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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 <cstdint>
     21 #include <iosfwd>
     22 
     23 #include <android-base/logging.h>
     24 
     25 #include "base/bit_utils.h"
     26 #include "read_barrier.h"
     27 
     28 namespace art {
     29 namespace mirror {
     30 class Object;
     31 }  // namespace mirror
     32 
     33 class Monitor;
     34 
     35 /* The lock value itself as stored in mirror::Object::monitor_.  The two most significant bits of
     36  * the state. The four possible states are fat locked, thin/unlocked, hash code, and forwarding
     37  * address.
     38  *
     39  * When the lock word is in the "thin" state and its bits are formatted as follows:
     40  *
     41  *  |33|2|2|222222221111|1111110000000000|
     42  *  |10|9|8|765432109876|5432109876543210|
     43  *  |00|m|r| lock count |thread id owner |
     44  *
     45  * When the lock word is in the "fat" state and its bits are formatted as follows:
     46  *
     47  *  |33|2|2|2222222211111111110000000000|
     48  *  |10|9|8|7654321098765432109876543210|
     49  *  |01|m|r| MonitorId                  |
     50  *
     51  * When the lock word is in hash state and its bits are formatted as follows:
     52  *
     53  *  |33|2|2|2222222211111111110000000000|
     54  *  |10|9|8|7654321098765432109876543210|
     55  *  |10|m|r| HashCode                   |
     56  *
     57  * When the lock word is in forwarding address state and its bits are formatted as follows:
     58  *
     59  *  |33|2|22222222211111111110000000000|
     60  *  |10|9|87654321098765432109876543210|
     61  *  |11|0| ForwardingAddress           |
     62  *
     63  * The `r` bit stores the read barrier state.
     64  * The `m` bit stores the mark bit state.
     65  */
     66 class LockWord {
     67  public:
     68   enum SizeShiftsAndMasks : uint32_t {  // private marker to avoid generate-operator-out.py from processing.
     69     // Number of bits to encode the state, currently just fat or thin/unlocked or hash code.
     70     kStateSize = 2,
     71     kReadBarrierStateSize = 1,
     72     kMarkBitStateSize = 1,
     73     // Number of bits to encode the thin lock owner.
     74     kThinLockOwnerSize = 16,
     75     // Remaining bits are the recursive lock count.
     76     kThinLockCountSize = 32 - kThinLockOwnerSize - kStateSize - kReadBarrierStateSize -
     77         kMarkBitStateSize,
     78     // Thin lock bits. Owner in lowest bits.
     79 
     80     kThinLockOwnerShift = 0,
     81     kThinLockOwnerMask = (1 << kThinLockOwnerSize) - 1,
     82     kThinLockMaxOwner = kThinLockOwnerMask,
     83     // Count in higher bits.
     84     kThinLockCountShift = kThinLockOwnerSize + kThinLockOwnerShift,
     85     kThinLockCountMask = (1 << kThinLockCountSize) - 1,
     86     kThinLockMaxCount = kThinLockCountMask,
     87     kThinLockCountOne = 1 << kThinLockCountShift,  // == 65536 (0x10000)
     88 
     89     // State in the highest bits.
     90     kStateShift = kReadBarrierStateSize + kThinLockCountSize + kThinLockCountShift +
     91         kMarkBitStateSize,
     92     kStateMask = (1 << kStateSize) - 1,
     93     kStateMaskShifted = kStateMask << kStateShift,
     94     kStateThinOrUnlocked = 0,
     95     kStateFat = 1,
     96     kStateHash = 2,
     97     kStateForwardingAddress = 3,
     98     kStateForwardingAddressShifted = kStateForwardingAddress << kStateShift,
     99     kStateForwardingAddressOverflow = (1 + kStateMask - kStateForwardingAddress) << kStateShift,
    100 
    101     // Read barrier bit.
    102     kReadBarrierStateShift = kThinLockCountSize + kThinLockCountShift,
    103     kReadBarrierStateMask = (1 << kReadBarrierStateSize) - 1,
    104     kReadBarrierStateMaskShifted = kReadBarrierStateMask << kReadBarrierStateShift,
    105     kReadBarrierStateMaskShiftedToggled = ~kReadBarrierStateMaskShifted,
    106 
    107     // Mark bit.
    108     kMarkBitStateShift = kReadBarrierStateSize + kReadBarrierStateShift,
    109     kMarkBitStateMask = (1 << kMarkBitStateSize) - 1,
    110     kMarkBitStateMaskShifted = kMarkBitStateMask << kMarkBitStateShift,
    111     kMarkBitStateMaskShiftedToggled = ~kMarkBitStateMaskShifted,
    112 
    113     // GC state is mark bit and read barrier state.
