src/v8/src/heap/slot-set.h - cobalt - Git at Google

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 #ifndef V8_SLOT_SET_H
 #define V8_SLOT_SET_H

 #include <map>
 #include <stack>

 #include "src/allocation.h"
 #include "src/base/atomic-utils.h"
 #include "src/base/bits.h"
 #include "src/utils.h"

 namespace v8 {
 namespace internal {

 enum SlotCallbackResult { KEEP_SLOT, REMOVE_SLOT };

 // Data structure for maintaining a set of slots in a standard (non-large)
 // page. The base address of the page must be set with SetPageStart before any
 // operation.
 // The data structure assumes that the slots are pointer size aligned and
 // splits the valid slot offset range into kBuckets buckets.
 // Each bucket is a bitmap with a bit corresponding to a single slot offset.
 class SlotSet : public Malloced {
  public:
   enum EmptyBucketMode {
     FREE_EMPTY_BUCKETS,     // An empty bucket will be deallocated immediately.
     PREFREE_EMPTY_BUCKETS,  // An empty bucket will be unlinked from the slot
                             // set, but deallocated on demand by a sweeper
                             // thread.
     KEEP_EMPTY_BUCKETS      // An empty bucket will be kept.
   };

   SlotSet() {
     for (int i = 0; i < kBuckets; i++) {
       StoreBucket(&buckets_[i], nullptr);
     }
   }

   ~SlotSet() {
     for (int i = 0; i < kBuckets; i++) {
       ReleaseBucket(i);
     }
     FreeToBeFreedBuckets();
   }

   void SetPageStart(Address page_start) { page_start_ = page_start; }

   // The slot offset specifies a slot at address page_start_ + slot_offset.
   // This method should only be called on the main thread because concurrent
   // allocation of the bucket is not thread-safe.
   //
   // AccessMode defines whether there can be concurrent access on the buckets
   // or not.
   template <AccessMode access_mode = AccessMode::ATOMIC>
   void Insert(int slot_offset) {
     int bucket_index, cell_index, bit_index;
     SlotToIndices(slot_offset, &bucket_index, &cell_index, &bit_index);
     Bucket bucket = LoadBucket<access_mode>(&buckets_[bucket_index]);
     if (bucket == nullptr) {
       bucket = AllocateBucket();
       if (!SwapInNewBucket<access_mode>(&buckets_[bucket_index], bucket)) {
         DeleteArray<uint32_t>(bucket);
         bucket = LoadBucket<access_mode>(&buckets_[bucket_index]);
       }
     }
     // Check that monotonicity is preserved, i.e., once a bucket is set we do
     // not free it concurrently.
     DCHECK_NOT_NULL(bucket);
     DCHECK_EQ(bucket, LoadBucket<access_mode>(&buckets_[bucket_index]));
     uint32_t mask = 1u << bit_index;
     if ((LoadCell<access_mode>(&bucket[cell_index]) & mask) == 0) {
       SetCellBits<access_mode>(&bucket[cell_index], mask);
     }
   }

   // The slot offset specifies a slot at address page_start_ + slot_offset.
   // Returns true if the set contains the slot.
   bool Contains(int slot_offset) {
     int bucket_index, cell_index, bit_index;
     SlotToIndices(slot_offset, &bucket_index, &cell_index, &bit_index);
     Bucket bucket = LoadBucket(&buckets_[bucket_index]);
     if (bucket == nullptr) return false;
     return (LoadCell(&bucket[cell_index]) & (1u << bit_index)) != 0;
   }

   // The slot offset specifies a slot at address page_start_ + slot_offset.
   void Remove(int slot_offset) {
     int bucket_index, cell_index, bit_index;
     SlotToIndices(slot_offset, &bucket_index, &cell_index, &bit_index);
     Bucket bucket = LoadBucket(&buckets_[bucket_index]);
     if (bucket != nullptr) {
       uint32_t cell = LoadCell(&bucket[cell_index]);
       uint32_t bit_mask = 1u << bit_index;
       if (cell & bit_mask) {
         ClearCellBits(&bucket[cell_index], bit_mask);
       }
     }
   }

   // The slot offsets specify a range of slots at addresses:
   // [page_start_ + start_offset ... page_start_ + end_offset).
   void RemoveRange(int start_offset, int end_offset, EmptyBucketMode mode) {
     CHECK_LE(end_offset, 1 << kPageSizeBits);
     DCHECK_LE(start_offset, end_offset);
     int start_bucket, start_cell, start_bit;
     SlotToIndices(start_offset, &start_bucket, &start_cell, &start_bit);
     int end_bucket, end_cell, end_bit;
     SlotToIndices(end_offset, &end_bucket, &end_cell, &end_bit);
     uint32_t start_mask = (1u << start_bit) - 1;
     uint32_t end_mask = ~((1u << end_bit) - 1);
     Bucket bucket;
     if (start_bucket == end_bucket && start_cell == end_cell) {
       bucket = LoadBucket(&buckets_[start_bucket]);
       if (bucket != nullptr) {
         ClearCellBits(&bucket[start_cell], ~(start_mask | end_mask));
       }
       return;
     }
     int current_bucket = start_bucket;
     int current_cell = start_cell;
     bucket = LoadBucket(&buckets_[current_bucket]);
     if (bucket != nullptr) {
       ClearCellBits(&bucket[current_cell], ~start_mask);
     }
     current_cell++;
     if (current_bucket < end_bucket) {
       if (bucket != nullptr) {
         ClearBucket(bucket, current_cell, kCellsPerBucket);
       }
       // The rest of the current bucket is cleared.
       // Move on to the next bucket.
       current_bucket++;
       current_cell = 0;
     }
     DCHECK(current_bucket == end_bucket ||
            (current_bucket < end_bucket && current_cell == 0));
     while (current_bucket < end_bucket) {
       if (mode == PREFREE_EMPTY_BUCKETS) {
         PreFreeEmptyBucket(current_bucket);
       } else if (mode == FREE_EMPTY_BUCKETS) {
         ReleaseBucket(current_bucket);
       } else {
         DCHECK(mode == KEEP_EMPTY_BUCKETS);
         bucket = LoadBucket(&buckets_[current_bucket]);
         if (bucket != nullptr) {
           ClearBucket(bucket, 0, kCellsPerBucket);
         }
       }
       current_bucket++;
     }
     // All buckets between start_bucket and end_bucket are cleared.
     bucket = LoadBucket(&buckets_[current_bucket]);
     DCHECK(current_bucket == end_bucket && current_cell <= end_cell);
     if (current_bucket == kBuckets || bucket == nullptr) {
       return;
     }
     while (current_cell < end_cell) {
       StoreCell(&bucket[current_cell], 0);
       current_cell++;
     }
     // All cells between start_cell and end_cell are cleared.
     DCHECK(current_bucket == end_bucket && current_cell == end_cell);
     ClearCellBits(&bucket[end_cell], ~end_mask);
   }

