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24
25 #ifndef SHARE_OOPS_MARKWORD_HPP
26 #define SHARE_OOPS_MARKWORD_HPP
27
28 #include "metaprogramming/primitiveConversions.hpp"
29 #include "layoutKind.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() || (EnableValhalla && is_larval_state()));
310 }
311
312 // WARNING: The following routines are used EXCLUSIVELY by
313 // synchronization functions. They are not really gc safe.
314 // They must get updated if markWord layout get changed.
315 markWord set_unlocked() const {
316 return markWord(value() | unlocked_value);
317 }
318 bool has_locker() const {
319 assert(LockingMode == LM_LEGACY, "should only be called with legacy stack locking");
320 return (value() & lock_mask_in_place) == locked_value;
321 }
322 BasicLock* locker() const {
323 assert(has_locker(), "check");
324 return (BasicLock*) value();
325 }
326
327 bool is_fast_locked() const {
328 assert(LockingMode == LM_LIGHTWEIGHT, "should only be called with new lightweight locking");
329 return (value() & lock_mask_in_place) == locked_value;
330 }
331 markWord set_fast_locked() const {
332 // Clear the lock_mask_in_place bits to set locked_value:
333 return markWord(value() & ~lock_mask_in_place);
334 }
335
336 bool has_monitor() const {
337 return ((value() & lock_mask_in_place) == monitor_value);
338 }
339 ObjectMonitor* monitor() const {
340 assert(has_monitor(), "check");
341 assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors");
342 // Use xor instead of &~ to provide one extra tag-bit check.
343 return (ObjectMonitor*) (value() ^ monitor_value);
344 }
345 bool has_displaced_mark_helper() const {
346 intptr_t lockbits = value() & lock_mask_in_place;
347 if (LockingMode == LM_LIGHTWEIGHT) {
348 return !UseObjectMonitorTable && lockbits == monitor_value;
349 }
350 // monitor (0b10) | stack-locked (0b00)?
351 return (lockbits & unlocked_value) == 0;
352 }
353 markWord displaced_mark_helper() const;
354 void set_displaced_mark_helper(markWord m) const;
355 markWord copy_set_hash(intptr_t hash) const {
356 uintptr_t tmp = value() & (~hash_mask_in_place);
357 tmp |= ((hash & hash_mask) << hash_shift);
358 return markWord(tmp);
359 }
360 // it is only used to be stored into BasicLock as the
361 // indicator that the lock is using heavyweight monitor
362 static markWord unused_mark() {
363 return markWord(marked_value);
364 }
365 // the following two functions create the markWord to be
366 // stored into object header, it encodes monitor info
367 static markWord encode(BasicLock* lock) {
368 return from_pointer(lock);
369 }
370 static markWord encode(ObjectMonitor* monitor) {
371 assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors");
372 uintptr_t tmp = (uintptr_t) monitor;
373 return markWord(tmp | monitor_value);
374 }
375
376 markWord set_has_monitor() const {
377 return markWord((value() & ~lock_mask_in_place) | monitor_value);
378 }
379
380 // used to encode pointers during GC
381 markWord clear_lock_bits() const { return markWord(value() & ~lock_mask_in_place); }
382
383 // age operations
384 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
385 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
386
387 uint age() const { return (uint) mask_bits(value() >> age_shift, age_mask); }
388 markWord set_age(uint v) const {
389 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
390 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
391 }
392 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
393
394 // hash operations
395 intptr_t hash() const {
396 return mask_bits(value() >> hash_shift, hash_mask);
397 }
398
399 bool has_no_hash() const {
400 return hash() == no_hash;
401 }
402
403 // private buffered value operations
404 markWord enter_larval_state() const {
405 return markWord(value() | larval_bit_in_place);
406 }
407 markWord exit_larval_state() const {
408 return markWord(value() & ~larval_bit_in_place);
409 }
410 bool is_larval_state() const {
411 return (mask_bits(value(), larval_mask_in_place) == larval_pattern);
412 }
413
414 #ifdef _LP64 // 64 bit encodings only
415 bool is_flat_array() const {
416 return (mask_bits(value(), flat_array_mask_in_place) == null_free_flat_array_pattern)
417 || (mask_bits(value(), flat_array_mask_in_place) == nullable_flat_array_pattern);
418 }
419
420 bool is_null_free_array() const {
421 return (mask_bits(value(), null_free_array_mask_in_place) == null_free_array_pattern);
422 }
423 #else
424 bool is_flat_array() const {
425 fatal("Should not ask this for mark word, ask oopDesc");
426 return false;
427 }
428
429 bool is_null_free_array() const {
430 fatal("Should not ask this for mark word, ask oopDesc");
431 return false;
432 }
433 #endif
434 inline Klass* klass() const;
435 inline Klass* klass_or_null() const;
436 inline Klass* klass_without_asserts() const;
437 inline narrowKlass narrow_klass() const;
438 inline markWord set_narrow_klass(narrowKlass narrow_klass) const;
439
440 // Prototype mark for initialization
441 static markWord prototype() {
442 return markWord( no_hash_in_place | no_lock_in_place );
443 }
444
445 static markWord inline_type_prototype() {
446 return markWord(inline_type_pattern);
447 }
448
449 #ifdef _LP64 // 64 bit encodings only
450 static markWord flat_array_prototype(LayoutKind lk);
451
452 static markWord null_free_array_prototype() {
453 return markWord(null_free_array_pattern);
454 }
455 #endif
456
457 // Debugging
458 void print_on(outputStream* st, bool print_monitor_info = true) const;
459
460 // Prepare address of oop for placement into mark
461 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
462
463 // Recover address of oop from encoded form used in mark
464 inline void* decode_pointer() const {
465 return (EnableValhalla && _value < static_prototype_value_max) ? nullptr :
466 (void*) (clear_lock_bits().value());
467 }
468
469 inline bool is_self_forwarded() const {
470 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
471 return mask_bits(value(), self_fwd_mask_in_place) != 0;
472 }
473
474 inline markWord set_self_forwarded() const {
475 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
476 return markWord(value() | self_fwd_mask_in_place);
477 }
478
479 inline markWord unset_self_forwarded() const {
480 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
481 return markWord(value() & ~self_fwd_mask_in_place);
482 }
483
484 inline oop forwardee() const {
485 return cast_to_oop(decode_pointer());
486 }
487 };
488
489 // Support atomic operations.
490 template<>
491 struct PrimitiveConversions::Translate<markWord> : public std::true_type {
492 typedef markWord Value;
493 typedef uintptr_t Decayed;
494
495 static Decayed decay(const Value& x) { return x.value(); }
496 static Value recover(Decayed x) { return Value(x); }
497 };
498
499 #endif // SHARE_OOPS_MARKWORD_HPP