js引擎zone怎么使用

   
     
 
    
    #ifndef V8_ZONE_H_#define V8_ZONE_H_namespace v8 { namespace internal {// The Zone supports very fast allocation of small chunks of// memory. The chunks cannot be deallocated individually, but instead// the Zone supports deallocating all chunks in one fast// operation. The Zone is used to hold temporary data structures like// the abstract syntax tree, which is deallocated after compilation.// Note: There is no need to initialize the Zone; the first time an// allocation is attempted, a segment of memory will be requested// through a call to malloc().// Note: The implementation is inherently not thread safe. Do not use// from multi-threaded code.class Zone { public:  // Allocate 'size' bytes of memory in the Zone; expands the Zone by  // allocating new segments of memory on demand using malloc().  // 分配size大小的内存  static inline void* New(int size);  // Delete all objects and free all memory allocated in the Zone.  // 一次性释放所有内存  static void DeleteAll(); private:  // All pointers returned from New() have this alignment.  static const int kAlignment = kPointerSize;  // Never allocate segments smaller than this size in bytes.  static const int kMinimumSegmentSize = 8 * KB;  // Never keep segments larger than this size in bytes around.  static const int kMaximumKeptSegmentSize = 64 * KB;  // The Zone is intentionally a singleton; you should not try to  // allocate instances of the class.  // 不能new 只能调用Zone::New  Zone() { UNREACHABLE(); }  // Expand the Zone to hold at least 'size' more bytes and allocate  // the bytes. Returns the address of the newly allocated chunk of  // memory in the Zone. Should only be called if there isn't enough  // room in the Zone already.  // 扩展内存  static Address NewExpand(int size);  // The free region in the current (front) segment is represented as  // the half-open interval [position, limit). The 'position' variable  // is guaranteed to be aligned as dictated by kAlignment.  // 管理内存的首地址和大小限制  static Address position_;  static Address limit_;};// ZoneObject is an abstraction that helps define classes of objects// allocated in the Zone. Use it as a base class; see ast.h.// 对Zone的封装new ZoneObject();即Zone::New(size);class ZoneObject { public:  // Allocate a new ZoneObject of 'size' bytes in the Zone.  void* operator new(size_t size) { return Zone::New(size); }  // Ideally, the delete operator should be private instead of  // public, but unfortuately the compiler sometimes synthesizes  // (unused) destructors for classes derived from ZoneObject, which  // require the operator to be visible. MSVC requires the delete  // operator to be public.  // ZoneObjects should never be deleted individually; use  // Zone::DeleteAll() to delete all zone objects in one go.  // 禁止delete该类的对象  void operator delete(void*, size_t) { UNREACHABLE(); }};/*  管理allow_allocation_字段，  new AssertNoZoneAllocation的时候，保存当前的allow_allocation_,  设置allow_allocation_为false，析构后，恢复allow_allocation_的值*/ class AssertNoZoneAllocation { public:  AssertNoZoneAllocation() : prev_(allow_allocation_) {    allow_allocation_ = false;  }  ~AssertNoZoneAllocation() { allow_allocation_ = prev_; }  static bool allow_allocation() { return allow_allocation_; } private:  bool prev_;  static bool allow_allocation_;};// The ZoneListAllocationPolicy is used to specialize the GenericList// implementation to allocate ZoneLists and their elements in the// Zone.class ZoneListAllocationPolicy { public:  // Allocate 'size' bytes of memory in the zone.  static void* New(int size) {  return Zone::New(size); }  // De-allocation attempts are silently ignored.  static void Delete(void* p) { }};// ZoneLists are growable lists with constant-time access to the// elements. The list itself and all its elements are allocated in the// Zone. ZoneLists cannot be deleted individually; you can delete all// objects in the Zone by calling Zone::DeleteAll().template<typename T>// ZoneList本质上是一个list，ZoneListAllocationPolicy是list里的内存管理器class ZoneList: public List<T, ZoneListAllocationPolicy> { public:  // Construct a new ZoneList with the given capacity; the length is  // always zero. The capacity must be non-negative.  explicit ZoneList(int capacity)      : List<T, ZoneListAllocationPolicy>(capacity) { }};} }  // namespace v8::internal#endif  // V8_ZONE_H_
   
     
 
    
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    #ifndef V8_ZONE_INL_H_#define V8_ZONE_INL_H_#include "zone.h"namespace v8 { namespace internal {inline void* Zone::New(int size) {  ASSERT(AssertNoZoneAllocation::allow_allocation());  // Round up the requested size to fit the alignment.  size = RoundUp(size, kAlignment);  // Check if the requested size is available without expanding.  // 当前的指针位置  Address result = position_;  /*    一开始position和limit都是0，所以会分配一个segment，后续还需要分配的时候，如果segment    里的内存还可以满足，则不需要再分配一个新的segment，在原来的分配就行，    分配size后超过了限制，扩容的时候也是分配一个segment  */  if ((position_ += size) > limit_) result = NewExpand(size);  // Check that the result has the proper alignment and return it.  ASSERT(IsAddressAligned(result, kAlignment, 0));  return reinterpret_cast<void*>(result);}} }  // namespace v8::internal#endif  // V8_ZONE_INL_H_
   