   // The slot offset specifies a slot at address page_start_ + slot_offset.
   bool Lookup(int slot_offset) {
     int bucket_index, cell_index, bit_index;
     SlotToIndices(slot_offset, &bucket_index, &cell_index, &bit_index);
     Bucket bucket = LoadBucket(&buckets_[bucket_index]);
     if (bucket == nullptr) return false;
     return (LoadCell(&bucket[cell_index]) & (1u << bit_index)) != 0;
   }

   // Iterate over all slots in the set and for each slot invoke the callback.
   // If the callback returns REMOVE_SLOT then the slot is removed from the set.
   // Returns the new number of slots.
   // This method should only be called on the main thread.
   //
   // Sample usage:
   // Iterate([](Address slot_address) {
   //    if (good(slot_address)) return KEEP_SLOT;
   //    else return REMOVE_SLOT;
   // });
   template <typename Callback>
   int Iterate(Callback callback, EmptyBucketMode mode) {
     int new_count = 0;
     for (int bucket_index = 0; bucket_index < kBuckets; bucket_index++) {
       Bucket bucket = LoadBucket(&buckets_[bucket_index]);
       if (bucket != nullptr) {
         int in_bucket_count = 0;
         int cell_offset = bucket_index * kBitsPerBucket;
         for (int i = 0; i < kCellsPerBucket; i++, cell_offset += kBitsPerCell) {
           uint32_t cell = LoadCell(&bucket[i]);
           if (cell) {
             uint32_t old_cell = cell;
             uint32_t mask = 0;
             while (cell) {
               int bit_offset = base::bits::CountTrailingZeros(cell);
               uint32_t bit_mask = 1u << bit_offset;
               uint32_t slot = (cell_offset + bit_offset) << kPointerSizeLog2;
               if (callback(page_start_ + slot) == KEEP_SLOT) {
                 ++in_bucket_count;
               } else {
                 mask |= bit_mask;
               }
               cell ^= bit_mask;
             }
             uint32_t new_cell = old_cell & ~mask;
             if (old_cell != new_cell) {
               ClearCellBits(&bucket[i], mask);
             }
           }
         }
         if (mode == PREFREE_EMPTY_BUCKETS && in_bucket_count == 0) {
           PreFreeEmptyBucket(bucket_index);
         }
         new_count += in_bucket_count;
       }
     }
     return new_count;
   }

   int NumberOfPreFreedEmptyBuckets() {
     base::LockGuard<base::Mutex> guard(&to_be_freed_buckets_mutex_);
     return static_cast<int>(to_be_freed_buckets_.size());
   }

   void PreFreeEmptyBuckets() {
     for (int bucket_index = 0; bucket_index < kBuckets; bucket_index++) {
       Bucket bucket = LoadBucket(&buckets_[bucket_index]);
       if (bucket != nullptr) {
         if (IsEmptyBucket(bucket)) {
           PreFreeEmptyBucket(bucket_index);
         }
       }
     }
   }

   void FreeEmptyBuckets() {
     for (int bucket_index = 0; bucket_index < kBuckets; bucket_index++) {
       Bucket bucket = LoadBucket(&buckets_[bucket_index]);
       if (bucket != nullptr) {
         if (IsEmptyBucket(bucket)) {
           ReleaseBucket(bucket_index);
         }
       }
     }
   }

   void FreeToBeFreedBuckets() {
     base::LockGuard<base::Mutex> guard(&to_be_freed_buckets_mutex_);
     while (!to_be_freed_buckets_.empty()) {
       Bucket top = to_be_freed_buckets_.top();
       to_be_freed_buckets_.pop();
       DeleteArray<uint32_t>(top);
     }
     DCHECK_EQ(0u, to_be_freed_buckets_.size());
   }

  private:
   typedef uint32_t* Bucket;
   static const int kMaxSlots = (1 << kPageSizeBits) / kPointerSize;
   static const int kCellsPerBucket = 32;
   static const int kCellsPerBucketLog2 = 5;
   static const int kBitsPerCell = 32;
   static const int kBitsPerCellLog2 = 5;
   static const int kBitsPerBucket = kCellsPerBucket * kBitsPerCell;
   static const int kBitsPerBucketLog2 = kCellsPerBucketLog2 + kBitsPerCellLog2;
   static const int kBuckets = kMaxSlots / kCellsPerBucket / kBitsPerCell;

