1 /*
2 * Copyright (c) 1997, 2021, 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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7 * published by the Free Software Foundation.
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13 * accompanied this code).
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23 */
24
25 #ifndef SHARE_OOPS_MARKWORD_HPP
26 #define SHARE_OOPS_MARKWORD_HPP
27
28 #include "gc/shared/gc_globals.hpp"
29 #include "metaprogramming/primitiveConversions.hpp"
30 #include "oops/oopsHierarchy.hpp"
31 #include "runtime/globals.hpp"
32
33 #include <type_traits>
34
35 // The markWord describes the header of an object.
36 //
37 // Bit-format of an object header (most significant first, big endian layout below):
38 //
39 // 32 bits:
40 // --------
41 // hash:25 ------------>| age:4 unused_gap:1 lock:2 (normal object)
42 //
43 // 64 bits:
44 // --------
45 // unused:25 hash:31 -->| unused_gap:1 age:4 unused_gap:1 lock:2 (normal object)
46 //
47 // 64 bits (with compact headers):
48 // -------------------------------
49 // nklass:32 hash:25 -->| unused_gap:1 age:4 self-fwded:1 lock:2 (normal object)
50 //
51 // - hash contains the identity hash value: largest value is
52 // 31 bits, see os::random(). Also, 64-bit vm's require
53 // a hash value no bigger than 32 bits because they will not
54 // properly generate a mask larger than that: see library_call.cpp
55 //
56 // - the two lock bits are used to describe three states: locked/unlocked and monitor.
57 //
58 // [ptr | 00] locked ptr points to real header on stack (stack-locking in use)
59 // [header | 00] locked locked regular object header (fast-locking in use)
60 // [header | 01] unlocked regular object header
61 // [ptr | 10] monitor inflated lock (header is swapped out)
62 // [ptr | 11] marked used to mark an object
63 // [0 ............ 0| 00] inflating inflation in progress (stack-locking in use)
64 //
65 // We assume that stack/thread pointers have the lowest two bits cleared.
66 //
67 // - INFLATING() is a distinguished markword value of all zeros that is
68 // used when inflating an existing stack-lock into an ObjectMonitor.
69 // See below for is_being_inflated() and INFLATING().
70
71 class BasicLock;
72 class ObjectMonitor;
73 class JavaThread;
74 class outputStream;
75
76 class markWord {
77 private:
78 uintptr_t _value;
79
80 public:
81 explicit markWord(uintptr_t value) : _value(value) {}
82
83 markWord() = default; // Doesn't initialize _value.
84
85 // It is critical for performance that this class be trivially
86 // destructable, copyable, and assignable.
87 ~markWord() = default;
88 markWord(const markWord&) = default;
89 markWord& operator=(const markWord&) = default;
90
91 static markWord from_pointer(void* ptr) {
92 return markWord((uintptr_t)ptr);
93 }
94 void* to_pointer() const {
95 return (void*)_value;
96 }
97
98 bool operator==(const markWord& other) const {
99 return _value == other._value;
100 }
101 bool operator!=(const markWord& other) const {
102 return !operator==(other);
103 }
104
105 // Conversion
106 uintptr_t value() const { return _value; }
107
108 // Constants
109 static const int age_bits = 4;
110 static const int lock_bits = 2;
111 static const int self_forwarded_bits = 1;
112 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - self_forwarded_bits;
113 static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
114 static const int hash_bits_compact = max_hash_bits > 25 ? 25 : max_hash_bits;
115 // Used only without compact headers.
116 static const int unused_gap_bits = LP64_ONLY(1) NOT_LP64(0);
117 #ifdef _LP64
118 // Used only with compact headers.
119 static const int klass_bits = 32;
120 #endif
121
122 static const int lock_shift = 0;
123 static const int self_forwarded_shift = lock_shift + lock_bits;
124 static const int age_shift = self_forwarded_shift + self_forwarded_bits;
125 static const int hash_shift = age_shift + age_bits + unused_gap_bits;
126 static const int hash_shift_compact = age_shift + age_bits;
127 #ifdef _LP64
128 // Used only with compact headers.
