates the markword is "BUSY".
300 // Lockword mutators that use a LD...CAS idiom should always
301 // check for and avoid overwriting a 0 value installed by some
302 // other thread. (They should spin or block instead. The 0 value
303 // is transient and *should* be short-lived).
304 // Fast-locking does not use INFLATING.
305 static markWord INFLATING() { return zero(); } // inflate-in-progress
306
307 // Should this header be preserved during GC?
308 bool must_be_preserved() const {
309 return (!is_unlocked() || !has_no_hash() ||
310 (EnableValhalla && (is_larval_state() || is_inline_type() || is_flat_array() || is_null_free_array())));
311 }
312
313 // WARNING: The following routines are used EXCLUSIVELY by
314 // synchronization functions. They are not really gc safe.
315 // They must get updated if markWord layout get changed.
316 markWord set_unlocked() const {
317 return markWord(value() | unlocked_value);
318 }
319 bool has_locker() const {
320 assert(LockingMode == LM_LEGACY, "should only be called with legacy stack locking");
321 return (value() & lock_mask_in_place) == locked_value;
322 }
323 BasicLock* locker() const {
324 assert(has_locker(), "check");
325 return (BasicLock*) value();
326 }
327
328 bool is_fast_locked() const {
329 assert(LockingMode == LM_LIGHTWEIGHT, "should only be called with new lightweight locking");
330 return (value() & lock_mask_in_place) == locked_value;
331 }
332 markWord set_fast_locked() const {
333 // Clear the lock_mask_in_place bits to set locked_value:
334 return markWord(value() & ~lock_mask_in_place);
335 }
336
337 bool has_monitor() const {
338 return ((value() & lock_mask_in_place) == monitor_value);
339 }
340 ObjectMonitor* monitor() const {
341 assert(has_monitor(), "check");
342 assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors");
343 // Use xor instead of &~ to provide one extra tag-bit check.
344 return (ObjectMonitor*) (value() ^ monitor_value);
345 }
346 bool has_displaced_mark_helper() const {
347 intptr_t lockbits = value() & lock_mask_in_place;
348 if (LockingMode == LM_LIGHTWEIGHT) {
349 return !UseObjectMonitorTable && lockbits == monitor_value;
350 }
351 // monitor (0b10) | stack-locked (0b00)?
352 return (lockbits & unlocked_value) == 0;
353 }
354 markWord displaced_mark_helper() const;
355 void set_displaced_mark_helper(markWord m) const;
356 markWord copy_set_hash(intptr_t hash) const {
357 uintptr_t tmp = value() & (~hash_mask_in_place);
358 tmp |= ((hash & hash_mask) << hash_shift);
359 return markWord(tmp);
360 }
361 // it is only used to be stored into BasicLock as the
362 // indicator that the lock is using heavyweight monitor
363 static markWord unused_mark() {
364 return markWord(marked_value);
365 }
366 // the following two functions create the markWord to be
367 // stored into object header, it encodes monitor info
368 static markWord encode(BasicLock* lock) {
369 return from_pointer(lock);
370 }
371 static markWord encode(ObjectMonitor* monitor) {
372 assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors");
373 uintptr_t tmp = (uintptr_t) monitor;
374 return markWord(tmp | monitor_value);
375 }
376
377 markWord set_has_monitor() const {
378 return markWord((value() & ~lock_mask_in_place) | monitor_value);
379 }
380
381 // used to encode pointers during GC
382 markWord clear_lock_bits() const { return markWord(value() & ~lock_mask_in_place); }
383
384 // age operations
385 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
386 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
387
388 uint age() const { return (uint) mask_bits(value() >> age_shift, age_mask); }
389 markWord set_age(uint v) const {
390 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
391 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
392 }
393 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
394
395 // hash operations
396 intptr_t hash() const {
397 return mask_bits(value() >> hash_shift, hash_mask);
398 }
399
400 bool has_no_hash() const {
401 return hash() == no_hash;
402 }
403
404 // private buffered value operations
405 markWord enter_larval_state() const {
406 return markWord(value() | larval_bit_in_place);
407 }
408 markWord exit_larval_state() const {
409 return markWord(value() & ~larval_bit_in_place);
410 }
411 bool is_larval_state() const {
412 return (mask_bits(value(), larval_mask_in_place) == larval_pattern);
413 }
414
415 bool is_flat_array() const {
416 #ifdef _LP64 // 64 bit encodings only
417 return (mask_bits(value(), flat_array_mask_in_place) == null_free_flat_array_pattern)
418 || (mask_bits(value(), flat_array_mask_in_place) == nullable_flat_array_pattern);
419 #else
420 return false;
421 #endif
422 }
423
424 bool is_null_free_array() const {
425 #ifdef _LP64 // 64 bit encodings only
426 return (mask_bits(value(), null_free_array_mask_in_place) == null_free_array_pattern);
427 #else
428 return false;
429 #endif
430 }
431
432 inline Klass* klass() const;
433 inline Klass* klass_or_null() const;
434 inline Klass* klass_without_asserts() const;
435 inline narrowKlass narrow_klass() const;
436 inline markWord set_narrow_klass(narrowKlass narrow_klass) const;
437
438 // Prototype mark for initialization
439 static markWord prototype() {
440 return markWord( no_hash_in_place | no_lock_in_place );
441 }
442
443 static markWord inline_type_prototype() {
444 return markWord(inline_type_pattern);
445 }
446
447 #ifdef _LP64 // 64 bit encodings only
448 static markWord flat_array_prototype(LayoutKind lk);
449
450 static markWord null_free_array_prototype() {
451 return markWord(null_free_array_pattern);
452 }
453 #endif
454
455 // Debugging
456 void print_on(outputStream* st, bool print_monitor_info = true) const;
457
458 // Prepare address of oop for placement into mark
459 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
460
461 // Recover address of oop from encoded form used in mark
462 inline void* decode_pointer() const {
463 return (EnableValhalla && _value < static_prototype_value_max) ? nullptr :
464 (void*) (clear_lock_bits().value());
465 }
466
467 inline bool is_self_forwarded() const {
468 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
469 return mask_bits(value(), self_fwd_mask_in_place) != 0;
470 }
471
472 inline markWord set_self_forwarded() const {
473 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
474 return markWord(value() | self_fwd_mask_in_place);
475 }
476
477 inline markWord unset_self_forwarded() const {
478 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
479 return markWord(value() & ~self_fwd_mask_in_place);
480 }
481
482 inline oop forwardee() const {
483 return cast_to_oop(decode_pointer());
484 }
485 };
486
487 // Support atomic operations.
488 template<>
489 struct PrimitiveConversions::Translate<markWord> : public std::true_type {
490 typedef markWord Value;
491 typedef uintptr_t Decayed;
492
493 static Decayed decay(const Value& x) { return x.value(); }
494 static Value recover(Decayed x) { return Value(x); }
495 };
496
497 #endif // SHARE_OOPS_MARKWORD_HPP