   Bucket AllocateBucket() {
     Bucket result = NewArray<uint32_t>(kCellsPerBucket);
     for (int i = 0; i < kCellsPerBucket; i++) {
       result[i] = 0;
     }
     return result;
   }

   void ClearBucket(Bucket bucket, int start_cell, int end_cell) {
     DCHECK_GE(start_cell, 0);
     DCHECK_LE(end_cell, kCellsPerBucket);
     int current_cell = start_cell;
     while (current_cell < kCellsPerBucket) {
       StoreCell(&bucket[current_cell], 0);
       current_cell++;
     }
   }

   void PreFreeEmptyBucket(int bucket_index) {
     Bucket bucket = LoadBucket(&buckets_[bucket_index]);
     if (bucket != nullptr) {
       base::LockGuard<base::Mutex> guard(&to_be_freed_buckets_mutex_);
       to_be_freed_buckets_.push(bucket);
       StoreBucket(&buckets_[bucket_index], nullptr);
     }
   }

   void ReleaseBucket(int bucket_index) {
     Bucket bucket = LoadBucket(&buckets_[bucket_index]);
     StoreBucket(&buckets_[bucket_index], nullptr);
     DeleteArray<uint32_t>(bucket);
   }

   template <AccessMode access_mode = AccessMode::ATOMIC>
   Bucket LoadBucket(Bucket* bucket) {
     if (access_mode == AccessMode::ATOMIC)
       return base::AsAtomicPointer::Acquire_Load(bucket);
     return *bucket;
   }

   template <AccessMode access_mode = AccessMode::ATOMIC>
   void StoreBucket(Bucket* bucket, Bucket value) {
     if (access_mode == AccessMode::ATOMIC) {
       base::AsAtomicPointer::Release_Store(bucket, value);
     } else {
       *bucket = value;
     }
   }

   bool IsEmptyBucket(Bucket bucket) {
     for (int i = 0; i < kCellsPerBucket; i++) {
       if (LoadCell(&bucket[i])) {
         return false;
       }
     }
     return true;
   }

   template <AccessMode access_mode = AccessMode::ATOMIC>
   bool SwapInNewBucket(Bucket* bucket, Bucket value) {
     if (access_mode == AccessMode::ATOMIC) {
       return base::AsAtomicPointer::Release_CompareAndSwap(bucket, nullptr,
                                                            value) == nullptr;
     } else {
       DCHECK_NULL(*bucket);
       *bucket = value;
       return true;
     }
   }

   template <AccessMode access_mode = AccessMode::ATOMIC>
   uint32_t LoadCell(uint32_t* cell) {
     if (access_mode == AccessMode::ATOMIC)
       return base::AsAtomic32::Acquire_Load(cell);
     return *cell;
   }

   void StoreCell(uint32_t* cell, uint32_t value) {
     base::AsAtomic32::Release_Store(cell, value);
   }

   void ClearCellBits(uint32_t* cell, uint32_t mask) {
     base::AsAtomic32::SetBits(cell, 0u, mask);
   }

   template <AccessMode access_mode = AccessMode::ATOMIC>
   void SetCellBits(uint32_t* cell, uint32_t mask) {
     if (access_mode == AccessMode::ATOMIC) {
       base::AsAtomic32::SetBits(cell, mask, mask);
     } else {
       *cell = (*cell & ~mask) | mask;
     }
   }

   // Converts the slot offset into bucket/cell/bit index.
   void SlotToIndices(int slot_offset, int* bucket_index, int* cell_index,
                      int* bit_index) {
     DCHECK_EQ(slot_offset % kPointerSize, 0);
     int slot = slot_offset >> kPointerSizeLog2;
     DCHECK(slot >= 0 && slot <= kMaxSlots);
     *bucket_index = slot >> kBitsPerBucketLog2;
     *cell_index = (slot >> kBitsPerCellLog2) & (kCellsPerBucket - 1);
     *bit_index = slot & (kBitsPerCell - 1);
   }

   Bucket buckets_[kBuckets];
   Address page_start_;
   base::Mutex to_be_freed_buckets_mutex_;
   std::stack<uint32_t*> to_be_freed_buckets_;
 };

 enum SlotType {
   EMBEDDED_OBJECT_SLOT,
   OBJECT_SLOT,
   CODE_TARGET_SLOT,
   CODE_ENTRY_SLOT,
   CLEARED_SLOT
 };

 // Data structure for maintaining a multiset of typed slots in a page.
 // Typed slots can only appear in Code and JSFunction objects, so
 // the maximum possible offset is limited by the LargePage::kMaxCodePageSize.
 // The implementation is a chain of chunks, where each chunks is an array of
 // encoded (slot type, slot offset) pairs.
 // There is no duplicate detection and we do not expect many duplicates because
 // typed slots contain V8 internal pointers that are not directly exposed to JS.
 class TypedSlotSet {
  public:
   enum IterationMode { PREFREE_EMPTY_CHUNKS, KEEP_EMPTY_CHUNKS };

   typedef std::pair<SlotType, uint32_t> TypeAndOffset;

   struct TypedSlot {
     TypedSlot() : type_and_offset_(0), host_offset_(0) {}

     TypedSlot(SlotType type, uint32_t host_offset, uint32_t offset)
         : type_and_offset_(TypeField::encode(type) |
                            OffsetField::encode(offset)),
           host_offset_(host_offset) {}

     bool operator==(const TypedSlot other) {
       return type_and_offset() == other.type_and_offset() &&
              host_offset() == other.host_offset();
     }

     bool operator!=(const TypedSlot other) { return !(*this == other); }

     SlotType type() const { return TypeField::decode(type_and_offset()); }

     uint32_t offset() const { return OffsetField::decode(type_and_offset()); }

     TypeAndOffset GetTypeAndOffset() const {
       uint32_t t_and_o = type_and_offset();
       return std::make_pair(TypeField::decode(t_and_o),
                             OffsetField::decode(t_and_o));
     }

     uint32_t type_and_offset() const {
       return base::AsAtomic32::Acquire_Load(&type_and_offset_);
     }

     uint32_t host_offset() const {
       return base::AsAtomic32::Acquire_Load(&host_offset_);
     }

     void Set(TypedSlot slot) {
       base::AsAtomic32::Release_Store(&type_and_offset_,
                                       slot.type_and_offset());
       base::AsAtomic32::Release_Store(&host_offset_, slot.host_offset());
     }

     void Clear() {
       base::AsAtomic32::Release_Store(
           &type_and_offset_,
           TypeField::encode(CLEARED_SLOT) | OffsetField::encode(0));
       base::AsAtomic32::Release_Store(&host_offset_, 0);
     }

     uint32_t type_and_offset_;
     uint32_t host_offset_;
   };
   static const int kMaxOffset = 1 << 29;

   explicit TypedSlotSet(Address page_start)
       : page_start_(page_start), top_(new Chunk(nullptr, kInitialBufferSize)) {}

