What could /possibly/ go wrong?

gistfile1.c

double
SECTION
__sin (double x)
{
  double xx, res, t, cor, y, s, c, sn, ssn, cs, ccs, xn, a, da, db, eps, xn1,
    xn2;
  mynumber u, v;
  int4 k, m, n;
  double retval = 0;

  SET_RESTORE_ROUND_53BIT (FE_TONEAREST);

  u.x = x;
  m = u.i[HIGH_HALF];
  k = 0x7fffffff & m;		/* no sign           */
  if (k < 0x3e500000)		/* if x->0 =>sin(x)=x */
    retval = x;
 /*---------------------------- 2^-26 < |x|< 0.25 ----------------------*/
  else if (k < 0x3fd00000)
    {
      xx = x * x;
      /* Taylor series.  */
      t = POLYNOMIAL (xx) * (xx * x);
      res = x + t;
      cor = (x - res) + t;
      retval = (res == res + 1.07 * cor) ? res : slow (x);
    }				/*  else  if (k < 0x3fd00000)    */
/*---------------------------- 0.25<|x|< 0.855469---------------------- */
  else if (k < 0x3feb6000)
    {
      u.x = (m > 0) ? big + x : big - x;
      y = (m > 0) ? x - (u.x - big) : x + (u.x - big);
      xx = y * y;
      s = y + y * xx * (sn3 + xx * sn5);
      c = xx * (cs2 + xx * (cs4 + xx * cs6));
      SINCOS_TABLE_LOOKUP (u, sn, ssn, cs, ccs);
      if (m <= 0)
        {
          sn = -sn;
	  ssn = -ssn;
	}
      cor = (ssn + s * ccs - sn * c) + cs * s;
      res = sn + cor;
      cor = (sn - res) + cor;
      retval = (res == res + 1.096 * cor) ? res : slow1 (x);
    }				/*   else  if (k < 0x3feb6000)    */

/*----------------------- 0.855469  <|x|<2.426265  ----------------------*/
  else if (k < 0x400368fd)
    {

      y = (m > 0) ? hp0 - x : hp0 + x;
      if (y >= 0)
	{
	  u.x = big + y;
	  y = (y - (u.x - big)) + hp1;
	}
      else
	{
	  u.x = big - y;
	  y = (-hp1) - (y + (u.x - big));
	}
      res = do_cos (u, y, &cor);
      retval = (res == res + 1.020 * cor) ? ((m > 0) ? res : -res) : slow2 (x);
    }				/*   else  if (k < 0x400368fd)    */

/*-------------------------- 2.426265<|x|< 105414350 ----------------------*/
  else if (k < 0x419921FB)
    {
      t = (x * hpinv + toint);
      xn = t - toint;
      v.x = t;
      y = (x - xn * mp1) - xn * mp2;
      n = v.i[LOW_HALF] & 3;
      da = xn * mp3;
      a = y - da;
      da = (y - a) - da;
      eps = ABS (x) * 1.2e-30;

      switch (n)
	{			/* quarter of unit circle */
	case 0:
	case 2:
	  xx = a * a;
	  if (n)
	    {
	      a = -a;
	      da = -da;
	    }
	  if (xx < 0.01588)
	    {
	      /* Taylor series.  */
	      res = TAYLOR_SIN (xx, a, da, cor);
	      cor = (cor > 0) ? 1.02 * cor + eps : 1.02 * cor - eps;
	      retval = (res == res + cor) ? res : sloww (a, da, x);
	    }
	  else
	    {
	      if (a > 0)
		m = 1;
	      else
		{
		  m = 0;
		  a = -a;
		  da = -da;
		}
	      u.x = big + a;
	      y = a - (u.x - big);
	      res = do_sin (u, y, da, &cor);
	      cor = (cor > 0) ? 1.035 * cor + eps : 1.035 * cor - eps;
	      retval = ((res == res + cor) ? ((m) ? res : -res)
			: sloww1 (a, da, x, m));
	    }
	  break;

	case 1:
	case 3:
	  if (a < 0)
	    {
	      a = -a;
	      da = -da;
	    }
	  u.x = big + a;
	  y = a - (u.x - big) + da;
	  res = do_cos (u, y, &cor);
	  cor = (cor > 0) ? 1.025 * cor + eps : 1.025 * cor - eps;
	  retval = ((res == res + cor) ? ((n & 2) ? -res : res)
		    : sloww2 (a, da, x, n));
	  break;
	}
    }				/*   else  if (k <  0x419921FB )    */

/*---------------------105414350 <|x|< 281474976710656 --------------------*/
  else if (k < 0x42F00000)
    {
      t = (x * hpinv + toint);
      xn = t - toint;
      v.x = t;
      xn1 = (xn + 8.0e22) - 8.0e22;
      xn2 = xn - xn1;
      y = ((((x - xn1 * mp1) - xn1 * mp2) - xn2 * mp1) - xn2 * mp2);
      n = v.i[LOW_HALF] & 3;
      da = xn1 * pp3;
      t = y - da;
      da = (y - t) - da;
      da = (da - xn2 * pp3) - xn * pp4;
      a = t + da;
      da = (t - a) + da;
      eps = 1.0e-24;

      switch (n)
	{
	case 0:
	case 2:
	  xx = a * a;
	  if (n)
	    {
	      a = -a;
	      da = -da;
	    }
	  if (xx < 0.01588)
	    {
	      /* Taylor series.  */
	      res = TAYLOR_SIN (xx, a, da, cor);
	      cor = (cor > 0) ? 1.02 * cor + eps : 1.02 * cor - eps;
	      retval = (res == res + cor) ? res : bsloww (a, da, x, n);
	    }
	  else
	    {
	      double t;
	      if (a > 0)
		{
		  m = 1;
		  t = a;
		  db = da;
		}
	      else
		{
		  m = 0;
		  t = -a;
		  db = -da;
		}
	      u.x = big + t;
	      y = t - (u.x - big);
	      res = do_sin (u, y, db, &cor);
	      cor = (cor > 0) ? 1.035 * cor + eps : 1.035 * cor - eps;
	      retval = ((res == res + cor) ? ((m) ? res : -res)
			: bsloww1 (a, da, x, n));
	    }
	  break;

	case 1:
	case 3:
	  if (a < 0)
	    {
	      a = -a;
	      da = -da;
	    }
	  u.x = big + a;
	  y = a - (u.x - big) + da;
	  res = do_cos (u, y, &cor);
	  cor = (cor > 0) ? 1.025 * cor + eps : 1.025 * cor - eps;
	  retval = ((res == res + cor) ? ((n & 2) ? -res : res)
		    : bsloww2 (a, da, x, n));
	  break;
	}
    }				/*   else  if (k <  0x42F00000 )   */

/* -----------------281474976710656 <|x| <2^1024----------------------------*/
  else if (k < 0x7ff00000)
    retval = reduce_and_compute (x, 0);

/*--------------------- |x| > 2^1024 ----------------------------------*/
  else
    {
      if (k == 0x7ff00000 && u.i[LOW_HALF] == 0)
	__set_errno (EDOM);
      retval = x / x;
    }

  return retval;
}