fastpowf: fast single precision power function

• Weak alias: powf
• C Prototype:
  • float fastpowf (float x, float y);
  • Inputs:
    • float x - the single precision base input value.
    • float y - the single precision exponent input value.
  • Outputs:
    • x raised to the power y.
• Fortran Function Interface:
  • REAL FASTPOWF(X,Y)
  • Inputs:
    • REAL X - the single precision base value.
    • REAL Y - the single precision exponent value.
  • Return Value:
    • X raised to the power Y.
• Notes:
  • fastpowf computes the x raised to the power y in single precision.
    This is a relaxed version of powf, suitable for use with fastmath compiler flags or applications not requiring full error handling. Denormal inputs will produce unpredictable results. Special case inputs produce C99 return values. The routine is accurate to better than 0.5 ulp over the valid input range.
• Special case return values:

  • Input x	Input y	Output
    +/- 0	y<0, odd integer	+/- infinity
    +/- 0	y<0, not odd integer	+infinity
    +/- 0	y>0, odd integer	+/- 0
    +/- 0	y>0, not odd integer	+0
    -1	+infinity	1
    +1	y (incl. NaN)	1
    x (incl. Nan)	+/- 0	1
    x<0	y, not integer	QNaN
    |x|<1	-infinity	+infinity
    |x|>1	-infinity	+0
    |x|<1	+infinity	+0
    |x|>1	+infinity	+infinity
    -infinity	y<0, odd integer	-0
    -infinity	y<0, not odd integer	+0
    -infinity	y>0, odd integer	-infinity
    -infinity	y>0, not odd integer	+infinity
    +infinity	y<0,	+0
    +infinity	y>0,	+infinity
    NaN	y nonzero,	NaN
    x<>1	NaN,	NaN

• Performance:
  • 175 cycles for most valid inputs.