lock_word.h revision 2ffb703bf431d74326c88266b4ddaf225eb3c6ad 
1/*
2 * Copyright (C) 2011 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#ifndef ART_RUNTIME_LOCK_WORD_H_
18#define ART_RUNTIME_LOCK_WORD_H_
19
20#include <cstdint>
21#include <iosfwd>
22
23#include "base/bit_utils.h"
24#include "base/logging.h"
25#include "read_barrier.h"
26
27namespace art {
28namespace mirror {
29class Object;
30}  // namespace mirror
31
32class Monitor;
33
34/* The lock value itself as stored in mirror::Object::monitor_.  The two most significant bits of
35 * the state. The four possible states are fat locked, thin/unlocked, hash code, and forwarding
36 * address. When the lock word is in the "thin" state and its bits are formatted as follows:
37 *
38 *  |33|2|2|222222221111|1111110000000000|
39 *  |10|9|8|765432109876|5432109876543210|
40 *  |00|m|r| lock count |thread id owner |
41 *
42 * When the lock word is in the "fat" state and its bits are formatted as follows:
43 *
44 *  |33|2|2|2222222211111111110000000000|
45 *  |10|9|8|7654321098765432109876543210|
46 *  |01|m|r| MonitorId                  |
47 *
48 * When the lock word is in hash state and its bits are formatted as follows:
49 *
50 *  |33|2|2|2222222211111111110000000000|
51 *  |10|9|8|7654321098765432109876543210|
52 *  |10|m|r| HashCode                   |
53 *
54 * When the lock word is in forwarding address state and its bits are formatted as follows:
55 *
56 *  |33|2|22222222211111111110000000000|
57 *  |10|9|87654321098765432109876543210|
58 *  |11|0| ForwardingAddress           |
59 *
60 * The `r` bit stores the read barrier state.
61 * The `m` bit stores the mark state.
62 */
63class LockWord {
64 public:
65  enum SizeShiftsAndMasks : uint32_t {  // private marker to avoid generate-operator-out.py from processing.
66    // Number of bits to encode the state, currently just fat or thin/unlocked or hash code.
67    kStateSize = 2,
68    kReadBarrierStateSize = 1,
69    kMarkBitStateSize = 1,
70    // Number of bits to encode the thin lock owner.
71    kThinLockOwnerSize = 16,
72    // Remaining bits are the recursive lock count.
73    kThinLockCountSize = 32 - kThinLockOwnerSize - kStateSize - kReadBarrierStateSize -
74        kMarkBitStateSize,
75    // Thin lock bits. Owner in lowest bits.
76
77    kThinLockOwnerShift = 0,
78    kThinLockOwnerMask = (1 << kThinLockOwnerSize) - 1,
79    kThinLockMaxOwner = kThinLockOwnerMask,
80    // Count in higher bits.
81    kThinLockCountShift = kThinLockOwnerSize + kThinLockOwnerShift,
82    kThinLockCountMask = (1 << kThinLockCountSize) - 1,
83    kThinLockMaxCount = kThinLockCountMask,
84    kThinLockCountOne = 1 << kThinLockCountShift,  // == 65536 (0x10000)
85
86    // State in the highest bits.
87    kStateShift = kReadBarrierStateSize + kThinLockCountSize + kThinLockCountShift +
88        kMarkBitStateSize,
89    kStateMask = (1 << kStateSize) - 1,
90    kStateMaskShifted = kStateMask << kStateShift,
91    kStateThinOrUnlocked = 0,
92    kStateFat = 1,
93    kStateHash = 2,
94    kStateForwardingAddress = 3,
95    kStateForwardingAddressShifted = kStateForwardingAddress << kStateShift,
96    kStateForwardingAddressOverflow = (1 + kStateMask - kStateForwardingAddress) << kStateShift,
97
98    // Read barrier bit.
99    kReadBarrierStateShift = kThinLockCountSize + kThinLockCountShift,
100    kReadBarrierStateMask = (1 << kReadBarrierStateSize) - 1,
101    kReadBarrierStateMaskShifted = kReadBarrierStateMask << kReadBarrierStateShift,
102    kReadBarrierStateMaskShiftedToggled = ~kReadBarrierStateMaskShifted,
103
104    // Mark bit.
105    kMarkBitStateShift = kReadBarrierStateSize + kReadBarrierStateShift,
106    kMarkBitStateMask = (1 << kMarkBitStateSize) - 1,
107    kMarkBitStateMaskShifted = kMarkBitStateMask << kMarkBitStateShift,
108    kMarkBitStateMaskShiftedToggled = ~kMarkBitStateMaskShifted,
109
110    // GC state is mark bit and read barrier state.
