prime/src/Runtime/s_ldexp.c

/* @(#)s_ldexp.c 1.2 95/01/04 */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunPro, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice
 * is preserved.
 * ====================================================
 */

#include "fdlibm.h"

static const double two54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
    twom54 = 5.55111512312578270212e-17,                /* 0x3C900000, 0x00000000 */
    big = 1.0e+300, tiny = 1.0e-300;

double ldexp(double x, int n) {
  _INT32 k, hx, lx;
  if (!isfinite(x) || x == 0.0)
    return x;

  hx = __HI(x);
  lx = __LO(x);
  k = (hx & 0x7ff00000) >> 20; /* extract exponent */
  if (k == 0) {                /* 0 or subnormal x */
    if ((lx | (hx & 0x7fffffff)) == 0)
      return x; /* +-0 */
    x *= two54;
    hx = __HI(x);
    k = ((hx & 0x7ff00000) >> 20) - 54;
    if (n < -50000)
      return tiny * x; /*underflow*/
  }
  if (k == 0x7ff)
    return x + x; /* NaN or Inf */
  k = k + n;
  if (k > 0x7fe)
    return big * copysign(big, x); /* overflow  */
  if (k > 0)                       /* normal result */
  {
    __HI(x) = (hx & 0x800fffff) | (k << 20);
    return x;
  }
  if (k <= -54)
    if (n > 50000)                   /* in case integer overflow in n+k */
      return big * copysign(big, x); /*overflow*/
    else
      return tiny * copysign(tiny, x); /*underflow*/
  k += 54;                             /* subnormal result */
  __HI(x) = (hx & 0x800fffff) | (k << 20);
  return x * twom54;
}