lock_word.h revision 6f198e3fde6fe0009c1f333c283c6d1cb4fa9b55 
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/bit_utils.h"
24#include "base/logging.h"
25#include "read_barrier.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|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 rb bits store the read barrier state.
61 */
62class LockWord {
63 public:
64  enum SizeShiftsAndMasks : uint32_t {  // 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 = 1,
68    kMarkBitStateSize = 1,
69    // Number of bits to encode the thin lock owner.
70    kThinLockOwnerSize = 16,
71    // Remaining bits are the recursive lock count.
72    kThinLockCountSize = 32 - kThinLockOwnerSize - kStateSize - kReadBarrierStateSize -
73        kMarkBitStateSize,
74    // Thin lock bits. Owner in lowest bits.
75
76    kThinLockOwnerShift = 0,
77    kThinLockOwnerMask = (1 << kThinLockOwnerSize) - 1,
78    kThinLockMaxOwner = kThinLockOwnerMask,
79    // Count in higher bits.
80    kThinLockCountShift = kThinLockOwnerSize + kThinLockOwnerShift,
81    kThinLockCountMask = (1 << kThinLockCountSize) - 1,
82    kThinLockMaxCount = kThinLockCountMask,
83    kThinLockCountOne = 1 << kThinLockCountShift,  // == 65536 (0x10000)
84
85    // State in the highest bits.
86    kStateShift = kReadBarrierStateSize + kThinLockCountSize + kThinLockCountShift +
87        kMarkBitStateSize,
88    kStateMask = (1 << kStateSize) - 1,
89    kStateMaskShifted = kStateMask << kStateShift,
90    kStateThinOrUnlocked = 0,
91    kStateFat = 1,
92    kStateHash = 2,
93    kStateForwardingAddress = 3,
94    kStateForwardingAddressShifted = kStateForwardingAddress << kStateShift,
95    kStateForwardingAddressOverflow = (1 + kStateMask - kStateForwardingAddress) << kStateShift,
96
97    // Read barrier bit.
98    kReadBarrierStateShift = kThinLockCountSize + kThinLockCountShift,
99    kReadBarrierStateMask = (1 << kReadBarrierStateSize) - 1,
100    kReadBarrierStateMaskShifted = kReadBarrierStateMask << kReadBarrierStateShift,
101    kReadBarrierStateMaskShiftedToggled = ~kReadBarrierStateMaskShifted,
102
103    // Mark bit.
104    kMarkBitStateShift = kReadBarrierStateSize + kReadBarrierStateShift,
105    kMarkBitStateMask = (1 << kMarkBitStateSize) - 1,
106    kMarkBitStateMaskShifted = kMarkBitStateMask << kMarkBitStateShift,
107    kMarkBitStateMaskShiftedToggled = ~kMarkBitStateMaskShifted,
108
109    // GC state is mark bit and read barrier state.
110    kGCStateSize = kReadBarrierStateSize + kMarkBitStateSize,
111    kGCStateShift = kReadBarrierStateShift,
112    kGCStateMaskShifted = kReadBarrierStateMaskShifted | kMarkBitStateMaskShifted,
113    kGCStateMaskShiftedToggled = ~kGCStateMaskShifted,
114
115    // When the state is kHashCode, the non-state bits hold the hashcode.
116    // Note Object.hashCode() has the hash code layout hardcoded.
117    kHashShift = 0,
118    kHashSize = 32 - kStateSize - kReadBarrierStateSize - kMarkBitStateSize,
119    kHashMask = (1 << kHashSize) - 1,
120    kMaxHash = kHashMask,
121
122    // Forwarding address shift.