111    kGCStateSize = kReadBarrierStateSize + kMarkBitStateSize,
112    kGCStateShift = kReadBarrierStateShift,
113    kGCStateMaskShifted = kReadBarrierStateMaskShifted | kMarkBitStateMaskShifted,
114    kGCStateMaskShiftedToggled = ~kGCStateMaskShifted,
115
116    // When the state is kHashCode, the non-state bits hold the hashcode.
117    // Note Object.hashCode() has the hash code layout hardcoded.
118    kHashShift = 0,
119    kHashSize = 32 - kStateSize - kReadBarrierStateSize - kMarkBitStateSize,
120    kHashMask = (1 << kHashSize) - 1,
121    kMaxHash = kHashMask,
122
123    // Forwarding address shift.
124    kForwardingAddressShift = kObjectAlignmentShift,
125
126    kMonitorIdShift = kHashShift,
127    kMonitorIdSize = kHashSize,
128    kMonitorIdMask = kHashMask,
129    kMonitorIdAlignmentShift = 32 - kMonitorIdSize,
130    kMonitorIdAlignment = 1 << kMonitorIdAlignmentShift,
131    kMaxMonitorId = kMaxHash
132  };
133
134  static LockWord FromThinLockId(uint32_t thread_id, uint32_t count, uint32_t gc_state) {
135    CHECK_LE(thread_id, static_cast<uint32_t>(kThinLockMaxOwner));
136    CHECK_LE(count, static_cast<uint32_t>(kThinLockMaxCount));
137    // DCHECK_EQ(gc_bits & kGCStateMaskToggled, 0U);
138    return LockWord((thread_id << kThinLockOwnerShift) |
139                    (count << kThinLockCountShift) |
140                    (gc_state << kGCStateShift) |
141                    (kStateThinOrUnlocked << kStateShift));
142  }
143
144  static LockWord FromForwardingAddress(size_t target) {
145    DCHECK_ALIGNED(target, (1 << kStateSize));
146    return LockWord((target >> kForwardingAddressShift) | kStateForwardingAddressShifted);
147  }
148
149  static LockWord FromHashCode(uint32_t hash_code, uint32_t gc_state) {
150    CHECK_LE(hash_code, static_cast<uint32_t>(kMaxHash));
151    // DCHECK_EQ(gc_bits & kGCStateMaskToggled, 0U);
152    return LockWord((hash_code << kHashShift) |
153                    (gc_state << kGCStateShift) |
154                    (kStateHash << kStateShift));
155  }
156
157  static LockWord FromDefault(uint32_t gc_state) {
158    return LockWord(gc_state << kGCStateShift);
159  }
160
161  static bool IsDefault(LockWord lw) {
162    return LockWord().GetValue() == lw.GetValue();
163  }
164
165  static LockWord Default() {
166    return LockWord();
167  }
168
169  enum LockState {
170    kUnlocked,    // No lock owners.
171    kThinLocked,  // Single uncontended owner.
172    kFatLocked,   // See associated monitor.
173    kHashCode,    // Lock word contains an identity hash.
174    kForwardingAddress,  // Lock word contains the forwarding address of an object.
175  };
176
177  LockState GetState() const {
178    CheckReadBarrierState();
179    if ((!kUseReadBarrier && UNLIKELY(value_ == 0)) ||
180        (kUseReadBarrier && UNLIKELY((value_ & kGCStateMaskShiftedToggled) == 0))) {
181      return kUnlocked;
182    } else {
183      uint32_t internal_state = (value_ >> kStateShift) & kStateMask;
184      switch (internal_state) {
185        case kStateThinOrUnlocked:
186          return kThinLocked;
187        case kStateHash:
188          return kHashCode;
189        case kStateForwardingAddress:
190          return kForwardingAddress;
191        default:
192          DCHECK_EQ(internal_state, static_cast<uint32_t>(kStateFat));
193          return kFatLocked;
194      }
195    }
196  }
197
198  uint32_t ReadBarrierState() const {
199    return (value_ >> kReadBarrierStateShift) & kReadBarrierStateMask;
200  }
201
202  uint32_t GCState() const {
203    return (value_ & kGCStateMaskShifted) >> kGCStateShift;
204  }
205
206  void SetReadBarrierState(uint32_t rb_state) {
207    DCHECK_EQ(rb_state & ~kReadBarrierStateMask, 0U);
208    DCHECK(rb_state == ReadBarrier::WhiteState() ||
209           rb_state == ReadBarrier::GrayState()) << rb_state;
210    DCHECK_NE(static_cast<uint32_t>(GetState()), static_cast<uint32_t>(kForwardingAddress));
211    // Clear and or the bits.
