lock_word.h revision 60f63f53c01cb38ca18a815603282e802a6cf918 
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 <iosfwd>
21#include <stdint.h>
22
23#include "base/logging.h"
24#include "read_barrier.h"
25#include "utils.h"
26
27namespace art {
28namespace mirror {
29  class 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|22|222222221111|1111110000000000|
39 *  |10|98|765432109876|5432109876543210|
40 *  |00|rb| 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|22|2222222211111111110000000000|
45 *  |10|98|7654321098765432109876543210|
46 *  |01|rb| MonitorId                  |
47 *
48 * When the lock word is in hash state and its bits are formatted as follows:
49 *
50 *  |33|22|2222222211111111110000000000|
51 *  |10|98|7654321098765432109876543210|
52 *  |10|rb| HashCode                   |
53 *
54 * When the lock word is in fowarding address state and its bits are formatted as follows:
55 *
56 *  |33|22|2222222211111111110000000000|
57 *  |10|98|7654321098765432109876543210|
58 *  |11| ForwardingAddress             |
59 *
60 * The rb bits store the read barrier state.
61 */
62class LockWord {
63 public:
64  enum SizeShiftsAndMasks {  // private marker to avoid generate-operator-out.py from processing.
65    // Number of bits to encode the state, currently just fat or thin/unlocked or hash code.
66    kStateSize = 2,
67    kReadBarrierStateSize = 2,
68    // Number of bits to encode the thin lock owner.
69    kThinLockOwnerSize = 16,
70    // Remaining bits are the recursive lock count.
71    kThinLockCountSize = 32 - kThinLockOwnerSize - kStateSize - kReadBarrierStateSize,
72    // Thin lock bits. Owner in lowest bits.
73
74    kThinLockOwnerShift = 0,
75    kThinLockOwnerMask = (1 << kThinLockOwnerSize) - 1,
76    kThinLockMaxOwner = kThinLockOwnerMask,
77    // Count in higher bits.
78    kThinLockCountShift = kThinLockOwnerSize + kThinLockOwnerShift,
79    kThinLockCountMask = (1 << kThinLockCountSize) - 1,
80    kThinLockMaxCount = kThinLockCountMask,
81    kThinLockCountOne = 1 << kThinLockCountShift,  // == 65536 (0x10000)
82
83    // State in the highest bits.
84    kStateShift = kReadBarrierStateSize + kThinLockCountSize + kThinLockCountShift,
85    kStateMask = (1 << kStateSize) - 1,
86    kStateMaskShifted = kStateMask << kStateShift,
87    kStateThinOrUnlocked = 0,
88    kStateFat = 1,
89    kStateHash = 2,
90    kStateForwardingAddress = 3,
91    kReadBarrierStateShift = kThinLockCountSize + kThinLockCountShift,
92    kReadBarrierStateMask = (1 << kReadBarrierStateSize) - 1,
93    kReadBarrierStateMaskShifted = kReadBarrierStateMask << kReadBarrierStateShift,
94    kReadBarrierStateMaskShiftedToggled = ~kReadBarrierStateMaskShifted,
95
96    // When the state is kHashCode, the non-state bits hold the hashcode.
97    // Note Object.hashCode() has the hash code layout hardcoded.
98    kHashShift = 0,
99    kHashSize = 32 - kStateSize - kReadBarrierStateSize,
100    kHashMask = (1 << kHashSize) - 1,
101    kMaxHash = kHashMask,
102
103    kMonitorIdShift = kHashShift,
104    kMonitorIdSize = kHashSize,
105    kMonitorIdMask = kHashMask,
106    kMonitorIdAlignmentShift = 32 - kMonitorIdSize,
107    kMonitorIdAlignment = 1 << kMonitorIdAlignmentShift,
108    kMaxMonitorId = kMaxHash
109  };
110
111  static LockWord FromThinLockId(uint32_t thread_id, uint32_t count, uint32_t rb_state) {
112    CHECK_LE(thread_id, static_cast<uint32_t>(kThinLockMaxOwner));
113    CHECK_LE(count, static_cast<uint32_t>(kThinLockMaxCount));
114    DCHECK_EQ(rb_state & ~kReadBarrierStateMask, 0U);
115    return LockWord((thread_id << kThinLockOwnerShift) | (count << kThinLockCountShift) |
116                    (rb_state << kReadBarrierStateShift) |
117                    (kStateThinOrUnlocked << kStateShift));
118  }
119
120  static LockWord FromForwardingAddress(size_t target) {
121    DCHECK(IsAligned < 1 << kStateSize>(target));
122    return LockWord((target >> kStateSize) | (kStateForwardingAddress << kStateShift));
123  }
124
125  static LockWord FromHashCode(uint32_t hash_code, uint32_t rb_state) {
126    CHECK_LE(hash_code, static_cast<uint32_t>(kMaxHash));
127    DCHECK_EQ(rb_state & ~kReadBarrierStateMask, 0U);
128    return LockWord((hash_code << kHashShift) |
129                    (rb_state << kReadBarrierStateShift) |
130                    (kStateHash << kStateShift));
131  }
132
133  static LockWord FromDefault(uint32_t rb_state) {
134    DCHECK_EQ(rb_state & ~kReadBarrierStateMask, 0U);
135    return LockWord(rb_state << kReadBarrierStateShift);
136  }
137
138  static bool IsDefault(LockWord lw) {
139    return LockWord().GetValue() == lw.GetValue();
140  }
141
142  static LockWord Default() {
143    return LockWord();
144  }
145
146  enum LockState {
147    kUnlocked,    // No lock owners.