     
 
    
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    #include "v8.h"#include "zone-inl.h"namespace v8 { namespace internal {Address Zone::position_ = 0;Address Zone::limit_ = 0;bool AssertNoZoneAllocation::allow_allocation_ = true;// Segments represent chunks of memory: They have starting address// (encoded in the this pointer) and a size in bytes. Segments are// chained together forming a LIFO structure with the newest segment// available as Segment::head(). Segments are allocated using malloc()// and de-allocated using free().class Segment { public:  // 下一个节点  Segment* next() const { return next_; }  // 断开指向下一个节点的指针  void clear_next() { next_ = NULL; }  // 内存总大小  int size() const { return size_; }  // 内存可用大小，前面有一个Segment对象  int capacity() const { return size_ - sizeof(Segment); }  // 内存的开始地址，即Segement对象  Address start() const { return address(sizeof(Segment)); }  // 结束地址即首地址加上size  Address end() const { return address(size_); }  // 返回第一个Segment节点  static Segment* head() { return head_; }  static void set_head(Segment* head) { head_ = head; }  // Creates a new segment, sets it size, and pushes it to the front  // of the segment chain. Returns the new segment.  // 新增一个Segment  static Segment* New(int size) {    Segment* result = reinterpret_cast<Segment*>(Malloced::New(size));    // 分配成功    if (result != NULL) {      // 头插入插入链表，size是分配的总大小      result->next_ = head_;      result->size_ = size;      head_ = result;    }    return result;  }  // Deletes the given segment. Does not touch the segment chain.  // 释放segment节点  static void Delete(Segment* segment) {    Malloced::Delete(segment);  } private:  // Computes the address of the nth byte in this segment.  // 首地址加上n个字节  Address address(int n) const {    return Address(this) + n;  }  // 管理所有segment节点的头指针  static Segment* head_;  // 每个segment节点的属性  Segment* next_;  int size_;};Segment* Segment::head_ = NULL;void Zone::DeleteAll() {#ifdef DEBUG  // Constant byte value used for zapping dead memory in debug mode.  static const unsigned char kZapDeadByte = 0xcd;#endif  // Find a segment with a suitable size to keep around.  Segment* keep = Segment::head();  // 到末节点或者小于kMaximumKeptSegmentSize大小的节点  while (keep != NULL && keep->size() > kMaximumKeptSegmentSize) {    keep = keep->next();  }  // Traverse the chained list of segments, zapping (in debug mode)  // and freeing every segment except the one we wish to keep.  Segment* current = Segment::head();  // 处理keep节点，其余节点的内存都被释放  while (current != NULL) {    Segment* next = current->next();    if (current == keep) {      // Unlink the segment we wish to keep from the list.      current->clear_next();    } else {#ifdef DEBUG      // Zap the entire current segment (including the header).      memset(current, kZapDeadByte, current->size());#endif      Segment::Delete(current);    }    current = next;  }  // If we have found a segment we want to keep, we must recompute the  // variables 'position' and 'limit' to prepare for future allocate  // attempts. Otherwise, we must clear the position and limit to  // force a new segment to be allocated on demand.  // 更新属性，有保留的内存则用于下次分配  if (keep != NULL) {    Address start = keep->start();    position_ = RoundUp(start, kAlignment);    limit_ = keep->end();#ifdef DEBUG    // Zap the contents of the kept segment (but not the header).    memset(start, kZapDeadByte, keep->capacity());#endif  } else {    position_ = limit_ = 0;  }  // Update the head segment to be the kept segment (if any).  // 更新头指针  Segment::set_head(keep);}Address Zone::NewExpand(int size) {  // Make sure the requested size is already properly aligned and that  // there isn't enough room in the Zone to satisfy the request.  ASSERT(size == RoundDown(size, kAlignment));  ASSERT(position_ + size > limit_);  // Compute the new segment size. We use a 'high water mark'  // strategy, where we increase the segment size every time we expand  // except that we employ a maximum segment size when we delete. This  // is to avoid excessive malloc() and free() overhead.  Segment* head = Segment::head();  int old_size = (head == NULL) ? 0 : head->size();  int new_size = sizeof(Segment) + kAlignment + size + (old_size << 1);  if (new_size < kMinimumSegmentSize) new_size = kMinimumSegmentSize;  // 分配一个新的segment节点插入到链表  Segment* segment = Segment::New(new_size);  if (segment == NULL) V8::FatalProcessOutOfMemory("Zone");  // Recompute 'top' and 'limit' based on the new segment.  Address result = RoundUp(segment->start(), kAlignment);  // 更新属性，下次分配的时候使用  position_ = result + size;  limit_ = segment->end();  ASSERT(position_ <= limit_);  return result;}} }  // namespace v8::internal
   
     
 
    
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