fastlog2: fast double precision base-2 logarithm function

• Weak alias: log2
• C Prototype:
  • double fastlog2 (double x);
  • Inputs:
    • double x - the double precision input value.
  • Outputs:
    • The base-2 logarithm of x.
• Fortran Function Interface:
  • DOUBLE PRECISION FASTLOG2(X)
  • Inputs:
    • DOUBLE PRECISION X - the double precision input value.
  • Return Value:
    • The base-2 logarithm of X.
• Notes:
  • fastlog2 computes the double precision base-2 logarithm of its argument x.
    
    This is a relaxed version of log2, 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:
  • 112 cycles for most valid inputs.