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