212    value_ &= ~(kReadBarrierStateMask << kReadBarrierStateShift);
213    value_ |= (rb_state & kReadBarrierStateMask) << kReadBarrierStateShift;
214  }
215
216
217  uint32_t MarkBitState() const {
218    return (value_ >> kMarkBitStateShift) & kMarkBitStateMask;
219  }
220
221  void SetMarkBitState(uint32_t mark_bit) {
222    DCHECK_EQ(mark_bit & ~kMarkBitStateMask, 0U);
223    DCHECK_NE(static_cast<uint32_t>(GetState()), static_cast<uint32_t>(kForwardingAddress));
224    // Clear and or the bits.
225    value_ &= kMarkBitStateMaskShiftedToggled;
226    value_ |= mark_bit << kMarkBitStateShift;
227  }
228
229  // Return the owner thin lock thread id.
230  uint32_t ThinLockOwner() const;
231
232  // Return the number of times a lock value has been locked.
233  uint32_t ThinLockCount() const;
234
235  // Return the Monitor encoded in a fat lock.
236  Monitor* FatLockMonitor() const;
237
238  // Return the forwarding address stored in the monitor.
239  size_t ForwardingAddress() const;
240
241  // Constructor a lock word for inflation to use a Monitor.
242  LockWord(Monitor* mon, uint32_t gc_state);
243
244  // Return the hash code stored in the lock word, must be kHashCode state.
245  int32_t GetHashCode() const;
246
247  template <bool kIncludeReadBarrierState>
248  static bool Equal(LockWord lw1, LockWord lw2) {
249    if (kIncludeReadBarrierState) {
250      return lw1.GetValue() == lw2.GetValue();
251    }
252    return lw1.GetValueWithoutGCState() == lw2.GetValueWithoutGCState();
253  }
254
255  void Dump(std::ostream& os) {
256    os << "LockWord:" << std::hex << value_;
257  }
258
259 private:
260  // Default constructor with no lock ownership.
261  LockWord();
262
263  explicit LockWord(uint32_t val) : value_(val) {
264    // Make sure adding the overflow causes an overflow.
265    constexpr uint64_t overflow = static_cast<uint64_t>(kStateForwardingAddressShifted) +
266        static_cast<uint64_t>(kStateForwardingAddressOverflow);
267    constexpr bool is_larger = overflow > static_cast<uint64_t>(0xFFFFFFFF);
268    static_assert(is_larger, "should have overflowed");
269    static_assert(
270         (~kStateForwardingAddress & kStateMask) == 0,
271        "READ_BARRIER_MARK_REG relies on the forwarding address state being only one bits");
272    CheckReadBarrierState();
273  }
274
275  // Disallow this in favor of explicit Equal() with the
276  // kIncludeReadBarrierState param to make clients be aware of the
277  // read barrier state.
278  bool operator==(const LockWord& rhs) = delete;
279
280  void CheckReadBarrierState() const {
281    if (kIsDebugBuild && ((value_ >> kStateShift) & kStateMask) != kStateForwardingAddress) {
282      uint32_t rb_state = ReadBarrierState();
283      if (!kUseReadBarrier) {
284        DCHECK_EQ(rb_state, 0U);
285      } else {
286        DCHECK(rb_state == ReadBarrier::WhiteState() ||
287               rb_state == ReadBarrier::GrayState()) << rb_state;
288      }
289    }
290  }
291
292  // Note GetValue() includes the read barrier bits and comparing (==)
293  // GetValue() between two lock words to compare the lock states may
294  // not work. Prefer Equal() or GetValueWithoutReadBarrierState().
295  uint32_t GetValue() const {
296    CheckReadBarrierState();
297    return value_;
298  }
299
300  uint32_t GetValueWithoutGCState() const {
301    CheckReadBarrierState();
302    return value_ & kGCStateMaskShiftedToggled;
303  }
304
305  // Only Object should be converting LockWords to/from uints.
306  friend class mirror::Object;
307
308  // The encoded value holding all the state.
309  uint32_t value_;
310};
311std::ostream& operator<<(std::ostream& os, const LockWord::LockState& code);
312
313}  // namespace art
314
315
316#endif  // ART_RUNTIME_LOCK_WORD_H_
317