1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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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 "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, UseObjectMonitorTable == false)
57 // [header | 10] monitor inflated lock (UseObjectMonitorTable == true)
58 // [ptr | 11] marked used to mark an object
59 // [0 ............ 0| 00] inflating inflation in progress (stack-locking in use)
60 //
61 // We assume that stack/thread pointers have the lowest two bits cleared.
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 class BasicLock;
68 class ObjectMonitor;
69 class JavaThread;
70 class outputStream;
71
72 class markWord {
73 private:
74 uintptr_t _value;
75
76 public:
77 explicit markWord(uintptr_t value) : _value(value) {}
78
79 markWord() = default; // Doesn't initialize _value.
80
81 // It is critical for performance that this class be trivially
82 // destructable, copyable, and assignable.
83 ~markWord() = default;
84 markWord(const markWord&) = default;
85 markWord& operator=(const markWord&) = default;
86
87 static markWord from_pointer(void* ptr) {
88 return markWord((uintptr_t)ptr);
89 }
90 void* to_pointer() const {
91 return (void*)_value;
92 }
93
94 bool operator==(const markWord& other) const {
95 return _value == other._value;
96 }
97 bool operator!=(const markWord& other) const {
98 return !operator==(other);
99 }
100
101 // Conversion
102 uintptr_t value() const { return _value; }
103
104 // Constants
105 static const int age_bits = 4;
106 static const int lock_bits = 2;
107 static const int first_unused_gap_bits = 1;
108 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - first_unused_gap_bits;
109 static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
110 static const int second_unused_gap_bits = LP64_ONLY(1) NOT_LP64(0);
111
112 static const int lock_shift = 0;
113 static const int age_shift = lock_bits + first_unused_gap_bits;
114 static const int hash_shift = age_shift + age_bits + second_unused_gap_bits;
115
116 static const uintptr_t lock_mask = right_n_bits(lock_bits);
117 static const uintptr_t lock_mask_in_place = lock_mask << lock_shift;
118 static const uintptr_t age_mask = right_n_bits(age_bits);
119 static const uintptr_t age_mask_in_place = age_mask << age_shift;
120 static const uintptr_t hash_mask = right_n_bits(hash_bits);
121 static const uintptr_t hash_mask_in_place = hash_mask << hash_shift;
122
123 static const uintptr_t locked_value = 0;
124 static const uintptr_t unlocked_value = 1;
125 static const uintptr_t monitor_value = 2;
126 static const uintptr_t marked_value = 3;
127
128 static const uintptr_t no_hash = 0 ; // no hash value assigned
129 static const uintptr_t no_hash_in_place = (uintptr_t)no_hash << hash_shift;
130 static const uintptr_t no_lock_in_place = unlocked_value;
131
132 static const uint max_age = age_mask;
133
134 // Creates a markWord with all bits set to zero.
135 static markWord zero() { return markWord(uintptr_t(0)); }
136
137 // lock accessors (note that these assume lock_shift == 0)
138 bool is_locked() const {
139 return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
140 }
141 bool is_unlocked() const {
142 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
143 }
144 bool is_marked() const {
145 return (mask_bits(value(), lock_mask_in_place) == marked_value);
146 }
147 bool is_forwarded() const {
148 return (mask_bits(value(), lock_mask_in_place) == marked_value);
149 }
150 bool is_neutral() const { // Not locked, or marked - a "clean" neutral state
151 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
152 }
153
154 // Special temporary state of the markWord while being inflated.
155 // Code that looks at mark outside a lock need to take this into account.
156 bool is_being_inflated() const { return (value() == 0); }
157
158 // Distinguished markword value - used when inflating over
159 // an existing stack-lock. 0 indicates the markword is "BUSY".
160 // Lockword mutators that use a LD...CAS idiom should always
161 // check for and avoid overwriting a 0 value installed by some
162 // other thread. (They should spin or block instead. The 0 value
163 // is transient and *should* be short-lived).
164 // Fast-locking does not use INFLATING.
165 static markWord INFLATING() { return zero(); } // inflate-in-progress
166
167 // Should this header be preserved during GC?
168 bool must_be_preserved(const oopDesc* obj) const {
169 return (!is_unlocked() || !has_no_hash());
170 }
171
172 // WARNING: The following routines are used EXCLUSIVELY by
173 // synchronization functions. They are not really gc safe.
