src/math/rint.c

   1 /* origin: FreeBSD /usr/src/lib/msun/src/s_rint.c */
   2 /*
   3  * ====================================================
   4  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
   5  *
   6  * Developed at SunPro, a Sun Microsystems, Inc. business.
   7  * Permission to use, copy, modify, and distribute this
   8  * software is freely granted, provided that this notice
   9  * is preserved.
  10  * ====================================================
  11  */
  12 /*
  13  * rint(x)
  14  * Return x rounded to integral value according to the prevailing
  15  * rounding mode.
  16  * Method:
  17  *      Using floating addition.
  18  * Exception:
  19  *      Inexact flag raised if x not equal to rint(x).
  20  */
  21
  22 #include "libm.h"
  23
  24 static const double
  25 TWO52[2] = {
  26   4.50359962737049600000e+15, /* 0x43300000, 0x00000000 */
  27  -4.50359962737049600000e+15, /* 0xC3300000, 0x00000000 */
  28 };
  29
  30 double rint(double x)
  31 {
  32         int32_t i0,j0,sx;
  33         uint32_t i,i1;
  34         double w,t;
  35
  36         EXTRACT_WORDS(i0, i1, x);
  37         // FIXME: signed shift
  38         sx = (i0>>31) & 1;
  39         j0 = ((i0>>20)&0x7ff) - 0x3ff;
  40         if (j0 < 20) {
  41                 if (j0 < 0) {
  42                         if (((i0&0x7fffffff)|i1) == 0)
  43                                 return x;
  44                         i1 |= i0 & 0x0fffff;
  45                         i0 &= 0xfffe0000;
  46                         i0 |= ((i1|-i1)>>12) & 0x80000;
  47                         SET_HIGH_WORD(x, i0);
  48                         STRICT_ASSIGN(double, w, TWO52[sx] + x);
  49                         t = w - TWO52[sx];
  50                         GET_HIGH_WORD(i0, t);
  51                         SET_HIGH_WORD(t, (i0&0x7fffffff)|(sx<<31));
  52                         return t;
  53                 } else {
  54                         i = 0x000fffff>>j0;
  55                         if (((i0&i)|i1) == 0)
  56                                 return x; /* x is integral */
  57                         i >>= 1;
  58                         if (((i0&i)|i1) != 0) {
  59                                 /*
  60                                  * Some bit is set after the 0.5 bit.  To avoid the
  61                                  * possibility of errors from double rounding in
  62                                  * w = TWO52[sx]+x, adjust the 0.25 bit to a lower
  63                                  * guard bit.  We do this for all j0<=51.  The
  64                                  * adjustment is trickiest for j0==18 and j0==19
  65                                  * since then it spans the word boundary.
  66                                  */
  67                                 if (j0 == 19)
  68                                         i1 = 0x40000000;
  69                                 else if (j0 == 18)
  70                                         i1 = 0x80000000;
  71                                 else
  72                                         i0 = (i0 & ~i)|(0x20000>>j0);
  73                         }
  74                 }
  75         } else if (j0 > 51) {
  76                 if (j0 == 0x400)
  77                         return x+x;  /* inf or NaN */
  78                 return x;            /* x is integral */
  79         } else {
  80                 i = (uint32_t)0xffffffff>>(j0-20);
  81                 if ((i1&i) == 0)
  82                         return x;    /* x is integral */
  83                 i >>= 1;
  84                 if ((i1&i) != 0)
  85                         i1 = (i1 & ~i)|(0x40000000>>(j0-20));
  86         }
  87         INSERT_WORDS(x, i0, i1);
  88         STRICT_ASSIGN(double, w, TWO52[sx] + x);
  89         return w - TWO52[sx];
  90 }