fastcos: fast double precision Cosine

• Weak alias: cos
• C Prototype:
  • double fastcos (double x);
  • Inputs:
    • double x - the double precision input value.
  • Outputs:
    • Cosine of x.
• Fortran Function Interface:
  • DOUBLE PRECISION FASTCOS(X)
  • Inputs:
    • DOUBLE PRECISION X - the double precision input value.
  • Return Value:
    • Cosine of X.
• Notes:
  • fastcos computes the Cosine function of its argument x.
    
    This is a relaxed version of cos, suitable for use with fastmath compiler flags or applications not requiring full error handling. Denormal inputs may produce unpredictable results. Error inputs produce C99 return values. The routine is accurate to better than 2 ulp over most of the valid input range.
• Special case return values:

  • Input	Output
    QNaN	same QNaN
    SNaN	same NaN converted to QNaN
    +infinity	QNaN
    -infinity	QNaN

• Performance:
  • 88 cycles for most valid inputs < 5e5.