129 static const int klass_shift = hash_shift_compact + hash_bits_compact;
130 #endif
131
132 static const uintptr_t lock_mask = right_n_bits(lock_bits);
133 static const uintptr_t lock_mask_in_place = lock_mask << lock_shift;
134 static const uintptr_t self_forwarded_mask = right_n_bits(self_forwarded_bits);
135 static const uintptr_t self_forwarded_mask_in_place = self_forwarded_mask << self_forwarded_shift;
136 static const uintptr_t age_mask = right_n_bits(age_bits);
137 static const uintptr_t age_mask_in_place = age_mask << age_shift;
138 static const uintptr_t hash_mask = right_n_bits(hash_bits);
139 static const uintptr_t hash_mask_in_place = hash_mask << hash_shift;
140 static const uintptr_t hash_mask_compact = right_n_bits(hash_bits_compact);
141 static const uintptr_t hash_mask_compact_in_place = hash_mask_compact << hash_shift_compact;
142 #ifdef _LP64
143 // Used only with compact headers.
144 static const uintptr_t klass_mask = right_n_bits(klass_bits);
145 static const uintptr_t klass_mask_in_place = klass_mask << klass_shift;
146 #endif
147
148
149 static const uintptr_t locked_value = 0;
150 static const uintptr_t unlocked_value = 1;
151 static const uintptr_t monitor_value = 2;
152 static const uintptr_t marked_value = 3;
153
154 static const uintptr_t no_hash = 0 ; // no hash value assigned
155 static const uintptr_t no_hash_in_place = (address_word)no_hash << hash_shift;
156 static const uintptr_t no_lock_in_place = unlocked_value;
157
158 static const uint max_age = age_mask;
159
160 // Creates a markWord with all bits set to zero.
161 static markWord zero() { return markWord(uintptr_t(0)); }
162
163 // lock accessors (note that these assume lock_shift == 0)
164 bool is_locked() const {
165 return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
166 }
167 bool is_unlocked() const {
168 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
169 }
170 bool is_marked() const {
171 return (mask_bits(value(), lock_mask_in_place) == marked_value);
172 }
173 bool is_neutral() const {
174 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
175 }
176
177 // Special temporary state of the markWord while being inflated.
178 // Code that looks at mark outside a lock need to take this into account.
179 bool is_being_inflated() const { return (value() == 0); }
180
181 // Distinguished markword value - used when inflating over
182 // an existing stack-lock. 0 indicates the markword is "BUSY".
183 // Lockword mutators that use a LD...CAS idiom should always
184 // check for and avoid overwriting a 0 value installed by some
185 // other thread. (They should spin or block instead. The 0 value
186 // is transient and *should* be short-lived).
187 // Fast-locking does not use INFLATING.
188 static markWord INFLATING() { return zero(); } // inflate-in-progress
189
190 // Should this header be preserved during GC?
191 bool must_be_preserved(const oopDesc* obj) const {
192 return (!is_unlocked() || !has_no_hash());
193 }
194
195 // WARNING: The following routines are used EXCLUSIVELY by
196 // synchronization functions. They are not really gc safe.
197 // They must get updated if markWord layout get changed.
198 markWord set_unlocked() const {
199 return markWord(value() | unlocked_value);
200 }
201 bool has_locker() const {
202 assert(LockingMode == LM_LEGACY, "should only be called with legacy stack locking");
203 return (value() & lock_mask_in_place) == locked_value;
204 }
205 BasicLock* locker() const {
206 assert(has_locker(), "check");
207 return (BasicLock*) value();
208 }
209
210 bool is_fast_locked() const {
211 assert(LockingMode == LM_LIGHTWEIGHT, "should only be called with new lightweight locking");
212 return (value() & lock_mask_in_place) == locked_value;
213 }
214 markWord set_fast_locked() const {
215 // Clear the lock_mask_in_place bits to set locked_value:
216 return markWord(value() & ~lock_mask_in_place);
217 }
218
219 bool has_monitor() const {
220 return ((value() & lock_mask_in_place) == monitor_value);
221 }
222 ObjectMonitor* monitor() const {
223 assert(has_monitor(), "check");
224 // Use xor instead of &~ to provide one extra tag-bit check.
