compiler/stdc/math/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: src/lib/msun/src/s_rint.c,v 1.13 2005/12/03 07:38:35 bde Exp $";
  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         uint32_t i,i1;
  41         volatile double w; /* N.b. - gcc optimises "w" away so use volatile to work around*/
  42         double t;
  43         EXTRACT_WORDS(i0,i1,x);
  44         sx = (i0>>31)&1;
  45         j0 = ((i0>>20)&0x7ff)-0x3ff;
  46         if(j0<20) {
  47             if(j0<0) {
  48                 if(((i0&0x7fffffff)|i1)==0) return x;
  49                 i1 |= (i0&0x0fffff);
  50                 i0 &= 0xfffe0000;
  51                 i0 |= ((i1|-i1)>>12)&0x80000;
  52                 SET_HIGH_WORD(x,i0);
  53                 w = TWO52[sx]+x;
  54                 t =  w-TWO52[sx];
  55                 GET_HIGH_WORD(i0,t);
  56                 SET_HIGH_WORD(t,(i0&0x7fffffff)|(sx<<31));
  57                 return t;
  58             } else {
  59                 i = (0x000fffff)>>j0;
  60                 if(((i0&i)|i1)==0) return x; /* x is integral */
  61                 i>>=1;
  62                 if(((i0&i)|i1)!=0) {
  63                     /*
  64                      * Some bit is set after the 0.5 bit.  To avoid the
  65                      * possibility of errors from double rounding in
  66                      * w = TWO52[sx]+x, adjust the 0.25 bit to a lower
  67                      * guard bit.  We do this for all j0<=51.  The
  68                      * adjustment is trickiest for j0==18 and j0==19
  69                      * since then it spans the word boundary.
  70                      */
  71                     if(j0==19) i1 = 0x40000000; else
  72                     if(j0==18) i1 = 0x80000000; else
  73                     i0 = (i0&(~i))|((0x20000)>>j0);
  74                 }
  75             }
  76         } else if (j0>51) {
  77             if(j0==0x400) return x+x;   /* inf or NaN */
  78             else return x;              /* x is integral */
  79         } else {
  80             i = ((uint32_t)(0xffffffff))>>(j0-20);
  81             if((i1&i)==0) return x;     /* x is integral */
  82             i>>=1;
  83             if((i1&i)!=0) i1 = (i1&(~i))|((0x40000000)>>(j0-20));
  84         }
  85         INSERT_WORDS(x,i0,i1);
  86         *(volatile double *)&w = TWO52[sx]+x;   /* clip any extra precision */
  87         return w-TWO52[sx];
  88 }