vrda_log2: Array double precision base-2 logarithm

• C Prototype:
  • void vrda_log2 (int n, double *x, double *y)

  • Inputs:

    • int	n	- the number of values in both the input and output arrays.
      double	*x	- pointer to the array of input values.
      double	*y	- pointer to the array of output values.

  • Outputs:
    • The base-2 logarithm of each x value, filled into the y array.
• Fortran Subroutine Interface:
  • SUBROUTINE VRDA_LOG2(N,X,Y)
  • Inputs:
    • INTEGER N - the number of values in both the input and output arrays.
    • DOUBLE PRECISION X(N) - array of double precision input values.
  • Outputs:
    • DOUBLE PRECISION Y(N) - array of base-2 logarithms of input values.
• Notes:
  • vrda_log2 computes the double precision base-2 logarithm for each element of an array of input arguments.
    This routine accepts an array of double precision input values, computes the base-2 log for each input value, and stores the result in the array pointed to by the y pointer input. It is the responsibility of the calling program to allocate/deallocate enough storage for the output array. 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. 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	Output
    +/- 0	-infinity
    negative	QNaN
    QNaN	same QNaN
    SNaN	same NaN converted to QNaN
    +infinity	+infinity
    -infinity	QNaN

• Performance:
  • 54 cycles per value for valid inputs, n = 24, longer for input values very close to 1.0.