   ~TypedSlotSet() {
     Chunk* chunk = load_top();
     while (chunk != nullptr) {
       Chunk* n = chunk->next();
       delete chunk;
       chunk = n;
     }
     FreeToBeFreedChunks();
   }

   // The slot offset specifies a slot at address page_start_ + offset.
   // This method can only be called on the main thread.
   void Insert(SlotType type, uint32_t host_offset, uint32_t offset) {
     TypedSlot slot(type, host_offset, offset);
     Chunk* top_chunk = load_top();
     if (!top_chunk) {
       top_chunk = new Chunk(nullptr, kInitialBufferSize);
       set_top(top_chunk);
     }
     if (!top_chunk->AddSlot(slot)) {
       Chunk* new_top_chunk =
           new Chunk(top_chunk, NextCapacity(top_chunk->capacity()));
       bool added = new_top_chunk->AddSlot(slot);
       set_top(new_top_chunk);
       DCHECK(added);
       USE(added);
     }
   }

   // Iterate over all slots in the set and for each slot invoke the callback.
   // If the callback returns REMOVE_SLOT then the slot is removed from the set.
   // Returns the new number of slots.
   //
   // Sample usage:
   // Iterate([](SlotType slot_type, Address slot_address) {
   //    if (good(slot_type, slot_address)) return KEEP_SLOT;
   //    else return REMOVE_SLOT;
   // });
   template <typename Callback>
   int Iterate(Callback callback, IterationMode mode) {
     STATIC_ASSERT(CLEARED_SLOT < 8);
     Chunk* chunk = load_top();
     Chunk* previous = nullptr;
     int new_count = 0;
     while (chunk != nullptr) {
       TypedSlot* buf = chunk->buffer();
       bool empty = true;
       for (int i = 0; i < chunk->count(); i++) {
         // Order is important here. We have to read out the slot type last to
         // observe the concurrent removal case consistently.
         Address host_addr = page_start_ + buf[i].host_offset();
         TypeAndOffset type_and_offset = buf[i].GetTypeAndOffset();
         SlotType type = type_and_offset.first;
         if (type != CLEARED_SLOT) {
           Address addr = page_start_ + type_and_offset.second;
           if (callback(type, host_addr, addr) == KEEP_SLOT) {
             new_count++;
             empty = false;
           } else {
             buf[i].Clear();
           }
         }
       }

       Chunk* n = chunk->next();
       if (mode == PREFREE_EMPTY_CHUNKS && empty) {
         // We remove the chunk from the list but let it still point its next
         // chunk to allow concurrent iteration.
         if (previous) {
           previous->set_next(n);
         } else {
           set_top(n);
         }
         base::LockGuard<base::Mutex> guard(&to_be_freed_chunks_mutex_);
         to_be_freed_chunks_.push(chunk);
       } else {
         previous = chunk;
       }
       chunk = n;
     }
     return new_count;
   }

   void FreeToBeFreedChunks() {
     base::LockGuard<base::Mutex> guard(&to_be_freed_chunks_mutex_);
     while (!to_be_freed_chunks_.empty()) {
       Chunk* top = to_be_freed_chunks_.top();
       to_be_freed_chunks_.pop();
       delete top;
     }
   }

   void RemoveInvaldSlots(std::map<uint32_t, uint32_t>& invalid_ranges) {
     Chunk* chunk = load_top();
     while (chunk != nullptr) {
       TypedSlot* buf = chunk->buffer();
       for (int i = 0; i < chunk->count(); i++) {
         uint32_t host_offset = buf[i].host_offset();
         std::map<uint32_t, uint32_t>::iterator upper_bound =
             invalid_ranges.upper_bound(host_offset);
         if (upper_bound == invalid_ranges.begin()) continue;
         // upper_bounds points to the invalid range after the given slot. Hence,
         // we have to go to the previous element.
         upper_bound--;
         DCHECK_LE(upper_bound->first, host_offset);
         if (upper_bound->second > host_offset) {
           buf[i].Clear();
         }
       }
       chunk = chunk->next();
     }
   }

  private:
   static const int kInitialBufferSize = 100;
   static const int kMaxBufferSize = 16 * KB;

   static int NextCapacity(int capacity) {
     return Min(kMaxBufferSize, capacity * 2);
   }

   class OffsetField : public BitField<int, 0, 29> {};
   class TypeField : public BitField<SlotType, 29, 3> {};

   struct Chunk : Malloced {
     explicit Chunk(Chunk* next_chunk, int chunk_capacity) {
       next_ = next_chunk;
       buffer_ = NewArray<TypedSlot>(chunk_capacity);
       capacity_ = chunk_capacity;
       count_ = 0;
     }

     ~Chunk() { DeleteArray(buffer_); }

     bool AddSlot(TypedSlot slot) {
       int current_count = count();
       if (current_count == capacity()) return false;
       TypedSlot* current_buffer = buffer();
       // Order is important here. We have to write the slot first before
       // increasing the counter to guarantee that a consistent state is
       // observed by concurrent threads.
       current_buffer[current_count].Set(slot);
       set_count(current_count + 1);
       return true;
     }

     Chunk* next() const { return base::AsAtomicPointer::Acquire_Load(&next_); }

     void set_next(Chunk* n) {
       return base::AsAtomicPointer::Release_Store(&next_, n);
     }

     TypedSlot* buffer() const { return buffer_; }

     int32_t capacity() const { return capacity_; }

     int32_t count() const { return base::AsAtomic32::Acquire_Load(&count_); }

     void set_count(int32_t new_value) {
       base::AsAtomic32::Release_Store(&count_, new_value);
     }

    private:
     Chunk* next_;
     TypedSlot* buffer_;
     int32_t capacity_;
     int32_t count_;
   };

   Chunk* load_top() { return base::AsAtomicPointer::Acquire_Load(&top_); }

   void set_top(Chunk* c) { base::AsAtomicPointer::Release_Store(&top_, c); }

   Address page_start_;
   Chunk* top_;
   base::Mutex to_be_freed_chunks_mutex_;
   std::stack<Chunk*> to_be_freed_chunks_;
 };