174 // They must get updated if markWord layout get changed.
175 markWord set_unlocked() const {
176 return markWord(value() | unlocked_value);
177 }
178 bool has_locker() const {
179 assert(LockingMode == LM_LEGACY, "should only be called with legacy stack locking");
180 return (value() & lock_mask_in_place) == locked_value;
181 }
182 BasicLock* locker() const {
183 assert(has_locker(), "check");
184 return (BasicLock*) value();
185 }
186
187 bool is_fast_locked() const {
188 assert(LockingMode == LM_LIGHTWEIGHT, "should only be called with new lightweight locking");
189 return (value() & lock_mask_in_place) == locked_value;
190 }
191 markWord set_fast_locked() const {
192 // Clear the lock_mask_in_place bits to set locked_value:
193 return markWord(value() & ~lock_mask_in_place);
194 }
195
196 bool has_monitor() const {
197 return ((value() & lock_mask_in_place) == monitor_value);
198 }
199 ObjectMonitor* monitor() const {
200 assert(has_monitor(), "check");
201 assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors");
202 // Use xor instead of &~ to provide one extra tag-bit check.
203 return (ObjectMonitor*) (value() ^ monitor_value);
204 }
205 bool has_displaced_mark_helper() const {
206 intptr_t lockbits = value() & lock_mask_in_place;
207 if (LockingMode == LM_LIGHTWEIGHT) {
208 return !UseObjectMonitorTable && lockbits == monitor_value;
209 }
210 // monitor (0b10) | stack-locked (0b00)?
211 return (lockbits & unlocked_value) == 0;
212 }
213 markWord displaced_mark_helper() const;
214 void set_displaced_mark_helper(markWord m) const;
215 markWord copy_set_hash(intptr_t hash) const {
216 uintptr_t tmp = value() & (~hash_mask_in_place);
217 tmp |= ((hash & hash_mask) << hash_shift);
218 return markWord(tmp);
219 }
220 // it is only used to be stored into BasicLock as the
221 // indicator that the lock is using heavyweight monitor
222 static markWord unused_mark() {
223 return markWord(marked_value);
224 }
225 // the following two functions create the markWord to be
226 // stored into object header, it encodes monitor info
227 static markWord encode(BasicLock* lock) {
228 return from_pointer(lock);
229 }
230 static markWord encode(ObjectMonitor* monitor) {
231 assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors");
232 uintptr_t tmp = (uintptr_t) monitor;
233 return markWord(tmp | monitor_value);
234 }
235
236 markWord set_has_monitor() const {
237 return markWord((value() & ~lock_mask_in_place) | monitor_value);
238 }
239
240 // used to encode pointers during GC
241 markWord clear_lock_bits() const { return markWord(value() & ~lock_mask_in_place); }
242
243 // age operations
244 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
245 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
246
247 uint age() const { return (uint) mask_bits(value() >> age_shift, age_mask); }
248 markWord set_age(uint v) const {
249 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
250 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
251 }
252 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
253
254 // hash operations
255 intptr_t hash() const {
256 return mask_bits(value() >> hash_shift, hash_mask);
257 }
258
259 bool has_no_hash() const {
260 return hash() == no_hash;
261 }
262
263 // Prototype mark for initialization
264 static markWord prototype() {
265 return markWord( no_hash_in_place | no_lock_in_place );
266 }
267
268 // Debugging
269 void print_on(outputStream* st, bool print_monitor_info = true) const;
270
271 // Prepare address of oop for placement into mark
272 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
273
274 // Recover address of oop from encoded form used in mark
275 inline void* decode_pointer() const { return (void*)clear_lock_bits().value(); }
276
277 inline oop forwardee() const {
278 return cast_to_oop(decode_pointer());
279 }
280 };
281
282 // Support atomic operations.
283 template<>
284 struct PrimitiveConversions::Translate<markWord> : public std::true_type {
285 typedef markWord Value;
286 typedef uintptr_t Decayed;
287
288 static Decayed decay(const Value& x) { return x.value(); }
289 static Value recover(Decayed x) { return Value(x); }
290 };
291
292 #endif // SHARE_OOPS_MARKWORD_HPP