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/compressedKlass.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 self-fwd:1 lock:2 (normal object)
42 //
43 // 64 bits:
44 // --------
45 // unused:22 hash:31 -->| unused_gap:4 age:4 self-fwd:1 lock:2 (normal object)
46 //
47 // 64 bits (with compact headers):
48 // -------------------------------
49 // klass:22 hash:31 -->| unused_gap:4 age:4 self-fwd: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, UseObjectMonitorTable == false)
62 // [header | 10] monitor inflated lock (UseObjectMonitorTable == true)
63 // [ptr | 11] marked used to mark an object
64 // [0 ............ 0| 00] inflating inflation in progress (stack-locking in use)
65 //
66 // We assume that stack/thread pointers have the lowest two bits cleared.
67 //
68 // - INFLATING() is a distinguished markword value of all zeros that is
69 // used when inflating an existing stack-lock into an ObjectMonitor.
70 // See below for is_being_inflated() and INFLATING().
71
72 class BasicLock;
73 class ObjectMonitor;
74 class JavaThread;
75 class outputStream;
76
77 class markWord {
78 private:
79 uintptr_t _value;
80
81 public:
82 explicit markWord(uintptr_t value) : _value(value) {}
83
84 markWord() = default; // Doesn't initialize _value.
85
86 // It is critical for performance that this class be trivially
87 // destructable, copyable, and assignable.
88 ~markWord() = default;
89 markWord(const markWord&) = default;
90 markWord& operator=(const markWord&) = default;
91
92 static markWord from_pointer(void* ptr) {
93 return markWord((uintptr_t)ptr);
94 }
95 void* to_pointer() const {
96 return (void*)_value;
97 }
98
99 bool operator==(const markWord& other) const {
100 return _value == other._value;
101 }
102 bool operator!=(const markWord& other) const {
103 return !operator==(other);
104 }
105
106 // Conversion
107 uintptr_t value() const { return _value; }
108
109 // Constants
110 static const int age_bits = 4;
111 static const int lock_bits = 2;
112 static const int self_fwd_bits = 1;
113 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - self_fwd_bits;
114 static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
115 static const int unused_gap_bits = LP64_ONLY(4) NOT_LP64(0); // Reserved for Valhalla.
116
117 static const int lock_shift = 0;
118 static const int self_fwd_shift = lock_shift + lock_bits;
119 static const int age_shift = self_fwd_shift + self_fwd_bits;
120 static const int hash_shift = age_shift + age_bits + unused_gap_bits;
121
122 static const uintptr_t lock_mask = right_n_bits(lock_bits);
123 static const uintptr_t lock_mask_in_place = lock_mask << lock_shift;
124 static const uintptr_t self_fwd_mask = right_n_bits(self_fwd_bits);
125 static const uintptr_t self_fwd_mask_in_place = self_fwd_mask << self_fwd_shift;
126 static const uintptr_t age_mask = right_n_bits(age_bits);
127 static const uintptr_t age_mask_in_place = age_mask << age_shift;
128 static const uintptr_t hash_mask = right_n_bits(hash_bits);
129 static const uintptr_t hash_mask_in_place = hash_mask << hash_shift;
130
131 #ifdef _LP64
132 // Used only with compact headers:
133 // We store the (narrow) Klass* in the bits 43 to 64.
134
135 // These are for bit-precise extraction of the narrow Klass* from the 64-bit Markword
136 static constexpr int klass_offset_in_bytes = 4;
137 static constexpr int klass_shift = hash_shift + hash_bits;
138 static constexpr int klass_shift_at_offset = klass_shift - klass_offset_in_bytes * BitsPerByte;
139 static constexpr int klass_bits = 22;
140 static constexpr uintptr_t klass_mask = right_n_bits(klass_bits);
141 static constexpr uintptr_t klass_mask_in_place = klass_mask << klass_shift;
142 #endif
143
144
145 static const uintptr_t locked_value = 0;
146 static const uintptr_t unlocked_value = 1;
147 static const uintptr_t monitor_value = 2;
148 static const uintptr_t marked_value = 3;
149
150 static const uintptr_t no_hash = 0 ; // no hash value assigned
151 static const uintptr_t no_hash_in_place = (uintptr_t)no_hash << hash_shift;
152 static const uintptr_t no_lock_in_place = unlocked_value;
153
154 static const uint max_age = age_mask;
155
156 // Creates a markWord with all bits set to zero.
