fastlogf: fast single precision natural logarithm function

• Weak alias: logf
• C Prototype:
  • float fastlogf (float x);
  • Inputs:
    • float x - the single precision input value.
  • Outputs:
    • The natural logarithm (base e) of x.
• Fortran Function Interface:
  • REAL FASTLOGF(X)
  • Inputs:
    • REAL X - the single precision input value.
  • Return Value:
    • The natural logarithm (base e) of X.
• Notes:
  • fastlogf computes the single precision natural logarithm of its argument x.
    
    This is a relaxed version of logf, 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:
  • 94 cycles for most valid inputs.
    85 cycles for .97 < x < 1.03