js引擎v8源码分析之HeapNumber（基于v8 0.1.5）

// 存储了数字的堆对象
class HeapNumber: public HeapObject {
 public:
  inline double value();
  inline void set_value(double value);

  static inline HeapNumber* cast(Object* obj);

  Object* HeapNumberToBoolean();

  // Layout description.
  // kSize之前的空间存储map对象的指针
  static const int kValueOffset = HeapObject::kSize;
  // kValueOffset - kSize之间存储数字的值，double型
  static const int kSize = kValueOffset + kDoubleSize;

 private:
  DISALLOW_IMPLICIT_CONSTRUCTORS(HeapNumber);
};

// 返回double类型的值
double HeapNumber::value() {
  return READ_DOUBLE_FIELD(this, kValueOffset);
}

// 写double值到对象
void HeapNumber::set_value(double value) {
  WRITE_DOUBLE_FIELD(this, kValueOffset, value);
}

Object* HeapNumber::HeapNumberToBoolean() {
  // NaN, +0, and -0 should return the false object
  switch (fpclassify(value())) {
    case FP_NAN:  // fall through
    case FP_ZERO: return Heap::false_value();
    default: return Heap::true_value();
  }
}

// 参考ieee对浮点数的定义
int fpclassify(double x) {
  // Use the MS-specific _fpclass() for classification.
  int flags = _fpclass(x);

  // 非0正值或非0负值
  if (flags & (_FPCLASS_PN | _FPCLASS_NN)) return FP_NORMAL;
  // 正数0或负数0
  if (flags & (_FPCLASS_PZ | _FPCLASS_NZ)) return FP_ZERO;
  // 非标准化的正或负值 
  if (flags & (_FPCLASS_PD | _FPCLASS_ND)) return FP_SUBNORMAL;
  // 正负无穷
  if (flags & (_FPCLASS_PINF | _FPCLASS_NINF)) return FP_INFINITE;

  // All cases should be covered by the code above.
  // 不是数值
  ASSERT(flags & (_FPCLASS_SNAN | _FPCLASS_QNAN));
  return FP_NAN;
}