123    kForwardingAddressShift = kObjectAlignmentShift,
124
125    kMonitorIdShift = kHashShift,
126    kMonitorIdSize = kHashSize,
127    kMonitorIdMask = kHashMask,
128    kMonitorIdAlignmentShift = 32 - kMonitorIdSize,
129    kMonitorIdAlignment = 1 << kMonitorIdAlignmentShift,
130    kMaxMonitorId = kMaxHash
131  };
132
133  static LockWord FromThinLockId(uint32_t thread_id, uint32_t count, uint32_t gc_state) {
134    CHECK_LE(thread_id, static_cast<uint32_t>(kThinLockMaxOwner));
135    CHECK_LE(count, static_cast<uint32_t>(kThinLockMaxCount));
136    // DCHECK_EQ(gc_bits & kGCStateMaskToggled, 0U);
137    return LockWord((thread_id << kThinLockOwnerShift) |
138                    (count << kThinLockCountShift) |
139                    (gc_state << kGCStateShift) |
140                    (kStateThinOrUnlocked << kStateShift));
141  }
142
143  static LockWord FromForwardingAddress(size_t target) {
144    DCHECK_ALIGNED(target, (1 << kStateSize));
145    return LockWord((target >> kForwardingAddressShift) | kStateForwardingAddressShifted);
146  }
147
148  static LockWord FromHashCode(uint32_t hash_code, uint32_t gc_state) {
149    CHECK_LE(hash_code, static_cast<uint32_t>(kMaxHash));
150    // DCHECK_EQ(gc_bits & kGCStateMaskToggled, 0U);
151    return LockWord((hash_code << kHashShift) |
152                    (gc_state << kGCStateShift) |
153                    (kStateHash << kStateShift));
154  }
155
156  static LockWord FromDefault(uint32_t gc_state) {
157    return LockWord(gc_state << kGCStateShift);
158  }
159
160  static bool IsDefault(LockWord lw) {
161    return LockWord().GetValue() == lw.GetValue();
162  }
163
164  static LockWord Default() {
165    return LockWord();
166  }
167
168  enum LockState {
169    kUnlocked,    // No lock owners.
170    kThinLocked,  // Single uncontended owner.
171    kFatLocked,   // See associated monitor.
172    kHashCode,    // Lock word contains an identity hash.
173    kForwardingAddress,  // Lock word contains the forwarding address of an object.
174  };
175
176  LockState GetState() const {
177    CheckReadBarrierState();
178    if ((!kUseReadBarrier && UNLIKELY(value_ == 0)) ||
179        (kUseReadBarrier && UNLIKELY((value_ & kGCStateMaskShiftedToggled) == 0))) {
180      return kUnlocked;
181    } else {
182      uint32_t internal_state = (value_ >> kStateShift) & kStateMask;
183      switch (internal_state) {
184        case kStateThinOrUnlocked:
185          return kThinLocked;
186        case kStateHash:
187          return kHashCode;
188        case kStateForwardingAddress:
189          return kForwardingAddress;
190        default:
191          DCHECK_EQ(internal_state, static_cast<uint32_t>(kStateFat));
192          return kFatLocked;
193      }
194    }
195  }
196
197  uint32_t ReadBarrierState() const {
198    return (value_ >> kReadBarrierStateShift) & kReadBarrierStateMask;
199  }
200
201  uint32_t GCState() const {
202    return (value_ & kGCStateMaskShifted) >> kGCStateShift;
203  }
204
205  void SetReadBarrierState(uint32_t rb_state) {
206    DCHECK_EQ(rb_state & ~kReadBarrierStateMask, 0U);
207    DCHECK(rb_state == ReadBarrier::WhiteState() ||
208           rb_state == ReadBarrier::GrayState()) << rb_state;
209    DCHECK_NE(static_cast<uint32_t>(GetState()), static_cast<uint32_t>(kForwardingAddress));
210    // Clear and or the bits.