    114     kGCStateSize = kReadBarrierStateSize + kMarkBitStateSize,
    115     kGCStateShift = kReadBarrierStateShift,
    116     kGCStateMaskShifted = kReadBarrierStateMaskShifted | kMarkBitStateMaskShifted,
    117     kGCStateMaskShiftedToggled = ~kGCStateMaskShifted,
    118 
    119     // When the state is kHashCode, the non-state bits hold the hashcode.
    120     // Note Object.hashCode() has the hash code layout hardcoded.
    121     kHashShift = 0,
    122     kHashSize = 32 - kStateSize - kReadBarrierStateSize - kMarkBitStateSize,
    123     kHashMask = (1 << kHashSize) - 1,
    124     kMaxHash = kHashMask,
    125 
    126     // Forwarding address shift.
    127     kForwardingAddressShift = kObjectAlignmentShift,
    128 
    129     kMonitorIdShift = kHashShift,
    130     kMonitorIdSize = kHashSize,
    131     kMonitorIdMask = kHashMask,
    132     kMonitorIdAlignmentShift = 32 - kMonitorIdSize,
    133     kMonitorIdAlignment = 1 << kMonitorIdAlignmentShift,
    134     kMaxMonitorId = kMaxHash
    135   };
    136 
    137   static LockWord FromThinLockId(uint32_t thread_id, uint32_t count, uint32_t gc_state) {
    138     CHECK_LE(thread_id, static_cast<uint32_t>(kThinLockMaxOwner));
    139     CHECK_LE(count, static_cast<uint32_t>(kThinLockMaxCount));
    140     // DCHECK_EQ(gc_bits & kGCStateMaskToggled, 0U);
    141     return LockWord((thread_id << kThinLockOwnerShift) |
    142                     (count << kThinLockCountShift) |
    143                     (gc_state << kGCStateShift) |
    144                     (kStateThinOrUnlocked << kStateShift));
    145   }
    146 
    147   static LockWord FromForwardingAddress(size_t target) {
    148     DCHECK_ALIGNED(target, (1 << kStateSize));
    149     return LockWord((target >> kForwardingAddressShift) | kStateForwardingAddressShifted);
    150   }
    151 
    152   static LockWord FromHashCode(uint32_t hash_code, uint32_t gc_state) {
    153     CHECK_LE(hash_code, static_cast<uint32_t>(kMaxHash));
    154     // DCHECK_EQ(gc_bits & kGCStateMaskToggled, 0U);
    155     return LockWord((hash_code << kHashShift) |
    156                     (gc_state << kGCStateShift) |
    157                     (kStateHash << kStateShift));
    158   }
    159 
    160   static LockWord FromDefault(uint32_t gc_state) {
    161     return LockWord(gc_state << kGCStateShift);
    162   }
    163 
    164   static bool IsDefault(LockWord lw) {
    165     return LockWord().GetValue() == lw.GetValue();
    166   }
    167 
    168   static LockWord Default() {
    169     return LockWord();
    170   }
    171 
    172   enum LockState {
    173     kUnlocked,    // No lock owners.
    174     kThinLocked,  // Single uncontended owner.
    175     kFatLocked,   // See associated monitor.
    176     kHashCode,    // Lock word contains an identity hash.
    177     kForwardingAddress,  // Lock word contains the forwarding address of an object.
    178   };
    179 
    180   LockState GetState() const {
    181     CheckReadBarrierState();
    182     if ((!kUseReadBarrier && UNLIKELY(value_ == 0)) ||
    183         (kUseReadBarrier && UNLIKELY((value_ & kGCStateMaskShiftedToggled) == 0))) {
    184       return kUnlocked;
    185     } else {
    186       uint32_t internal_state = (value_ >> kStateShift) & kStateMask;
    187       switch (internal_state) {
    188         case kStateThinOrUnlocked:
    189           return kThinLocked;
    190         case kStateHash:
    191           return kHashCode;
    192         case kStateForwardingAddress:
    193           return kForwardingAddress;
    194         default:
    195           DCHECK_EQ(internal_state, static_cast<uint32_t>(kStateFat));
    196           return kFatLocked;
    197       }
    198     }
    199   }
    200 
    201   uint32_t ReadBarrierState() const {
    202     return (value_ >> kReadBarrierStateShift) & kReadBarrierStateMask;
    203   }
    204 
    205   uint32_t GCState() const {
    206     return (value_ & kGCStateMaskShifted) >> kGCStateShift;
    207   }
    208 
    209   void SetReadBarrierState(uint32_t rb_state) {
    210     DCHECK_EQ(rb_state & ~kReadBarrierStateMask, 0U);
    211     DCHECK(rb_state == ReadBarrier::WhiteState() ||
    212            rb_state == ReadBarrier::GrayState()) << rb_state;
    213     DCHECK_NE(static_cast<uint32_t>(GetState()), static_cast<uint32_t>(kForwardingAddress));
    214     // Clear and or the bits.