 }  // namespace internal
 }  // namespace v8

 #endif  // V8_SLOT_SET_H
	// Copyright 2016 the V8 project authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	#ifndef V8_SLOT_SET_H
	#define V8_SLOT_SET_H

	#include <map>
	#include <stack>

	#include "src/allocation.h"
	#include "src/base/atomic-utils.h"
	#include "src/base/bits.h"
	#include "src/utils.h"

	namespace v8 {
	namespace internal {

	enum SlotCallbackResult { KEEP_SLOT, REMOVE_SLOT };

	// Data structure for maintaining a set of slots in a standard (non-large)
	// page. The base address of the page must be set with SetPageStart before any
	// operation.
	// The data structure assumes that the slots are pointer size aligned and
	// splits the valid slot offset range into kBuckets buckets.
	// Each bucket is a bitmap with a bit corresponding to a single slot offset.
	class SlotSet : public Malloced {
	public:
	enum EmptyBucketMode {
	FREE_EMPTY_BUCKETS, // An empty bucket will be deallocated immediately.
	PREFREE_EMPTY_BUCKETS, // An empty bucket will be unlinked from the slot
	// set, but deallocated on demand by a sweeper
	// thread.
	KEEP_EMPTY_BUCKETS // An empty bucket will be kept.
	};

	SlotSet() {
	for (int i = 0; i < kBuckets; i++) {
	StoreBucket(&buckets_[i], nullptr);
	}
	}

	~SlotSet() {
	for (int i = 0; i < kBuckets; i++) {
	ReleaseBucket(i);
	}
	FreeToBeFreedBuckets();
	}

	void SetPageStart(Address page_start) { page_start_ = page_start; }

	// The slot offset specifies a slot at address page_start_ + slot_offset.
	// This method should only be called on the main thread because concurrent
	// allocation of the bucket is not thread-safe.
	//
	// AccessMode defines whether there can be concurrent access on the buckets
	// or not.
	template <AccessMode access_mode = AccessMode::ATOMIC>
	void Insert(int slot_offset) {
	int bucket_index, cell_index, bit_index;
	SlotToIndices(slot_offset, &bucket_index, &cell_index, &bit_index);
	Bucket bucket = LoadBucket<access_mode>(&buckets_[bucket_index]);
	if (bucket == nullptr) {
	bucket = AllocateBucket();
	if (!SwapInNewBucket<access_mode>(&buckets_[bucket_index], bucket)) {
	DeleteArray<uint32_t>(bucket);
	bucket = LoadBucket<access_mode>(&buckets_[bucket_index]);
	}
	}
	// Check that monotonicity is preserved, i.e., once a bucket is set we do
	// not free it concurrently.
	DCHECK_NOT_NULL(bucket);
	DCHECK_EQ(bucket, LoadBucket<access_mode>(&buckets_[bucket_index]));
	uint32_t mask = 1u << bit_index;
	if ((LoadCell<access_mode>(&bucket[cell_index]) & mask) == 0) {
	SetCellBits<access_mode>(&bucket[cell_index], mask);
	}
	}

	// The slot offset specifies a slot at address page_start_ + slot_offset.
	// Returns true if the set contains the slot.
	bool Contains(int slot_offset) {
	int bucket_index, cell_index, bit_index;
	SlotToIndices(slot_offset, &bucket_index, &cell_index, &bit_index);
	Bucket bucket = LoadBucket(&buckets_[bucket_index]);
	if (bucket == nullptr) return false;
	return (LoadCell(&bucket[cell_index]) & (1u << bit_index)) != 0;
	}

	// The slot offset specifies a slot at address page_start_ + slot_offset.
	void Remove(int slot_offset) {
	int bucket_index, cell_index, bit_index;
	SlotToIndices(slot_offset, &bucket_index, &cell_index, &bit_index);
	Bucket bucket = LoadBucket(&buckets_[bucket_index]);
	if (bucket != nullptr) {
	uint32_t cell = LoadCell(&bucket[cell_index]);
	uint32_t bit_mask = 1u << bit_index;
	if (cell & bit_mask) {
	ClearCellBits(&bucket[cell_index], bit_mask);
	}
	}
	}

	// The slot offsets specify a range of slots at addresses:
	// [page_start_ + start_offset ... page_start_ + end_offset).
	void RemoveRange(int start_offset, int end_offset, EmptyBucketMode mode) {
	CHECK_LE(end_offset, 1 << kPageSizeBits);
	DCHECK_LE(start_offset, end_offset);
	int start_bucket, start_cell, start_bit;
	SlotToIndices(start_offset, &start_bucket, &start_cell, &start_bit);
	int end_bucket, end_cell, end_bit;
	SlotToIndices(end_offset, &end_bucket, &end_cell, &end_bit);
	uint32_t start_mask = (1u << start_bit) - 1;
	uint32_t end_mask = ~((1u << end_bit) - 1);
	Bucket bucket;
	if (start_bucket == end_bucket && start_cell == end_cell) {
	bucket = LoadBucket(&buckets_[start_bucket]);
	if (bucket != nullptr) {
	ClearCellBits(&bucket[start_cell], ~(start_mask \| end_mask));
	}
	return;
	}
	int current_bucket = start_bucket;
	int current_cell = start_cell;
	bucket = LoadBucket(&buckets_[current_bucket]);
	if (bucket != nullptr) {
	ClearCellBits(&bucket[current_cell], ~start_mask);
	}
	current_cell++;
	if (current_bucket < end_bucket) {
	if (bucket != nullptr) {
	ClearBucket(bucket, current_cell, kCellsPerBucket);
	}
	// The rest of the current bucket is cleared.
	// Move on to the next bucket.
	current_bucket++;
	current_cell = 0;
	}
	DCHECK(current_bucket == end_bucket \|\|
	(current_bucket < end_bucket && current_cell == 0));
	while (current_bucket < end_bucket) {
	if (mode == PREFREE_EMPTY_BUCKETS) {
	PreFreeEmptyBucket(current_bucket);
	} else if (mode == FREE_EMPTY_BUCKETS) {
	ReleaseBucket(current_bucket);
	} else {
	DCHECK(mode == KEEP_EMPTY_BUCKETS);
	bucket = LoadBucket(&buckets_[current_bucket]);
	if (bucket != nullptr) {
	ClearBucket(bucket, 0, kCellsPerBucket);
	}
	}
	current_bucket++;
	}
	// All buckets between start_bucket and end_bucket are cleared.
	bucket = LoadBucket(&buckets_[current_bucket]);
	DCHECK(current_bucket == end_bucket && current_cell <= end_cell);
	if (current_bucket == kBuckets \|\| bucket == nullptr) {
	return;
	}
	while (current_cell < end_cell) {
	StoreCell(&bucket[current_cell], 0);
	current_cell++;
	}
	// All cells between start_cell and end_cell are cleared.
	DCHECK(current_bucket == end_bucket && current_cell == end_cell);
	ClearCellBits(&bucket[end_cell], ~end_mask);
	}

