1 /*
2 * Copyright (c) 1997, 2023, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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5 * This code is free software; you can redistribute it and/or modify it
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23 */
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)
57 // [ptr | 11] marked used to mark an object
58 // [0 ............ 0| 00] inflating inflation in progress (stack-locking in use)
59 //
60 // We assume that stack/thread pointers have the lowest two bits cleared.
61 //
62 // - INFLATING() is a distinguished markword value of all zeros that is
63 // used when inflating an existing stack-lock into an ObjectMonitor.
64 // See below for is_being_inflated() and INFLATING().
65
66 class BasicLock;
67 class ObjectMonitor;
68 class JavaThread;
69 class outputStream;
70
71 class markWord {
72 private:
73 uintptr_t _value;
74
75 public:
76 explicit markWord(uintptr_t value) : _value(value) {}
77
78 markWord() = default; // Doesn't initialize _value.
79
80 // It is critical for performance that this class be trivially
81 // destructable, copyable, and assignable.
82 ~markWord() = default;
83 markWord(const markWord&) = default;
84 markWord& operator=(const markWord&) = default;
85
86 static markWord from_pointer(void* ptr) {
87 return markWord((uintptr_t)ptr);
88 }
89 void* to_pointer() const {
90 return (void*)_value;
91 }
92
93 bool operator==(const markWord& other) const {
94 return _value == other._value;
95 }
96 bool operator!=(const markWord& other) const {
97 return !operator==(other);
98 }
99
100 // Conversion
101 uintptr_t value() const { return _value; }
102
103 // Constants
104 static const int age_bits = 4;
105 static const int lock_bits = 2;
106 static const int first_unused_gap_bits = 1;
107 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - first_unused_gap_bits;
108 static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
109 static const int second_unused_gap_bits = LP64_ONLY(1) NOT_LP64(0);
110
111 static const int lock_shift = 0;
112 static const int age_shift = lock_bits + first_unused_gap_bits;
113 static const int hash_shift = age_shift + age_bits + second_unused_gap_bits;
114
115 static const uintptr_t lock_mask = right_n_bits(lock_bits);
116 static const uintptr_t lock_mask_in_place = lock_mask << lock_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 static const uintptr_t locked_value = 0;
123 static const uintptr_t unlocked_value = 1;
124 static const uintptr_t monitor_value = 2;
125 static const uintptr_t marked_value = 3;
126
127 static const uintptr_t no_hash = 0 ; // no hash value assigned
128 static const uintptr_t no_hash_in_place = (uintptr_t)no_hash << hash_shift;
129 static const uintptr_t no_lock_in_place = unlocked_value;
130
131 static const uint max_age = age_mask;
132
133 // Creates a markWord with all bits set to zero.
134 static markWord zero() { return markWord(uintptr_t(0)); }
135
136 // lock accessors (note that these assume lock_shift == 0)
137 bool is_locked() const {
138 return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
139 }
140 bool is_unlocked() const {
141 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
142 }
143 bool is_marked() const {
144 return (mask_bits(value(), lock_mask_in_place) == marked_value);
145 }
146 bool is_neutral() const {
147 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
148 }
149
150 // Special temporary state of the markWord while being inflated.
151 // Code that looks at mark outside a lock need to take this into account.
152 bool is_being_inflated() const { return (value() == 0); }
153
154 // Distinguished markword value - used when inflating over
155 // an existing stack-lock. 0 indicates the markword is "BUSY".
156 // Lockword mutators that use a LD...CAS idiom should always
157 // check for and avoid overwriting a 0 value installed by some
158 // other thread. (They should spin or block instead. The 0 value
159 // is transient and *should* be short-lived).
160 // Fast-locking does not use INFLATING.
161 static markWord INFLATING() { return zero(); } // inflate-in-progress
162
163 // Should this header be preserved during GC?