211    value_ &= ~(kReadBarrierStateMask << kReadBarrierStateShift);
212    value_ |= (rb_state & kReadBarrierStateMask) << kReadBarrierStateShift;
213  }
214
215
216  uint32_t MarkBitState() const {
217    return (value_ >> kMarkBitStateShift) & kMarkBitStateMask;
218  }
219
220  void SetMarkBitState(uint32_t mark_bit) {
221    DCHECK_EQ(mark_bit & ~kMarkBitStateMask, 0U);
222    DCHECK_NE(static_cast<uint32_t>(GetState()), static_cast<uint32_t>(kForwardingAddress));
223    // Clear and or the bits.
224    value_ &= kMarkBitStateMaskShiftedToggled;
225    value_ |= mark_bit << kMarkBitStateShift;
226  }
227
228  // Return the owner thin lock thread id.
229  uint32_t ThinLockOwner() const;
230
231  // Return the number of times a lock value has been locked.
232  uint32_t ThinLockCount() const;
233
234  // Return the Monitor encoded in a fat lock.
235  Monitor* FatLockMonitor() const;
236
237  // Return the forwarding address stored in the monitor.
238  size_t ForwardingAddress() const;
239
240  // Constructor a lock word for inflation to use a Monitor.
241  LockWord(Monitor* mon, uint32_t gc_state);
242
243  // Return the hash code stored in the lock word, must be kHashCode state.
244  int32_t GetHashCode() const;
245
246  template <bool kIncludeReadBarrierState>
247  static bool Equal(LockWord lw1, LockWord lw2) {
248    if (kIncludeReadBarrierState) {
249      return lw1.GetValue() == lw2.GetValue();
250    }
251    return lw1.GetValueWithoutGCState() == lw2.GetValueWithoutGCState();
252  }
253
254  void Dump(std::ostream& os) {
255    os << "LockWord:" << std::hex << value_;
256  }
257
258 private:
259  // Default constructor with no lock ownership.
260  LockWord();
261
262  explicit LockWord(uint32_t val) : value_(val) {
263    // Make sure adding the overflow causes an overflow.
264    constexpr uint64_t overflow = static_cast<uint64_t>(kStateForwardingAddressShifted) +
265        static_cast<uint64_t>(kStateForwardingAddressOverflow);
266    constexpr bool is_larger = overflow > static_cast<uint64_t>(0xFFFFFFFF);
267    static_assert(is_larger, "should have overflowed");
268    static_assert(
269         (~kStateForwardingAddress & kStateMask) == 0,
270        "READ_BARRIER_MARK_REG relies on the forwarding address state being only one bits");
271    CheckReadBarrierState();
272  }
273
274  // Disallow this in favor of explicit Equal() with the
275  // kIncludeReadBarrierState param to make clients be aware of the
276  // read barrier state.
277  bool operator==(const LockWord& rhs) = delete;
278
279  void CheckReadBarrierState() const {
280    if (kIsDebugBuild && ((value_ >> kStateShift) & kStateMask) != kStateForwardingAddress) {
281      uint32_t rb_state = ReadBarrierState();
282      if (!kUseReadBarrier) {
283        DCHECK_EQ(rb_state, 0U);
284      } else {
285        DCHECK(rb_state == ReadBarrier::WhiteState() ||
286               rb_state == ReadBarrier::GrayState()) << rb_state;
287      }
288    }
289  }
290
291  // Note GetValue() includes the read barrier bits and comparing (==)
292  // GetValue() between two lock words to compare the lock states may
293  // not work. Prefer Equal() or GetValueWithoutReadBarrierState().
294  uint32_t GetValue() const {
295    CheckReadBarrierState();
296    return value_;
297  }
298
299  uint32_t GetValueWithoutGCState() const {
300    CheckReadBarrierState();
301    return value_ & kGCStateMaskShiftedToggled;
302  }
303
304  // Only Object should be converting LockWords to/from uints.
305  friend class mirror::Object;
306
307  // The encoded value holding all the state.
308  uint32_t value_;
309};
310std::ostream& operator<<(std::ostream& os, const LockWord::LockState& code);
311
312}  // namespace art
313
314
315#endif  // ART_RUNTIME_LOCK_WORD_H_
316