How to analyze HeapNumber for js engine v8 source code
In this issue, the editor will bring you about how to carry out js engine v8 source code analysis HeapNumber. The article is rich in content and analyzes and describes for you from a professional point of view. I hope you can get something after reading this article.
A HeapNumber is an object that holds a large plastic shape. Smi saves plastic surgery in V8, but it only has 31 bits, and those with more than 31 bits need to use HeapNumber.
/ / A heap object with stored numbers
Class HeapNumber: public HeapObject {
Public:
Inline double value ()
Inline void set_value (double value)
Static inline HeapNumber* cast (Object* obj)
Object* HeapNumberToBoolean ()
/ / Layout description.
/ / the space before kSize stores pointers to map objects
Static const int kValueOffset = HeapObject::kSize
/ / the value of the number stored between kValueOffset-kSize.
Static const int kSize = kValueOffset + kDoubleSize
Private:
DISALLOW_IMPLICIT_CONSTRUCTORS (HeapNumber)
}
The following is implemented.
1 access value
/ / returns the value of type double
Double HeapNumber::value () {
Return READ_DOUBLE_FIELD (this, kValueOffset)
}
/ / write double value to object
Void HeapNumber::set_value (double value) {
WRITE_DOUBLE_FIELD (this, kValueOffset, value)
}
2 digits are converted to Boolean values. 0 and NAN are false, and the rest are true.
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 ()
}
}
/ / refer to the definition of floating point number in ieee.
Int fpclassify (double x) {
/ / Use the MS-specific _ fpclass () for classification.
Int flags = _ fpclass (x)
/ / non-0 positive or non-0 negative
If (flags & (_ FPCLASS_PN | _ FPCLASS_NN)) return FP_NORMAL
/ / positive or negative 0
If (flags & (_ FPCLASS_PZ | _ FPCLASS_NZ)) return FP_ZERO
/ / non-standardized positive or negative values
If (flags & (_ FPCLASS_PD | _ FPCLASS_ND)) return FP_SUBNORMAL
/ / positive and negative infinity
If (flags & (_ FPCLASS_PINF | _ FPCLASS_NINF)) return FP_INFINITE
/ / All cases should be covered by the code above.
/ / not a numeric value
ASSERT (flags & (_ FPCLASS_SNAN | _ FPCLASS_QNAN))
Return FP_NAN
}
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