	// The slot offset specifies a slot at address page_start_ + slot_offset.
	bool Lookup(int slot_offset) {
	int bucket_index, cell_index, bit_index;
	SlotToIndices(slot_offset, &bucket_index, &cell_index, &bit_index);
	Bucket bucket = LoadBucket(&buckets_[bucket_index]);
	if (bucket == nullptr) return false;
	return (LoadCell(&bucket[cell_index]) & (1u << bit_index)) != 0;
	}

	// Iterate over all slots in the set and for each slot invoke the callback.
	// If the callback returns REMOVE_SLOT then the slot is removed from the set.
	// Returns the new number of slots.
	// This method should only be called on the main thread.
	//
	// Sample usage:
	// Iterate([](Address slot_address) {
	// if (good(slot_address)) return KEEP_SLOT;
	// else return REMOVE_SLOT;
	// });
	template <typename Callback>
	int Iterate(Callback callback, EmptyBucketMode mode) {
	int new_count = 0;
	for (int bucket_index = 0; bucket_index < kBuckets; bucket_index++) {
	Bucket bucket = LoadBucket(&buckets_[bucket_index]);
	if (bucket != nullptr) {
	int in_bucket_count = 0;
	int cell_offset = bucket_index * kBitsPerBucket;
	for (int i = 0; i < kCellsPerBucket; i++, cell_offset += kBitsPerCell) {
	uint32_t cell = LoadCell(&bucket[i]);
	if (cell) {
	uint32_t old_cell = cell;
	uint32_t mask = 0;
	while (cell) {
	int bit_offset = base::bits::CountTrailingZeros(cell);
	uint32_t bit_mask = 1u << bit_offset;
	uint32_t slot = (cell_offset + bit_offset) << kPointerSizeLog2;
	if (callback(page_start_ + slot) == KEEP_SLOT) {
	++in_bucket_count;
	} else {
	mask \|= bit_mask;
	}
	cell ^= bit_mask;
	}
	uint32_t new_cell = old_cell & ~mask;
	if (old_cell != new_cell) {
	ClearCellBits(&bucket[i], mask);
	}
	}
	}
	if (mode == PREFREE_EMPTY_BUCKETS && in_bucket_count == 0) {
	PreFreeEmptyBucket(bucket_index);
	}
	new_count += in_bucket_count;
	}
	}
	return new_count;
	}

	int NumberOfPreFreedEmptyBuckets() {
	base::LockGuard<base::Mutex> guard(&to_be_freed_buckets_mutex_);
	return static_cast<int>(to_be_freed_buckets_.size());
	}

	void PreFreeEmptyBuckets() {
	for (int bucket_index = 0; bucket_index < kBuckets; bucket_index++) {
	Bucket bucket = LoadBucket(&buckets_[bucket_index]);
	if (bucket != nullptr) {
	if (IsEmptyBucket(bucket)) {
	PreFreeEmptyBucket(bucket_index);
	}
	}
	}
	}

	void FreeEmptyBuckets() {
	for (int bucket_index = 0; bucket_index < kBuckets; bucket_index++) {
	Bucket bucket = LoadBucket(&buckets_[bucket_index]);
	if (bucket != nullptr) {
	if (IsEmptyBucket(bucket)) {
	ReleaseBucket(bucket_index);
	}
	}
	}
	}

	void FreeToBeFreedBuckets() {
	base::LockGuard<base::Mutex> guard(&to_be_freed_buckets_mutex_);
	while (!to_be_freed_buckets_.empty()) {
	Bucket top = to_be_freed_buckets_.top();
	to_be_freed_buckets_.pop();
	DeleteArray<uint32_t>(top);
	}
	DCHECK_EQ(0u, to_be_freed_buckets_.size());
	}

	private:
	typedef uint32_t* Bucket;
	static const int kMaxSlots = (1 << kPageSizeBits) / kPointerSize;
	static const int kCellsPerBucket = 32;
	static const int kCellsPerBucketLog2 = 5;
	static const int kBitsPerCell = 32;
	static const int kBitsPerCellLog2 = 5;
	static const int kBitsPerBucket = kCellsPerBucket * kBitsPerCell;
	static const int kBitsPerBucketLog2 = kCellsPerBucketLog2 + kBitsPerCellLog2;
	static const int kBuckets = kMaxSlots / kCellsPerBucket / kBitsPerCell;

