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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|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  */
     63 class 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 };
    311 std::ostream& operator<<(std::ostream& os, const LockWord::LockState& code);
    312 
    313 }  // namespace art
    314 
    315 
    316 #endif  // ART_RUNTIME_LOCK_WORD_H_
    317