src/v8/src/address-map.h

// Copyright 2015 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_ADDRESS_MAP_H_
#define V8_ADDRESS_MAP_H_

#include "include/v8.h"
#include "src/assert-scope.h"
#include "src/base/hashmap.h"
#include "src/objects.h"

namespace v8 {
namespace internal {

template <typename Type>
class PointerToIndexHashMap
    : public base::TemplateHashMapImpl<uintptr_t, uint32_t,
                                       base::KeyEqualityMatcher<intptr_t>,
                                       base::DefaultAllocationPolicy> {
 public:
  typedef base::TemplateHashMapEntry<uintptr_t, uint32_t> Entry;

  inline void Set(Type value, uint32_t index) {
    uintptr_t key = Key(value);
    LookupOrInsert(key, Hash(key))->value = index;
  }

  inline Maybe<uint32_t> Get(Type value) const {
    uintptr_t key = Key(value);
    Entry* entry = Lookup(key, Hash(key));
    if (entry == nullptr) return Nothing<uint32_t>();
    return Just(entry->value);
  }

 private:
  static uintptr_t Key(Type value) {
    return reinterpret_cast<uintptr_t>(value);
  }

  static uint32_t Hash(uintptr_t key) { return static_cast<uint32_t>(key); }
};

class AddressToIndexHashMap : public PointerToIndexHashMap<Address> {};
class HeapObjectToIndexHashMap : public PointerToIndexHashMap<HeapObject*> {};

class RootIndexMap {
 public:
  explicit RootIndexMap(Isolate* isolate);

  static const int kInvalidRootIndex = -1;

  int Lookup(HeapObject* obj) {
    Maybe<uint32_t> maybe_index = map_->Get(obj);
    return maybe_index.IsJust() ? maybe_index.FromJust() : kInvalidRootIndex;
  }

 private:
  HeapObjectToIndexHashMap* map_;

  DISALLOW_COPY_AND_ASSIGN(RootIndexMap);
};

class SerializerReference {
 public:
  SerializerReference() : bitfield_(Special(kInvalidValue)) {}

  static SerializerReference FromBitfield(uint32_t bitfield) {
    return SerializerReference(bitfield);
  }

  static SerializerReference BackReference(AllocationSpace space,
                                           uint32_t chunk_index,
                                           uint32_t chunk_offset) {
    DCHECK(IsAligned(chunk_offset, kObjectAlignment));
    DCHECK_NE(LO_SPACE, space);
    return SerializerReference(
        SpaceBits::encode(space) | ChunkIndexBits::encode(chunk_index) |
        ChunkOffsetBits::encode(chunk_offset >> kObjectAlignmentBits));
  }

  static SerializerReference MapReference(uint32_t index) {
    return SerializerReference(SpaceBits::encode(MAP_SPACE) |
                               ValueIndexBits::encode(index));
  }

  static SerializerReference OffHeapBackingStoreReference(uint32_t index) {
    return SerializerReference(SpaceBits::encode(kExternalSpace) |
                               ValueIndexBits::encode(index));
  }

  static SerializerReference LargeObjectReference(uint32_t index) {
    return SerializerReference(SpaceBits::encode(LO_SPACE) |
                               ValueIndexBits::encode(index));
  }

  static SerializerReference AttachedReference(uint32_t index) {
    return SerializerReference(SpaceBits::encode(kAttachedReferenceSpace) |
                               ValueIndexBits::encode(index));
  }

  static SerializerReference DummyReference() {
    return SerializerReference(Special(kDummyValue));
  }

  bool is_valid() const { return bitfield_ != Special(kInvalidValue); }

  bool is_back_reference() const {
    return SpaceBits::decode(bitfield_) <= LAST_SPACE;
  }