    215     value_ &= ~(kReadBarrierStateMask << kReadBarrierStateShift);
    216     value_ |= (rb_state & kReadBarrierStateMask) << kReadBarrierStateShift;
    217   }
    218 
    219 
    220   uint32_t MarkBitState() const {
    221     return (value_ >> kMarkBitStateShift) & kMarkBitStateMask;
    222   }
    223 
    224   void SetMarkBitState(uint32_t mark_bit) {
    225     DCHECK_EQ(mark_bit & ~kMarkBitStateMask, 0U);
    226     DCHECK_NE(static_cast<uint32_t>(GetState()), static_cast<uint32_t>(kForwardingAddress));
    227     // Clear and or the bits.
    228     value_ &= kMarkBitStateMaskShiftedToggled;
    229     value_ |= mark_bit << kMarkBitStateShift;
    230   }
    231 
    232   // Return the owner thin lock thread id.
    233   uint32_t ThinLockOwner() const;
    234 
    235   // Return the number of times a lock value has been locked.
    236   uint32_t ThinLockCount() const;
    237 
    238   // Return the Monitor encoded in a fat lock.
    239   Monitor* FatLockMonitor() const;
    240 
    241   // Return the forwarding address stored in the monitor.
    242   size_t ForwardingAddress() const;
    243 
    244   // Constructor a lock word for inflation to use a Monitor.
    245   LockWord(Monitor* mon, uint32_t gc_state);
    246 
    247   // Return the hash code stored in the lock word, must be kHashCode state.
    248   int32_t GetHashCode() const;
    249 
    250   template <bool kIncludeReadBarrierState>
    251   static bool Equal(LockWord lw1, LockWord lw2) {
    252     if (kIncludeReadBarrierState) {
    253       return lw1.GetValue() == lw2.GetValue();
    254     }
    255     return lw1.GetValueWithoutGCState() == lw2.GetValueWithoutGCState();
    256   }
    257 
    258   void Dump(std::ostream& os) {
    259     os << "LockWord:" << std::hex << value_;
    260   }
    261 
    262  private:
    263   // Default constructor with no lock ownership.
    264   LockWord();
    265 
    266   explicit LockWord(uint32_t val) : value_(val) {
    267     // Make sure adding the overflow causes an overflow.
    268     constexpr uint64_t overflow = static_cast<uint64_t>(kStateForwardingAddressShifted) +
    269         static_cast<uint64_t>(kStateForwardingAddressOverflow);
    270     constexpr bool is_larger = overflow > static_cast<uint64_t>(0xFFFFFFFF);
    271     static_assert(is_larger, "should have overflowed");
    272     static_assert(
    273          (~kStateForwardingAddress & kStateMask) == 0,
    274         "READ_BARRIER_MARK_REG relies on the forwarding address state being only one bits");
    275     CheckReadBarrierState();
    276   }
    277 
    278   // Disallow this in favor of explicit Equal() with the
    279   // kIncludeReadBarrierState param to make clients be aware of the
    280   // read barrier state.
    281   bool operator==(const LockWord& rhs) = delete;
    282 
    283   void CheckReadBarrierState() const {
    284     if (kIsDebugBuild && ((value_ >> kStateShift) & kStateMask) != kStateForwardingAddress) {
    285       uint32_t rb_state = ReadBarrierState();
    286       if (!kUseReadBarrier) {
    287         DCHECK_EQ(rb_state, 0U);
    288       } else {
    289         DCHECK(rb_state == ReadBarrier::WhiteState() ||
    290                rb_state == ReadBarrier::GrayState()) << rb_state;
    291       }
    292     }
    293   }
    294 
    295   // Note GetValue() includes the read barrier bits and comparing (==)
    296   // GetValue() between two lock words to compare the lock states may
    297   // not work. Prefer Equal() or GetValueWithoutReadBarrierState().
    298   uint32_t GetValue() const {
    299     CheckReadBarrierState();
    300     return value_;
    301   }
    302 
    303   uint32_t GetValueWithoutGCState() const {
    304     CheckReadBarrierState();
    305     return value_ & kGCStateMaskShiftedToggled;
    306   }
    307 
    308   // Only Object should be converting LockWords to/from uints.
    309   friend class mirror::Object;
    310 
    311   // The encoded value holding all the state.
    312   uint32_t value_;
    313 };
    314 std::ostream& operator<<(std::ostream& os, const LockWord::LockState& code);
    315 
    316 }  // namespace art
    317 
    318 
    319 #endif  // ART_RUNTIME_LOCK_WORD_H_
    320