fastpow: fast double precision power function

• Weak alias: pow
• C Prototype:
  • double fastpow (double x, double y);
  • Inputs:
    • double x - the double precision base input value.
    • double y - the double precision exponent input value.
  • Outputs:
    • x raised to the power y.
• Fortran Function Interface:
  • DOUBLE PRECISION FASTPOW(X,Y)
  • Inputs:
    • DOUBLE PRECISION X - the base value.
    • DOUBLE PRECISION Y - the exponent value.
  • Return Value:
    • X raised to the power Y.
• Notes:
  • fastpow computes the x raised to the power y in double precision.
    This is a relaxed version of pow, 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 1 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:
  • 200 cycles for most valid inputs.