164 bool must_be_preserved(const oopDesc* obj) const {
165 return (!is_unlocked() || !has_no_hash());
166 }
167
168 // WARNING: The following routines are used EXCLUSIVELY by
169 // synchronization functions. They are not really gc safe.
170 // They must get updated if markWord layout get changed.
171 markWord set_unlocked() const {
172 return markWord(value() | unlocked_value);
173 }
174 bool has_locker() const {
175 assert(LockingMode == LM_LEGACY, "should only be called with legacy stack locking");
176 return (value() & lock_mask_in_place) == locked_value;
177 }
178 BasicLock* locker() const {
179 assert(has_locker(), "check");
180 return (BasicLock*) value();
181 }
182
183 bool is_fast_locked() const {
184 assert(LockingMode == LM_LIGHTWEIGHT, "should only be called with new lightweight locking");
185 return (value() & lock_mask_in_place) == locked_value;
186 }
187 markWord set_fast_locked() const {
188 // Clear the lock_mask_in_place bits to set locked_value:
189 return markWord(value() & ~lock_mask_in_place);
190 }
191
192 bool has_monitor() const {
193 return ((value() & lock_mask_in_place) == monitor_value);
194 }
195 ObjectMonitor* monitor() const {
196 assert(has_monitor(), "check");
197 // Use xor instead of &~ to provide one extra tag-bit check.
198 return (ObjectMonitor*) (value() ^ monitor_value);
199 }
200 bool has_displaced_mark_helper() const {
201 intptr_t lockbits = value() & lock_mask_in_place;
202 return LockingMode == LM_LIGHTWEIGHT ? lockbits == monitor_value // monitor?
203 : (lockbits & unlocked_value) == 0; // monitor | stack-locked?
204 }
205 markWord displaced_mark_helper() const;
206 void set_displaced_mark_helper(markWord m) const;
207 markWord copy_set_hash(intptr_t hash) const {
208 uintptr_t tmp = value() & (~hash_mask_in_place);
209 tmp |= ((hash & hash_mask) << hash_shift);
210 return markWord(tmp);
211 }
212 // it is only used to be stored into BasicLock as the
213 // indicator that the lock is using heavyweight monitor
214 static markWord unused_mark() {
215 return markWord(marked_value);
216 }
217 // the following two functions create the markWord to be
218 // stored into object header, it encodes monitor info
219 static markWord encode(BasicLock* lock) {
220 return from_pointer(lock);
221 }
222 static markWord encode(ObjectMonitor* monitor) {
223 uintptr_t tmp = (uintptr_t) monitor;
224 return markWord(tmp | monitor_value);
225 }
226
227 // used to encode pointers during GC
228 markWord clear_lock_bits() { return markWord(value() & ~lock_mask_in_place); }
229
230 // age operations
231 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
232 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
233
234 uint age() const { return (uint) mask_bits(value() >> age_shift, age_mask); }
235 markWord set_age(uint v) const {
236 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
237 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
238 }
239 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
240
241 // hash operations
242 intptr_t hash() const {
243 return mask_bits(value() >> hash_shift, hash_mask);
244 }
245
246 bool has_no_hash() const {
247 return hash() == no_hash;
248 }
249
250 // Prototype mark for initialization
251 static markWord prototype() {
252 return markWord( no_hash_in_place | no_lock_in_place );
253 }
254
255 // Debugging
256 void print_on(outputStream* st, bool print_monitor_info = true) const;
257
258 // Prepare address of oop for placement into mark
259 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
260
261 // Recover address of oop from encoded form used in mark
262 inline void* decode_pointer() { return (void*)clear_lock_bits().value(); }
263 };
264
265 // Support atomic operations.
266 template<>
267 struct PrimitiveConversions::Translate<markWord> : public std::true_type {
268 typedef markWord Value;
269 typedef uintptr_t Decayed;
270
271 static Decayed decay(const Value& x) { return x.value(); }
272 static Value recover(Decayed x) { return Value(x); }
273 };
274
275 #endif // SHARE_OOPS_MARKWORD_HPP