js引擎zone怎么使用

这篇文章主要讲解了“js引擎zone怎么使用”，文中的讲解内容简单清晰，易于学习与理解，下面请大家跟着小编的思路慢慢深入，一起来研究和学习“js引擎zone怎么使用”吧！

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zone也是用于内存管理的，不过他是增量分配，一次销毁的。下面是结构图。

zone.h

   
     
 
    
   

#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
// ZoneList本质上是一个list，ZoneListAllocationPolicy是list里的内存管理器
class ZoneList: public List {
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(capacity) { }
};


} }  // namespace v8::internal

#endif  // V8_ZONE_H_
   
     
 
    
    

zone-inl.h

   
     
 
    
   

#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(result);
}


} }  // namespace v8::internal

#endif  // V8_ZONE_INL_H_


   
     
 
    
    

zone.cc      

   
     
 
    
   

#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(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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