225 return (ObjectMonitor*) (value() ^ monitor_value);
226 }
227 bool has_displaced_mark_helper() const {
228 intptr_t lockbits = value() & lock_mask_in_place;
229 return LockingMode == LM_LIGHTWEIGHT ? lockbits == monitor_value // monitor?
230 : (lockbits & unlocked_value) == 0; // monitor | stack-locked?
231 }
232 markWord displaced_mark_helper() const;
233 void set_displaced_mark_helper(markWord m) const;
234 markWord copy_set_hash(intptr_t hash) const {
235 if (UseCompactObjectHeaders) {
236 uintptr_t tmp = value() & (~hash_mask_compact_in_place);
237 tmp |= ((hash & hash_mask_compact) << hash_shift_compact);
238 return markWord(tmp);
239 } else {
240 uintptr_t tmp = value() & (~hash_mask_in_place);
241 tmp |= ((hash & hash_mask) << hash_shift);
242 return markWord(tmp);
243 }
244 }
245 // it is only used to be stored into BasicLock as the
246 // indicator that the lock is using heavyweight monitor
247 static markWord unused_mark() {
248 return markWord(marked_value);
249 }
250 // the following two functions create the markWord to be
251 // stored into object header, it encodes monitor info
252 static markWord encode(BasicLock* lock) {
253 return from_pointer(lock);
254 }
255 static markWord encode(ObjectMonitor* monitor) {
256 uintptr_t tmp = (uintptr_t) monitor;
257 return markWord(tmp | monitor_value);
258 }
259
260 // used to encode pointers during GC
261 markWord clear_lock_bits() { return markWord(value() & ~lock_mask_in_place); }
262
263 // age operations
264 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
265 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
266
267 uint age() const { return mask_bits(value() >> age_shift, age_mask); }
268 markWord set_age(uint v) const {
269 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
270 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
271 }
272 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
273
274 // hash operations
275 intptr_t hash() const {
276 if (UseCompactObjectHeaders) {
277 return mask_bits(value() >> hash_shift_compact, hash_mask_compact);
278 } else {
279 return mask_bits(value() >> hash_shift, hash_mask);
280 }
281 }
282
283 bool has_no_hash() const {
284 return hash() == no_hash;
285 }
286
287 #ifdef _LP64
288 inline markWord actual_mark() const;
289 inline Klass* klass() const;
290 inline Klass* klass_or_null() const;
291 inline narrowKlass narrow_klass() const;
292 inline markWord set_narrow_klass(narrowKlass nklass) const;
293 inline markWord set_klass(Klass* klass) const;
294 #endif
295
296 // Prototype mark for initialization
297 static markWord prototype() {
298 return markWord( no_hash_in_place | no_lock_in_place );
299 }
300
301 // Debugging
302 void print_on(outputStream* st, bool print_monitor_info = true) const;
303
304 // Prepare address of oop for placement into mark
305 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
306
307 // Recover address of oop from encoded form used in mark
308 inline void* decode_pointer() { return (void*)clear_lock_bits().value(); }
309
310 #ifdef _LP64
311 inline bool self_forwarded() const {
312 bool self_fwd = mask_bits(value(), self_forwarded_mask_in_place) != 0;
313 assert(!self_fwd || UseAltGCForwarding, "Only set self-fwd bit when using alt GC forwarding");
314 return self_fwd;
315 }
316
317 inline markWord set_self_forwarded() const {
318 assert(UseAltGCForwarding, "Only call this with alt GC forwarding");
319 return markWord(value() | self_forwarded_mask_in_place | marked_value);
320 }
321 #endif
322 };
323
324 // Support atomic operations.
325 template<>
326 struct PrimitiveConversions::Translate<markWord> : public std::true_type {
327 typedef markWord Value;
328 typedef uintptr_t Decayed;
329
330 static Decayed decay(const Value& x) { return x.value(); }
331 static Value recover(Decayed x) { return Value(x); }
332 };
333
334 #endif // SHARE_OOPS_MARKWORD_HPP