	Bucket AllocateBucket() {
	Bucket result = NewArray<uint32_t>(kCellsPerBucket);
	for (int i = 0; i < kCellsPerBucket; i++) {
	result[i] = 0;
	}
	return result;
	}

	void ClearBucket(Bucket bucket, int start_cell, int end_cell) {
	DCHECK_GE(start_cell, 0);
	DCHECK_LE(end_cell, kCellsPerBucket);
	int current_cell = start_cell;
	while (current_cell < kCellsPerBucket) {
	StoreCell(&bucket[current_cell], 0);
	current_cell++;
	}
	}

	void PreFreeEmptyBucket(int bucket_index) {
	Bucket bucket = LoadBucket(&buckets_[bucket_index]);
	if (bucket != nullptr) {
	base::LockGuard<base::Mutex> guard(&to_be_freed_buckets_mutex_);
	to_be_freed_buckets_.push(bucket);
	StoreBucket(&buckets_[bucket_index], nullptr);
	}
	}

	void ReleaseBucket(int bucket_index) {
	Bucket bucket = LoadBucket(&buckets_[bucket_index]);
	StoreBucket(&buckets_[bucket_index], nullptr);
	DeleteArray<uint32_t>(bucket);
	}

	template <AccessMode access_mode = AccessMode::ATOMIC>
	Bucket LoadBucket(Bucket* bucket) {
	if (access_mode == AccessMode::ATOMIC)
	return base::AsAtomicPointer::Acquire_Load(bucket);
	return *bucket;
	}

	template <AccessMode access_mode = AccessMode::ATOMIC>
	void StoreBucket(Bucket* bucket, Bucket value) {
	if (access_mode == AccessMode::ATOMIC) {
	base::AsAtomicPointer::Release_Store(bucket, value);
	} else {
	*bucket = value;
	}
	}

	bool IsEmptyBucket(Bucket bucket) {
	for (int i = 0; i < kCellsPerBucket; i++) {
	if (LoadCell(&bucket[i])) {
	return false;
	}
	}
	return true;
	}

	template <AccessMode access_mode = AccessMode::ATOMIC>
	bool SwapInNewBucket(Bucket* bucket, Bucket value) {
	if (access_mode == AccessMode::ATOMIC) {
	return base::AsAtomicPointer::Release_CompareAndSwap(bucket, nullptr,
	value) == nullptr;
	} else {
	DCHECK_NULL(*bucket);
	*bucket = value;
	return true;
	}
	}

	template <AccessMode access_mode = AccessMode::ATOMIC>
	uint32_t LoadCell(uint32_t* cell) {
	if (access_mode == AccessMode::ATOMIC)
	return base::AsAtomic32::Acquire_Load(cell);
	return *cell;
	}

	void StoreCell(uint32_t* cell, uint32_t value) {
	base::AsAtomic32::Release_Store(cell, value);
	}

	void ClearCellBits(uint32_t* cell, uint32_t mask) {
	base::AsAtomic32::SetBits(cell, 0u, mask);
	}

	template <AccessMode access_mode = AccessMode::ATOMIC>
	void SetCellBits(uint32_t* cell, uint32_t mask) {
	if (access_mode == AccessMode::ATOMIC) {
	base::AsAtomic32::SetBits(cell, mask, mask);
	} else {
	cell = (cell & ~mask) \| mask;
	}
	}

	// Converts the slot offset into bucket/cell/bit index.
	void SlotToIndices(int slot_offset, int* bucket_index, int* cell_index,
	int* bit_index) {
	DCHECK_EQ(slot_offset % kPointerSize, 0);
	int slot = slot_offset >> kPointerSizeLog2;
	DCHECK(slot >= 0 && slot <= kMaxSlots);
	*bucket_index = slot >> kBitsPerBucketLog2;
	*cell_index = (slot >> kBitsPerCellLog2) & (kCellsPerBucket - 1);
	*bit_index = slot & (kBitsPerCell - 1);
	}

	Bucket buckets_[kBuckets];
	Address page_start_;
	base::Mutex to_be_freed_buckets_mutex_;
	std::stack<uint32_t*> to_be_freed_buckets_;
	};

	enum SlotType {
	EMBEDDED_OBJECT_SLOT,
	OBJECT_SLOT,
	CODE_TARGET_SLOT,
	CODE_ENTRY_SLOT,
	CLEARED_SLOT
	};

	// Data structure for maintaining a multiset of typed slots in a page.
	// Typed slots can only appear in Code and JSFunction objects, so
	// the maximum possible offset is limited by the LargePage::kMaxCodePageSize.
	// The implementation is a chain of chunks, where each chunks is an array of
	// encoded (slot type, slot offset) pairs.
	// There is no duplicate detection and we do not expect many duplicates because
	// typed slots contain V8 internal pointers that are not directly exposed to JS.
	class TypedSlotSet {
	public:
	enum IterationMode { PREFREE_EMPTY_CHUNKS, KEEP_EMPTY_CHUNKS };

	typedef std::pair<SlotType, uint32_t> TypeAndOffset;

	struct TypedSlot {
	TypedSlot() : type_and_offset_(0), host_offset_(0) {}

	TypedSlot(SlotType type, uint32_t host_offset, uint32_t offset)
	: type_and_offset_(TypeField::encode(type) \|
	OffsetField::encode(offset)),
	host_offset_(host_offset) {}

	bool operator==(const TypedSlot other) {
	return type_and_offset() == other.type_and_offset() &&
	host_offset() == other.host_offset();
	}

	bool operator!=(const TypedSlot other) { return !(*this == other); }

	SlotType type() const { return TypeField::decode(type_and_offset()); }

	uint32_t offset() const { return OffsetField::decode(type_and_offset()); }

	TypeAndOffset GetTypeAndOffset() const {
	uint32_t t_and_o = type_and_offset();
	return std::make_pair(TypeField::decode(t_and_o),
	OffsetField::decode(t_and_o));
	}

	uint32_t type_and_offset() const {
	return base::AsAtomic32::Acquire_Load(&type_and_offset_);
	}

	uint32_t host_offset() const {
	return base::AsAtomic32::Acquire_Load(&host_offset_);
	}

	void Set(TypedSlot slot) {
	base::AsAtomic32::Release_Store(&type_and_offset_,
	slot.type_and_offset());
	base::AsAtomic32::Release_Store(&host_offset_, slot.host_offset());
	}

	void Clear() {
	base::AsAtomic32::Release_Store(
	&type_and_offset_,
	TypeField::encode(CLEARED_SLOT) \| OffsetField::encode(0));
	base::AsAtomic32::Release_Store(&host_offset_, 0);
	}

	uint32_t type_and_offset_;
	uint32_t host_offset_;
	};
	static const int kMaxOffset = 1 << 29;

	explicit TypedSlotSet(Address page_start)
	: page_start_(page_start), top_(new Chunk(nullptr, kInitialBufferSize)) {}