148    kThinLocked,  // Single uncontended owner.
149    kFatLocked,   // See associated monitor.
150    kHashCode,    // Lock word contains an identity hash.
151    kForwardingAddress,  // Lock word contains the forwarding address of an object.
152  };
153
154  LockState GetState() const {
155    CheckReadBarrierState();
156    if ((!kUseReadBarrier && UNLIKELY(value_ == 0)) ||
157        (kUseReadBarrier && UNLIKELY((value_ & kReadBarrierStateMaskShiftedToggled) == 0))) {
158      return kUnlocked;
159    } else {
160      uint32_t internal_state = (value_ >> kStateShift) & kStateMask;
161      switch (internal_state) {
162        case kStateThinOrUnlocked:
163          return kThinLocked;
164        case kStateHash:
165          return kHashCode;
166        case kStateForwardingAddress:
167          return kForwardingAddress;
168        default:
169          DCHECK_EQ(internal_state, static_cast<uint32_t>(kStateFat));
170          return kFatLocked;
171      }
172    }
173  }
174
175  uint32_t ReadBarrierState() const {
176    return (value_ >> kReadBarrierStateShift) & kReadBarrierStateMask;
177  }
178
179  void SetReadBarrierState(uint32_t rb_state) {
180    DCHECK_EQ(rb_state & ~kReadBarrierStateMask, 0U);
181    DCHECK_NE(static_cast<uint32_t>(GetState()), static_cast<uint32_t>(kForwardingAddress));
182    // Clear and or the bits.
183    value_ &= ~(kReadBarrierStateMask << kReadBarrierStateShift);
184    value_ |= (rb_state & kReadBarrierStateMask) << kReadBarrierStateShift;
185  }
186
187  // Return the owner thin lock thread id.
188  uint32_t ThinLockOwner() const;
189
190  // Return the number of times a lock value has been locked.
191  uint32_t ThinLockCount() const;
192
193  // Return the Monitor encoded in a fat lock.
194  Monitor* FatLockMonitor() const;
195
196  // Return the forwarding address stored in the monitor.
197  size_t ForwardingAddress() const;
198
199  // Constructor a lock word for inflation to use a Monitor.
200  explicit LockWord(Monitor* mon, uint32_t rb_state);
201
202  // Return the hash code stored in the lock word, must be kHashCode state.
203  int32_t GetHashCode() const;
204
205  template <bool kIncludeReadBarrierState>
206  static bool Equal(LockWord lw1, LockWord lw2) {
207    if (kIncludeReadBarrierState) {
208      return lw1.GetValue() == lw2.GetValue();
209    }
210    return lw1.GetValueWithoutReadBarrierState() == lw2.GetValueWithoutReadBarrierState();
211  }
212
213 private:
214  // Default constructor with no lock ownership.
215  LockWord();
216
217  explicit LockWord(uint32_t val) : value_(val) {
218    CheckReadBarrierState();
219  }
220
221  // Disallow this in favor of explicit Equal() with the
222  // kIncludeReadBarrierState param to make clients be aware of the
223  // read barrier state.
224  bool operator==(const LockWord& rhs) = delete;
225
226  void CheckReadBarrierState() const {
227    if (kIsDebugBuild && ((value_ >> kStateShift) & kStateMask) != kStateForwardingAddress) {
228      uint32_t rb_state = ReadBarrierState();
229      if (!kUseReadBarrier) {
230        DCHECK_EQ(rb_state, 0U);
231      } else {
232        DCHECK(rb_state == ReadBarrier::white_ptr_ ||
233               rb_state == ReadBarrier::gray_ptr_ ||
234               rb_state == ReadBarrier::black_ptr_) << rb_state;
235      }
236    }
237  }
238
239  // Note GetValue() includes the read barrier bits and comparing (==)
240  // GetValue() between two lock words to compare the lock states may
241  // not work. Prefer Equal() or GetValueWithoutReadBarrierState().
242  uint32_t GetValue() const {
243    CheckReadBarrierState();
244    return value_;
245  }
246
247  uint32_t GetValueWithoutReadBarrierState() const {
248    CheckReadBarrierState();
249    return value_ & ~(kReadBarrierStateMask << kReadBarrierStateShift);
250  }
251
252  // Only Object should be converting LockWords to/from uints.
253  friend class mirror::Object;
254
255  // The encoded value holding all the state.
256  uint32_t value_;
257};
258std::ostream& operator<<(std::ostream& os, const LockWord::LockState& code);
259
260}  // namespace art
261
262
263#endif  // ART_RUNTIME_LOCK_WORD_H_
264