lib/msun/src/s_rint.c

   1 /* @(#)s_rint.c 5.1 93/09/24 */
   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 #ifndef lint
  14 static char rcsid[] = "$FreeBSD$";
  15 #endif
  16
  17 /*
  18  * rint(x)
  19  * Return x rounded to integral value according to the prevailing
  20  * rounding mode.
  21  * Method:
  22  *      Using floating addition.
  23  * Exception:
  24  *      Inexact flag raised if x not equal to rint(x).
  25  */
  26
  27 #include "math.h"
  28 #include "math_private.h"
  29
  30 static const double
  31 TWO52[2]={
  32   4.50359962737049600000e+15, /* 0x43300000, 0x00000000 */
  33  -4.50359962737049600000e+15, /* 0xC3300000, 0x00000000 */
  34 };
  35
  36 double
  37 rint(double x)
  38 {
  39         int32_t i0,j0,sx;
  40         u_int32_t i,i1;
  41         double w,t;
  42         EXTRACT_WORDS(i0,i1,x);
  43         sx = (i0>>31)&1;
  44         j0 = ((i0>>20)&0x7ff)-0x3ff;
  45         if(j0<20) {
  46             if(j0<0) {
  47                 if(((i0&0x7fffffff)|i1)==0) return x;
  48                 i1 |= (i0&0x0fffff);
  49                 i0 &= 0xfffe0000;
  50                 i0 |= ((i1|-i1)>>12)&0x80000;
  51                 SET_HIGH_WORD(x,i0);
  52                 w = TWO52[sx]+x;
  53                 t =  w-TWO52[sx];
  54                 GET_HIGH_WORD(i0,t);
  55                 SET_HIGH_WORD(t,(i0&0x7fffffff)|(sx<<31));
  56                 return t;
  57             } else {
  58                 i = (0x000fffff)>>j0;
  59                 if(((i0&i)|i1)==0) return x; /* x is integral */
  60                 i>>=1;
  61                 if(((i0&i)|i1)!=0) {
  62                     /*
  63                      * Some bit is set after the 0.5 bit.  To avoid the
  64                      * possibility of errors from double rounding in
  65                      * w = TWO52[sx]+x, adjust the 0.25 bit to a lower
  66                      * guard bit.  We do this for all j0<=51.  The
  67                      * adjustment is trickiest for j0==18 and j0==19
  68                      * since then it spans the word boundary.
  69                      */
  70                     if(j0==19) i1 = 0x40000000; else
  71                     if(j0==18) i1 = 0x80000000; else
  72                     i0 = (i0&(~i))|((0x20000)>>j0);
  73                 }
  74             }
  75         } else if (j0>51) {
  76             if(j0==0x400) return x+x;   /* inf or NaN */
  77             else return x;              /* x is integral */
  78         } else {
  79             i = ((u_int32_t)(0xffffffff))>>(j0-20);
  80             if((i1&i)==0) return x;     /* x is integral */
  81             i>>=1;
  82             if((i1&i)!=0) i1 = (i1&(~i))|((0x40000000)>>(j0-20));
  83         }
  84         INSERT_WORDS(x,i0,i1);
  85         *(volatile double *)&w = TWO52[sx]+x;   /* clip any extra precision */
  86         return w-TWO52[sx];
  87 }