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 "cppstdlib/type_traits.hpp"
29 #include "metaprogramming/primitiveConversions.hpp"
30 #include "oops/compressedKlass.hpp"
31 #include "oops/oopsHierarchy.hpp"
32 #include "runtime/globals.hpp"
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 self-fwd:1 lock:2 (normal object)
41 //
42 // 64 bits:
43 // --------
44 // unused:22 hash:31 -->| unused_gap:4 age:4 self-fwd:1 lock:2 (normal object)
45 //
46 // 64 bits (with compact headers):
47 // -------------------------------
48 // klass:22 hash:31 -->| unused_gap:4 age:4 self-fwd:1 lock:2 (normal object)
49 //
50 // - hash contains the identity hash value: largest value is
51 // 31 bits, see os::random(). Also, 64-bit vm's require
52 // a hash value no bigger than 32 bits because they will not
53 // properly generate a mask larger than that: see library_call.cpp
54 //
55 // - the two lock bits are used to describe three states: locked/unlocked and monitor.
56 //
57 // [header | 00] locked locked regular object header (fast-locking in use)
58 // [header | 01] unlocked regular object header
59 // [ptr | 10] monitor inflated lock (header is swapped out, UseObjectMonitorTable == false)
60 // [header | 10] monitor inflated lock (UseObjectMonitorTable == true)
61 // [ptr | 11] marked used to mark an object
62
63 class BasicLock;
64 class ObjectMonitor;
65 class JavaThread;
66 class outputStream;
67
68 class markWord {
69 private:
70 uintptr_t _value;
71
72 public:
73 explicit markWord(uintptr_t value) : _value(value) {}
74
75 markWord() = default; // Doesn't initialize _value.
76
77 // It is critical for performance that this class be trivially
78 // destructable, copyable, and assignable.
79 ~markWord() = default;
80 markWord(const markWord&) = default;
81 markWord& operator=(const markWord&) = default;
82
83 static markWord from_pointer(void* ptr) {
84 return markWord((uintptr_t)ptr);
85 }
86 void* to_pointer() const {
87 return (void*)_value;
88 }
89
90 bool operator==(const markWord& other) const {
91 return _value == other._value;
92 }
93 bool operator!=(const markWord& other) const {
94 return !operator==(other);
95 }
96
97 // Conversion
98 uintptr_t value() const { return _value; }
99
100 // Constants
101 static const int age_bits = 4;
102 static const int lock_bits = 2;
103 static const int self_fwd_bits = 1;
104 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - self_fwd_bits;
105 static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
106 static const int unused_gap_bits = LP64_ONLY(4) NOT_LP64(0); // Reserved for Valhalla.
107
108 static const int lock_shift = 0;
109 static const int self_fwd_shift = lock_shift + lock_bits;
110 static const int age_shift = self_fwd_shift + self_fwd_bits;
111 static const int hash_shift = age_shift + age_bits + unused_gap_bits;
112
113 static const uintptr_t lock_mask = right_n_bits(lock_bits);
114 static const uintptr_t lock_mask_in_place = lock_mask << lock_shift;
115 static const uintptr_t self_fwd_mask = right_n_bits(self_fwd_bits);
116 static const uintptr_t self_fwd_mask_in_place = self_fwd_mask << self_fwd_shift;
117 static const uintptr_t age_mask = right_n_bits(age_bits);
118 static const uintptr_t age_mask_in_place = age_mask << age_shift;
119 static const uintptr_t hash_mask = right_n_bits(hash_bits);
120 static const uintptr_t hash_mask_in_place = hash_mask << hash_shift;
121
122 #ifdef _LP64
123 // Used only with compact headers:
124 // We store the (narrow) Klass* in the bits 43 to 64.
125
126 // These are for bit-precise extraction of the narrow Klass* from the 64-bit Markword
127 static constexpr int klass_offset_in_bytes = 4;
128 static constexpr int klass_shift = hash_shift + hash_bits;
129 static constexpr int klass_shift_at_offset = klass_shift - klass_offset_in_bytes * BitsPerByte;
130 static constexpr int klass_bits = 22;
131 static constexpr uintptr_t klass_mask = right_n_bits(klass_bits);
132 static constexpr uintptr_t klass_mask_in_place = klass_mask << klass_shift;
133 #endif
134
135
136 static const uintptr_t locked_value = 0;
137 static const uintptr_t unlocked_value = 1;
138 static const uintptr_t monitor_value = 2;
139 static const uintptr_t marked_value = 3;
140
141 static const uintptr_t no_hash = 0 ; // no hash value assigned
142 static const uintptr_t no_hash_in_place = (uintptr_t)no_hash << hash_shift;
143 static const uintptr_t no_lock_in_place = unlocked_value;
144
145 static const uint max_age = age_mask;
146
147 // Creates a markWord with all bits set to zero.
