fastlog: fast double precision natural logarithm function

• Weak alias: log
• C Prototype:
  • double fastlog (double x);
  • Inputs:
    • double x - the double precision input value.
  • Outputs:
    • The natural logarithm (base e) of x.
• Fortran Function Interface:
  • DOUBLE PRECISION FASTLOG(X)
  • Inputs:
    • DOUBLE PRECISION X - the double precision input value.
  • Return Value:
    • The natural logarithm (base e) of X.
• Notes:
  • fastlog computes the double precision natural logarithm of its argument x.
    
    This is a relaxed version of log, 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 1 ulp over the valid input range.
• Special case return values:

  • Input	Output
    +/- 0	-infinity
    negative	QNaN
    QNaN	same QNaN
    SNaN	same NaN converted to QNaN
    +infinity	+infinity
    -infinity	QNaN

• Performance:
  • 97 cycles for most valid inputs.
    86 cycles for .97 < x < 1.03