  AllocationSpace space() const {
    DCHECK(is_back_reference());
    return static_cast<AllocationSpace>(SpaceBits::decode(bitfield_));
  }

  uint32_t chunk_offset() const {
    DCHECK(is_back_reference());
    return ChunkOffsetBits::decode(bitfield_) << kObjectAlignmentBits;
  }

  uint32_t map_index() const {
    DCHECK(is_back_reference());
    return ValueIndexBits::decode(bitfield_);
  }

  bool is_off_heap_backing_store_reference() const {
    return SpaceBits::decode(bitfield_) == kExternalSpace;
  }

  uint32_t off_heap_backing_store_index() const {
    DCHECK(is_off_heap_backing_store_reference());
    return ValueIndexBits::decode(bitfield_);
  }

  uint32_t large_object_index() const {
    DCHECK(is_back_reference());
    return ValueIndexBits::decode(bitfield_);
  }

  uint32_t chunk_index() const {
    DCHECK(is_back_reference());
    return ChunkIndexBits::decode(bitfield_);
  }

  uint32_t back_reference() const {
    DCHECK(is_back_reference());
    return bitfield_ & (ChunkOffsetBits::kMask | ChunkIndexBits::kMask);
  }

  bool is_attached_reference() const {
    return SpaceBits::decode(bitfield_) == kAttachedReferenceSpace;
  }

  int attached_reference_index() const {
    DCHECK(is_attached_reference());
    return ValueIndexBits::decode(bitfield_);
  }

 private:
  explicit SerializerReference(uint32_t bitfield) : bitfield_(bitfield) {}

  inline static uint32_t Special(int value) {
    return SpaceBits::encode(kSpecialValueSpace) |
           ValueIndexBits::encode(value);
  }

  // We use the 32-bit bitfield to encode either a back reference, a special
  // value, or an attached reference index.
  // Back reference:
  //   [ Space index             ] [ Chunk index ] [ Chunk offset ]
  //   [ LO_SPACE                ] [ large object index           ]
  // Special value
  //   [ kSpecialValueSpace      ] [ Special value index          ]
  // Attached reference
  //   [ kAttachedReferenceSpace ] [ Attached reference index     ]
  // External
  //   [ kExternalSpace          ] [ External reference index     ]

  static const int kChunkOffsetSize = kPageSizeBits - kObjectAlignmentBits;
  static const int kChunkIndexSize = 32 - kChunkOffsetSize - kSpaceTagSize;
  static const int kValueIndexSize = kChunkOffsetSize + kChunkIndexSize;

  static const int kSpecialValueSpace = LAST_SPACE + 1;
  static const int kAttachedReferenceSpace = kSpecialValueSpace + 1;
  static const int kExternalSpace = kAttachedReferenceSpace + 1;
  STATIC_ASSERT(kExternalSpace < (1 << kSpaceTagSize));

  static const int kInvalidValue = 0;
  static const int kDummyValue = 1;

  // The chunk offset can also be used to encode the index of special values.
  class ChunkOffsetBits : public BitField<uint32_t, 0, kChunkOffsetSize> {};
  class ChunkIndexBits
      : public BitField<uint32_t, ChunkOffsetBits::kNext, kChunkIndexSize> {};
  class ValueIndexBits : public BitField<uint32_t, 0, kValueIndexSize> {};
  STATIC_ASSERT(ChunkIndexBits::kNext == ValueIndexBits::kNext);
  class SpaceBits : public BitField<int, kValueIndexSize, kSpaceTagSize> {};
  STATIC_ASSERT(SpaceBits::kNext == 32);

  uint32_t bitfield_;

  friend class SerializerReferenceMap;
};

// Mapping objects to their location after deserialization.
// This is used during building, but not at runtime by V8.
class SerializerReferenceMap {
 public:
  SerializerReferenceMap()
      : no_allocation_(), map_(), attached_reference_index_(0) {}

  SerializerReference Lookup(void* obj) {
    Maybe<uint32_t> maybe_index = map_.Get(obj);
    return maybe_index.IsJust() ? SerializerReference(maybe_index.FromJust())
                                : SerializerReference();
  }

  void Add(void* obj, SerializerReference b) {
    DCHECK(b.is_valid());
    DCHECK(map_.Get(obj).IsNothing());
    map_.Set(obj, b.bitfield_);
  }

  SerializerReference AddAttachedReference(HeapObject* attached_reference) {
    SerializerReference reference =
        SerializerReference::AttachedReference(attached_reference_index_++);
    Add(attached_reference, reference);
    return reference;
  }

 private:
  DisallowHeapAllocation no_allocation_;
  PointerToIndexHashMap<void*> map_;
  int attached_reference_index_;
  DISALLOW_COPY_AND_ASSIGN(SerializerReferenceMap);
};

}  // namespace internal
}  // namespace v8

#endif  // V8_ADDRESS_MAP_H_