148 static markWord zero() { return markWord(uintptr_t(0)); }
149
150 // lock accessors (note that these assume lock_shift == 0)
151 bool is_locked() const {
152 return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
153 }
154 bool is_unlocked() const {
155 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
156 }
157 bool is_marked() const {
158 return (mask_bits(value(), lock_mask_in_place) == marked_value);
159 }
160 bool is_forwarded() const {
161 // Returns true for normal forwarded (0b011) and self-forwarded (0b1xx).
162 return mask_bits(value(), lock_mask_in_place | self_fwd_mask_in_place) >= static_cast<intptr_t>(marked_value);
163 }
164 bool is_neutral() const { // Not locked, or marked - a "clean" neutral state
165 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
166 }
167
168 // Should this header be preserved during GC?
169 bool must_be_preserved() const {
170 return (!is_unlocked() || !has_no_hash());
171 }
172
173 // WARNING: The following routines are used EXCLUSIVELY by
174 // synchronization functions. They are not really gc safe.
175 // They must get updated if markWord layout get changed.
176 markWord set_unlocked() const {
177 return markWord(value() | unlocked_value);
178 }
179
180 bool is_fast_locked() const {
181 return (value() & lock_mask_in_place) == locked_value;
182 }
183 markWord set_fast_locked() const {
184 // Clear the lock_mask_in_place bits to set locked_value:
185 return markWord(value() & ~lock_mask_in_place);
186 }
187
188 bool has_monitor() const {
189 return ((value() & lock_mask_in_place) == monitor_value);
190 }
191 markWord set_has_monitor() const {
192 return markWord((value() & ~lock_mask_in_place) | monitor_value);
193 }
194 ObjectMonitor* monitor() const {
195 assert(has_monitor(), "check");
196 assert(!UseObjectMonitorTable, "Locking with OM table does not use markWord for monitors");
197 // Use xor instead of &~ to provide one extra tag-bit check.
198 return (ObjectMonitor*) (value() ^ monitor_value);
199 }
200
201 static markWord encode(ObjectMonitor* monitor) {
202 assert(!UseObjectMonitorTable, "Locking with OM table does not use markWord for monitors");
203 uintptr_t tmp = (uintptr_t) monitor;
204 return markWord(tmp | monitor_value);
205 }
206
207 bool has_displaced_mark_helper() const {
208 intptr_t lockbits = value() & lock_mask_in_place;
209 return !UseObjectMonitorTable && lockbits == monitor_value;
210 }
211 markWord displaced_mark_helper() const;
212 void set_displaced_mark_helper(markWord m) const;
213
214 // used to encode pointers during GC
215 markWord clear_lock_bits() const { return markWord(value() & ~lock_mask_in_place); }
216
217 // age operations
218 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
219 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
220
221 uint age() const { return (uint) mask_bits(value() >> age_shift, age_mask); }
222 markWord set_age(uint v) const {
223 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
224 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
225 }
226 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
227
228 // hash operations
229 intptr_t hash() const {
230 return mask_bits(value() >> hash_shift, hash_mask);
231 }
232
233 bool has_no_hash() const {
234 return hash() == no_hash;
235 }
236
237 markWord copy_set_hash(intptr_t hash) const {
238 uintptr_t tmp = value() & (~hash_mask_in_place);
239 tmp |= ((hash & hash_mask) << hash_shift);
240 return markWord(tmp);
241 }
242
243 inline Klass* klass() const;
244 inline Klass* klass_or_null() const;
245 inline Klass* klass_without_asserts() const;
246 inline narrowKlass narrow_klass() const;
247 inline markWord set_narrow_klass(narrowKlass narrow_klass) const;
248
249 // Prototype mark for initialization
250 static markWord prototype() {
251 return markWord( no_hash_in_place | no_lock_in_place );
252 }
253
254 // Debugging
255 void print_on(outputStream* st, bool print_monitor_info = true) const;
256
257 // Prepare address of oop for placement into mark
258 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
259
260 // Recover address of oop from encoded form used in mark
261 inline void* decode_pointer() const { return (void*)clear_lock_bits().value(); }
262
263 inline bool is_self_forwarded() const {
264 return mask_bits(value(), self_fwd_mask_in_place) != 0;
265 }
266
267 inline markWord set_self_forwarded() const {
268 return markWord(value() | self_fwd_mask_in_place);
269 }
270
271 inline markWord unset_self_forwarded() const {
272 return markWord(value() & ~self_fwd_mask_in_place);
273 }
274
275 inline oop forwardee() const {
276 return cast_to_oop(decode_pointer());
277 }
278 };
279
280 // Support atomic operations.
281 template<>
282 struct PrimitiveConversions::Translate<markWord> : public std::true_type {
283 typedef markWord Value;
284 typedef uintptr_t Decayed;
285
286 static Decayed decay(const Value& x) { return x.value(); }
287 static Value recover(Decayed x) { return Value(x); }
288 };
289
290 #endif // SHARE_OOPS_MARKWORD_HPP