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      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 
     27 namespace art {
     28 namespace mirror {
     29   class Object;
     30 }  // namespace mirror
     31 
     32 class 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  */
     62 class 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_ALIGNED(target, (1 << kStateSize));
    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   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   void Dump(std::ostream& os) {
    214     os << "LockWord:" << std::hex << value_;
    215   }
    216 
    217  private:
    218   // Default constructor with no lock ownership.
    219   LockWord();
    220 
    221   explicit LockWord(uint32_t val) : value_(val) {
    222     CheckReadBarrierState();
    223   }
    224 
    225   // Disallow this in favor of explicit Equal() with the
    226   // kIncludeReadBarrierState param to make clients be aware of the
    227   // read barrier state.
    228   bool operator==(const LockWord& rhs) = delete;
    229 
    230   void CheckReadBarrierState() const {
    231     if (kIsDebugBuild && ((value_ >> kStateShift) & kStateMask) != kStateForwardingAddress) {
    232       uint32_t rb_state = ReadBarrierState();
    233       if (!kUseReadBarrier) {
    234         DCHECK_EQ(rb_state, 0U);
    235       } else {
    236         DCHECK(rb_state == ReadBarrier::white_ptr_ ||
    237                rb_state == ReadBarrier::gray_ptr_ ||
    238                rb_state == ReadBarrier::black_ptr_) << rb_state;
    239       }
    240     }
    241   }
    242 
    243   // Note GetValue() includes the read barrier bits and comparing (==)
    244   // GetValue() between two lock words to compare the lock states may
    245   // not work. Prefer Equal() or GetValueWithoutReadBarrierState().
    246   uint32_t GetValue() const {
    247     CheckReadBarrierState();
    248     return value_;
    249   }
    250 
    251   uint32_t GetValueWithoutReadBarrierState() const {
    252     CheckReadBarrierState();
    253     return value_ & ~(kReadBarrierStateMask << kReadBarrierStateShift);
    254   }
    255 
    256   // Only Object should be converting LockWords to/from uints.
    257   friend class mirror::Object;
    258 
    259   // The encoded value holding all the state.
    260   uint32_t value_;
    261 };
    262 std::ostream& operator<<(std::ostream& os, const LockWord::LockState& code);
    263 
    264 }  // namespace art
    265 
    266 
    267 #endif  // ART_RUNTIME_LOCK_WORD_H_
    268