157 static markWord zero() { return markWord(uintptr_t(0)); }
158
159 // lock accessors (note that these assume lock_shift == 0)
160 bool is_locked() const {
161 return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
162 }
163 bool is_unlocked() const {
164 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
165 }
166 bool is_marked() const {
167 return (mask_bits(value(), lock_mask_in_place) == marked_value);
168 }
169 bool is_forwarded() const {
170 // Returns true for normal forwarded (0b011) and self-forwarded (0b1xx).
171 return mask_bits(value(), lock_mask_in_place | self_fwd_mask_in_place) >= static_cast<intptr_t>(marked_value);
172 }
173 bool is_neutral() const { // Not locked, or marked - a "clean" neutral state
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 assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors");
225 // Use xor instead of &~ to provide one extra tag-bit check.
226 return (ObjectMonitor*) (value() ^ monitor_value);
227 }
228 bool has_displaced_mark_helper() const {
229 intptr_t lockbits = value() & lock_mask_in_place;
230 if (LockingMode == LM_LIGHTWEIGHT) {
231 return !UseObjectMonitorTable && lockbits == monitor_value;
232 }
233 // monitor (0b10) | stack-locked (0b00)?
234 return (lockbits & unlocked_value) == 0;
235 }
236 markWord displaced_mark_helper() const;
237 void set_displaced_mark_helper(markWord m) const;
238 markWord copy_set_hash(intptr_t hash) const {
239 uintptr_t tmp = value() & (~hash_mask_in_place);
240 tmp |= ((hash & hash_mask) << hash_shift);
241 return markWord(tmp);
242 }
243 // it is only used to be stored into BasicLock as the
244 // indicator that the lock is using heavyweight monitor
245 static markWord unused_mark() {
246 return markWord(marked_value);
247 }
248 // the following two functions create the markWord to be
249 // stored into object header, it encodes monitor info
250 static markWord encode(BasicLock* lock) {
251 return from_pointer(lock);
252 }
253 static markWord encode(ObjectMonitor* monitor) {
254 assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors");
255 uintptr_t tmp = (uintptr_t) monitor;
256 return markWord(tmp | monitor_value);
257 }
258
259 markWord set_has_monitor() const {
260 return markWord((value() & ~lock_mask_in_place) | monitor_value);
261 }
262
263 // used to encode pointers during GC
264 markWord clear_lock_bits() const { return markWord(value() & ~lock_mask_in_place); }
265
266 // age operations
267 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
268 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
269
270 uint age() const { return (uint) mask_bits(value() >> age_shift, age_mask); }
271 markWord set_age(uint v) const {
272 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
273 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
274 }
275 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
276
277 // hash operations
278 intptr_t hash() const {
279 return mask_bits(value() >> hash_shift, hash_mask);
280 }
281
282 bool has_no_hash() const {
283 return hash() == no_hash;
284 }
285
286 inline Klass* klass() const;
287 inline Klass* klass_or_null() const;
288 inline Klass* klass_without_asserts() const;
289 inline narrowKlass narrow_klass() const;
290 inline markWord set_narrow_klass(narrowKlass narrow_klass) const;
291
292 // Prototype mark for initialization
293 static markWord prototype() {
294 return markWord( no_hash_in_place | no_lock_in_place );
295 }
296
297 // Debugging
298 void print_on(outputStream* st, bool print_monitor_info = true) const;
299
300 // Prepare address of oop for placement into mark
301 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
302
303 // Recover address of oop from encoded form used in mark
304 inline void* decode_pointer() const { return (void*)clear_lock_bits().value(); }
305
306 inline bool is_self_forwarded() const {
307 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
308 return mask_bits(value(), self_fwd_mask_in_place) != 0;
309 }
310
311 inline markWord set_self_forwarded() const {
312 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
313 return markWord(value() | self_fwd_mask_in_place);
314 }
315
316 inline markWord unset_self_forwarded() const {
317 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
318 return markWord(value() & ~self_fwd_mask_in_place);
319 }
320
321 inline oop forwardee() const {
322 return cast_to_oop(decode_pointer());
323 }
324 };
325
326 // Support atomic operations.
327 template<>
328 struct PrimitiveConversions::Translate<markWord> : public std::true_type {
329 typedef markWord Value;
330 typedef uintptr_t Decayed;
331
332 static Decayed decay(const Value& x) { return x.value(); }
333 static Value recover(Decayed x) { return Value(x); }
334 };
335
336 #endif // SHARE_OOPS_MARKWORD_HPP