// v8/src/objects/js-array.h // line 19
// The JSArray describes JavaScript Arrays
// Such an array can be in one of two modes:
// - fast, backing storage is a FixedArray and length <= elements.length();
// Please note: push and pop can be used to grow and shrink the array.
// - slow, backing storage is a HashTable with numbers as keys.
class JSArray : public JSObject {
public:
// [length]: The length property.
DECL_ACCESSORS(length, Object)
//...
// v8/src/objects/js-array.h // line 105
// Number of element slots to pre-allocate for an empty array.
static const int kPreallocatedArrayElements = 4;
// v8/src/objects/js-objects.h // line 537
static const uint32_t kMinAddedElementsCapacity = 16;
// v8/src/objects/js-objects.h // line 540 // 计算扩容后的容量
// Computes the new capacity when expanding the elements of a JSObject.
static uint32_t NewElementsCapacity(uint32_t old_capacity) {
// (old_capacity + 50%) + kMinAddedElementsCapacity
return old_capacity + (old_capacity >> 1) + kMinAddedElementsCapacity;
}
// v8/src/code-stub-assembler.cc // line 5137 // 扩容的实现
Node* CodeStubAssembler::CalculateNewElementsCapacity(Node* old_capacity,
ParameterMode mode) {
CSA_SLOW_ASSERT(this, MatchesParameterMode(old_capacity, mode));
Node* half_old_capacity = WordOrSmiShr(old_capacity, 1, mode);
Node* new_capacity = IntPtrOrSmiAdd(half_old_capacity, old_capacity, mode);
Node* padding =
IntPtrOrSmiConstant(JSObject::kMinAddedElementsCapacity, mode);
return IntPtrOrSmiAdd(new_capacity, padding, mode);
}
// v8/src/code-stub-assembler.cc // line 5202 // 内存的拷贝
// Allocate the new backing store.
Node* new_elements = AllocateFixedArray(to_kind, new_capacity, mode);
// Copy the elements from the old elements store to the new.
// The size-check above guarantees that the |new_elements| is allocated
// in new space so we can skip the write barrier.
CopyFixedArrayElements(from_kind, elements, to_kind, new_elements, capacity,
new_capacity, SKIP_WRITE_BARRIER, mode);
StoreObjectField(object, JSObject::kElementsOffset, new_elements);
// v8/src/elements.cc // line 783
if (2 * length + JSObject::kMinAddedElementsCapacity <= capacity) {
// If more than half the elements won't be used, trim the array.
// Do not trim from short arrays to prevent frequent trimming on
// repeated pop operations.
// Leave some space to allow for subsequent push operations.
int elements_to_trim = length + 1 == old_length
? (capacity - length) / 2
: capacity - length;
isolate->heap()->RightTrimFixedArray(*backing_store, elements_to_trim);
// Fill the non-trimmed elements with holes.
BackingStore::cast(*backing_store)
->FillWithHoles(length,
std::min(old_length, capacity - elements_to_trim));
} else {
// Otherwise, fill the unused tail with holes.
BackingStore::cast(*backing_store)->FillWithHoles(length, old_length);
}
// v8/src/objects/js-objects-inl.h // line 992
static inline bool ShouldConvertToSlowElements(JSObject object,
uint32_t capacity,
uint32_t index,
uint32_t* new_capacity) {
STATIC_ASSERT(JSObject::kMaxUncheckedOldFastElementsLength <=
JSObject::kMaxUncheckedFastElementsLength);
if (index < capacity) {
*new_capacity = capacity;
return false;
}
if (index - capacity >= JSObject::kMaxGap) return true; // 第二种转换
*new_capacity = JSObject::NewElementsCapacity(index + 1);
DCHECK_LT(index, *new_capacity);
// TODO(ulan): Check if it works with young large objects.
if (*new_capacity <= JSObject::kMaxUncheckedOldFastElementsLength ||
(*new_capacity <= JSObject::kMaxUncheckedFastElementsLength &&
ObjectInYoungGeneration(object))) {
return false;
}
// If the fast-case backing storage takes up much more memory than a
// dictionary backing storage would, the object should have slow elements.
int used_elements = object->GetFastElementsUsage();
uint32_t size_threshold = NumberDictionary::kPreferFastElementsSizeFactor *
NumberDictionary::ComputeCapacity(used_elements) *
NumberDictionary::kEntrySize;
return size_threshold <= *new_capacity; // 第一种转换
}
// v8/src/objects/js-objects.h // line 738
// JSObject::kMaxGap 常量
// Maximal gap that can be introduced by adding an element beyond
// the current elements length.
static const uint32_t kMaxGap = 1024;
// v8/src/objects/dictionary.h // line 362
// NumberDictionary::kPreferFastElementsSizeFactor 常量
// JSObjects prefer dictionary elements if the dictionary saves this much
// memory compared to a fast elements backing store.
static const uint32_t kPreferFastElementsSizeFactor = 3;
// v8/src/objects/hash-table-inl.h // line 76
// NumberDictionary::ComputeCapacity(used_elements)
// NumberDictionary 继承于 Dictionary 再继承于 HashTable
// static
int HashTableBase::ComputeCapacity(int at_least_space_for) {
// Add 50% slack to make slot collisions sufficiently unlikely.
// See matching computation in HashTable::HasSufficientCapacityToAdd().
// Must be kept in sync with CodeStubAssembler::HashTableComputeCapacity().
int raw_cap = at_least_space_for + (at_least_space_for >> 1);
int capacity = base::bits::RoundUpToPowerOfTwo32(raw_cap);
return Max(capacity, kMinCapacity);
}
// v8/src/objects/dictionary.h // line 260
// NumberDictionary::kEntrySize 常量
// NumberDictionary 继承 Dictionary 传入 NumberDictionaryShape作为Shape 继承HashTable
// HashTable 中定义 static const int kEntrySize = Shape::kEntrySize;
static const int kEntrySize = 3;
