mingw-w64-crt/math/powi.def.h

/*
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*/

/* IEEE 754 - Elementary Functions - Special Cases
 * powi (x, +/-0) is 1 for any x (even a zero, quiet NaN, or infinity)
 * powi (+1, y) is 1 for any y (even a quiet NaN)
 * powi (+/-0, y) is +/-oo and signals the divideByZero exception for y an odd integer < 0
 * powi (+/-0, y) is +oo and signals the divideByZero exception for finite y < 0 and not an odd integer
 * powi (+/-0, y) is +/-0 for finite y > 0 an odd integer
 * powi (+/-0, y) is +0 for finite y > 0 and not an odd integer
 powi (-inf, y) = +0 for y<0 and not an odd integer
 powi (-inf, y) = -inf for y an odd integer > 0
 powi (-inf, y) = +inf for y>0 and not an odd integer
 powi (+/-inf, y) is +/-0 with no exception for y an odd integer < 0
 powi (+/-inf, y) is +0 with no exception for finite y < 0 and not an odd integer
 powi (+/-inf, y) is +/-inf with no exception for finite y > 0 an odd integer
 powi (+/-inf, y) is +inf with no exception for finite y > 0 and not an odd integer
 powi (x, y) signals the invalid operation exception for finite x < 0 and finite non-integer y.
 
 For x /= 0: lim y->oo (1/x)^y results as: for |x| < 1 that sgn(x)*0 and for |x| > 0 that sgn(x)*Infinity

*/
#include "../complex/complex_internal.h"
#include <errno.h>
#include <limits.h>
#include <fenv.h>
#include <math.h>
#include <errno.h>

static __FLT_TYPE do_powi_iter(__FLT_TYPE d, int y)
{
  unsigned int u = (unsigned int) y;
  __FLT_TYPE rslt = ((u & 1) != 0) ? d : __FLT_CST(1.0);
  u >>= 1;
  do
    {
      d *= d;
      if ((u & 1) != 0)
        rslt *= d;
      u >>= 1;
    }
  while (u > 0);
  return rslt;
}

__FLT_TYPE __cdecl
__FLT_ABI(__powi) (__FLT_TYPE x, int y);

__FLT_TYPE __cdecl
__FLT_ABI(__powi) (__FLT_TYPE x, int y)
{
  int x_class = fpclassify (x);
  int odd_y = y & 1;
  int recip = 0;
  __FLT_TYPE d, rslt;

  if (y == 0 || x == __FLT_CST(1.0))
    return __FLT_CST(1.0);
  else if (x_class == FP_NAN)
    {
      rslt = (signbit(x) ? -__FLT_NAN : __FLT_NAN);
      __FLT_RPT_DOMAIN ("__powi", x, (__FLT_TYPE) y, rslt);
      return rslt;
    }
  else if (x_class == FP_ZERO)
    {
      if (y >= 0)
	{
	  if (!odd_y || !signbit (x))
	    return __FLT_CST (0.0);
	  return -__FLT_CST(0.0);
	}

      if (!odd_y || !signbit (x))
	return __FLT_HUGE_VAL;
      return (signbit(x) ? -__FLT_HUGE_VAL : __FLT_HUGE_VAL);
    }
  else if (x_class == FP_INFINITE)
    {
      /* pow( -inf, y) = +0 for y<0 and not an odd integer,  */
      if (signbit(x) && y < 0 && !odd_y)
	return __FLT_CST(0.0);
      /* pow( -inf, y) = -inf for y an odd integer > 0.  */
      if (signbit(x) && y >= 0 && odd_y)
	return -__FLT_HUGE_VAL;
      /* pow( -inf, y) = +inf for y>0 and not an odd integer.  */
      if (signbit(x) && y >= 0 && !odd_y)
	return __FLT_HUGE_VAL;
      /* pow (+/-inf, y) is +/-0 with no exception for y an odd integer < 0. */
      if (y < 0)
      {
        /* pow (+/-inf, y) is +0 with no exception for finite y < 0 and not an odd integer.  */
	return (odd_y && signbit(x) ? -__FLT_CST(0.0) : __FLT_CST(0.0));
      }
      /* pow (+/-inf, y) is +/-inf with no exception for finite y > 0 an odd integer.  */
      /* pow (+/-inf, y) is +inf with no exception for finite y > 0 and not an odd integer.  */
      return (odd_y && signbit(x) ? -__FLT_HUGE_VAL : __FLT_HUGE_VAL);
    }

  d = __FLT_ABI(fabs) (x);

  if (y < 0)
    {
      /* By default, do the reciprocal of the result. */
      recip = 1;
      y = -y;
    }

  if (!y)
    rslt = __FLT_CST(1.0);
  else if (y == 1)
    rslt = d;
  else
    {
      rslt = do_powi_iter(d, y);
      if (recip && fpclassify(rslt) == FP_INFINITE && d > __FLT_CST(1.0))
        {
          /* Uncommon case - we had overflow, but we're going to calculate
             the reciprocal. If this happened, redo the calculation by doing
             the reciprocal upfront instead. Instead of trying to calculate
             whether this will happen, we prefer keeping the default case
             cheap. */
          d = __FLT_CST(1.0) / d;
          recip = 0;
          rslt = do_powi_iter(d, y);
        }
    }
  if (recip)
    rslt = __FLT_CST(1.0) / rslt;
  if (signbit (x) && odd_y)
    rslt = -rslt;
  return rslt;
}