	~TypedSlotSet() {
	Chunk* chunk = load_top();
	while (chunk != nullptr) {
	Chunk* n = chunk->next();
	delete chunk;
	chunk = n;
	}
	FreeToBeFreedChunks();
	}

	// The slot offset specifies a slot at address page_start_ + offset.
	// This method can only be called on the main thread.
	void Insert(SlotType type, uint32_t host_offset, uint32_t offset) {
	TypedSlot slot(type, host_offset, offset);
	Chunk* top_chunk = load_top();
	if (!top_chunk) {
	top_chunk = new Chunk(nullptr, kInitialBufferSize);
	set_top(top_chunk);
	}
	if (!top_chunk->AddSlot(slot)) {
	Chunk* new_top_chunk =
	new Chunk(top_chunk, NextCapacity(top_chunk->capacity()));
	bool added = new_top_chunk->AddSlot(slot);
	set_top(new_top_chunk);
	DCHECK(added);
	USE(added);
	}
	}

	// Iterate over all slots in the set and for each slot invoke the callback.
	// If the callback returns REMOVE_SLOT then the slot is removed from the set.
	// Returns the new number of slots.
	//
	// Sample usage:
	// Iterate([](SlotType slot_type, Address slot_address) {
	// if (good(slot_type, slot_address)) return KEEP_SLOT;
	// else return REMOVE_SLOT;
	// });
	template <typename Callback>
	int Iterate(Callback callback, IterationMode mode) {
	STATIC_ASSERT(CLEARED_SLOT < 8);
	Chunk* chunk = load_top();
	Chunk* previous = nullptr;
	int new_count = 0;
	while (chunk != nullptr) {
	TypedSlot* buf = chunk->buffer();
	bool empty = true;
	for (int i = 0; i < chunk->count(); i++) {
	// Order is important here. We have to read out the slot type last to
	// observe the concurrent removal case consistently.
	Address host_addr = page_start_ + buf[i].host_offset();
	TypeAndOffset type_and_offset = buf[i].GetTypeAndOffset();
	SlotType type = type_and_offset.first;
	if (type != CLEARED_SLOT) {
	Address addr = page_start_ + type_and_offset.second;
	if (callback(type, host_addr, addr) == KEEP_SLOT) {
	new_count++;
	empty = false;
	} else {
	buf[i].Clear();
	}
	}
	}

	Chunk* n = chunk->next();
	if (mode == PREFREE_EMPTY_CHUNKS && empty) {
	// We remove the chunk from the list but let it still point its next
	// chunk to allow concurrent iteration.
	if (previous) {
	previous->set_next(n);
	} else {
	set_top(n);
	}
	base::LockGuard<base::Mutex> guard(&to_be_freed_chunks_mutex_);
	to_be_freed_chunks_.push(chunk);
	} else {
	previous = chunk;
	}
	chunk = n;
	}
	return new_count;
	}

	void FreeToBeFreedChunks() {
	base::LockGuard<base::Mutex> guard(&to_be_freed_chunks_mutex_);
	while (!to_be_freed_chunks_.empty()) {
	Chunk* top = to_be_freed_chunks_.top();
	to_be_freed_chunks_.pop();
	delete top;
	}
	}

	void RemoveInvaldSlots(std::map<uint32_t, uint32_t>& invalid_ranges) {
	Chunk* chunk = load_top();
	while (chunk != nullptr) {
	TypedSlot* buf = chunk->buffer();
	for (int i = 0; i < chunk->count(); i++) {
	uint32_t host_offset = buf[i].host_offset();
	std::map<uint32_t, uint32_t>::iterator upper_bound =
	invalid_ranges.upper_bound(host_offset);
	if (upper_bound == invalid_ranges.begin()) continue;
	// upper_bounds points to the invalid range after the given slot. Hence,
	// we have to go to the previous element.
	upper_bound--;
	DCHECK_LE(upper_bound->first, host_offset);
	if (upper_bound->second > host_offset) {
	buf[i].Clear();
	}
	}
	chunk = chunk->next();
	}
	}

	private:
	static const int kInitialBufferSize = 100;
	static const int kMaxBufferSize = 16 * KB;

	static int NextCapacity(int capacity) {
	return Min(kMaxBufferSize, capacity * 2);
	}

	class OffsetField : public BitField<int, 0, 29> {};
	class TypeField : public BitField<SlotType, 29, 3> {};

	struct Chunk : Malloced {
	explicit Chunk(Chunk* next_chunk, int chunk_capacity) {
	next_ = next_chunk;
	buffer_ = NewArray<TypedSlot>(chunk_capacity);
	capacity_ = chunk_capacity;
	count_ = 0;
	}

	~Chunk() { DeleteArray(buffer_); }

	bool AddSlot(TypedSlot slot) {
	int current_count = count();
	if (current_count == capacity()) return false;
	TypedSlot* current_buffer = buffer();
	// Order is important here. We have to write the slot first before
	// increasing the counter to guarantee that a consistent state is
	// observed by concurrent threads.
	current_buffer[current_count].Set(slot);
	set_count(current_count + 1);
	return true;
	}

	Chunk* next() const { return base::AsAtomicPointer::Acquire_Load(&next_); }

	void set_next(Chunk* n) {
	return base::AsAtomicPointer::Release_Store(&next_, n);
	}

	TypedSlot* buffer() const { return buffer_; }

	int32_t capacity() const { return capacity_; }

	int32_t count() const { return base::AsAtomic32::Acquire_Load(&count_); }

	void set_count(int32_t new_value) {
	base::AsAtomic32::Release_Store(&count_, new_value);
	}

	private:
	Chunk* next_;
	TypedSlot* buffer_;
	int32_t capacity_;
	int32_t count_;
	};

	Chunk* load_top() { return base::AsAtomicPointer::Acquire_Load(&top_); }

	void set_top(Chunk* c) { base::AsAtomicPointer::Release_Store(&top_, c); }

	Address page_start_;
	Chunk* top_;
	base::Mutex to_be_freed_chunks_mutex_;
	std::stack<Chunk*> to_be_freed_chunks_;
	};

	} // namespace internal
	} // namespace v8

	#endif // V8_SLOT_SET_H