lib/libm/src/s_rint.c

   1 /* @(#)s_rint.c 5.1 93/09/24 */
   2 /* $FreeBSD: head/lib/msun/src/s_rint.c 176451 2008-02-22 02:30:36Z das $ */
   3 /*
   4  * ====================================================
   5  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
   6  *
   7  * Developed at SunPro, a Sun Microsystems, Inc. business.
   8  * Permission to use, copy, modify, and distribute this
   9  * software is freely granted, provided that this notice
  10  * is preserved.
  11  * ====================================================
  12  */
  13
  14 /*
  15  * rint(x)
  16  * Return x rounded to integral value according to the prevailing
  17  * rounding mode.
  18  * Method:
  19  *      Using floating addition.
  20  * Exception:
  21  *      Inexact flag raised if x not equal to rint(x).
  22  */
  23
  24 #include <float.h>
  25
  26 #include "math.h"
  27 #include "math_private.h"
  28
  29 static const double
  30 TWO52[2]={
  31   4.50359962737049600000e+15, /* 0x43300000, 0x00000000 */
  32  -4.50359962737049600000e+15, /* 0xC3300000, 0x00000000 */
  33 };
  34
  35 double
  36 rint(double x)
  37 {
  38         int32_t i0,j0,sx;
  39         u_int32_t i,i1;
  40         double w,t;
  41         EXTRACT_WORDS(i0,i1,x);
  42         sx = (i0>>31)&1;
  43         j0 = ((i0>>20)&0x7ff)-0x3ff;
  44         if(j0<20) {
  45             if(j0<0) {
  46                 if(((i0&0x7fffffff)|i1)==0) return x;
  47                 i1 |= (i0&0x0fffff);
  48                 i0 &= 0xfffe0000;
  49                 i0 |= ((i1|-i1)>>12)&0x80000;
  50                 SET_HIGH_WORD(x,i0);
  51                 STRICT_ASSIGN(double,w,TWO52[sx]+x);
  52                 t =  w-TWO52[sx];
  53                 GET_HIGH_WORD(i0,t);
  54                 SET_HIGH_WORD(t,(i0&0x7fffffff)|(sx<<31));
  55                 return t;
  56             } else {
  57                 i = (0x000fffff)>>j0;
  58                 if(((i0&i)|i1)==0) return x; /* x is integral */
  59                 i>>=1;
  60                 if(((i0&i)|i1)!=0) {
  61                     /*
  62                      * Some bit is set after the 0.5 bit.  To avoid the
  63                      * possibility of errors from double rounding in
  64                      * w = TWO52[sx]+x, adjust the 0.25 bit to a lower
  65                      * guard bit.  We do this for all j0<=51.  The
  66                      * adjustment is trickiest for j0==18 and j0==19
  67                      * since then it spans the word boundary.
  68                      */
  69                     if(j0==19) i1 = 0x40000000; else
  70                     if(j0==18) i1 = 0x80000000; else
  71                     i0 = (i0&(~i))|((0x20000)>>j0);
  72                 }
  73             }
  74         } else if (j0>51) {
  75             if(j0==0x400) return x+x;   /* inf or NaN */
  76             else return x;              /* x is integral */
  77         } else {
  78             i = ((u_int32_t)(0xffffffff))>>(j0-20);
  79             if((i1&i)==0) return x;     /* x is integral */
  80             i>>=1;
  81             if((i1&i)!=0) i1 = (i1&(~i))|((0x40000000)>>(j0-20));
  82         }
  83         INSERT_WORDS(x,i0,i1);
  84         STRICT_ASSIGN(double,w,TWO52[sx]+x);
  85         return w-TWO52[sx];
  86 }
  87
  88 #if (LDBL_MANT_DIG == 53)
  89 __weak_reference(rint, rintl);
  90 #endif