musl/src/math/s_rint.c

/* @(#)s_rint.c 5.1 93/09/24 */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunPro, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice
 * is preserved.
 * ====================================================
 */

/*
 * rint(x)
 * Return x rounded to integral value according to the prevailing
 * rounding mode.
 * Method:
 *      Using floating addition.
 * Exception:
 *      Inexact flag raised if x not equal to rint(x).
 */

#include <math.h>
#include "math_private.h"

/*
 * TWO23 is long double instead of double to avoid a bug in gcc.  Without
 * this, gcc thinks that TWO23[sx]+x and w-TWO23[sx] already have double
 * precision and doesn't clip them to double precision when they are
 * assigned and returned.
 */
static const long double
TWO52[2]={
  4.50359962737049600000e+15, /* 0x43300000, 0x00000000 */
 -4.50359962737049600000e+15, /* 0xC3300000, 0x00000000 */
};

double
rint(double x)
{
        int32_t i0,j0,sx;
        uint32_t i,i1;
        double w,t;
        EXTRACT_WORDS(i0,i1,x);
        sx = (i0>>31)&1;
        j0 = ((i0>>20)&0x7ff)-0x3ff;
        if(j0<20) {
            if(j0<0) {
                if(((i0&0x7fffffff)|i1)==0) return x;
                i1 |= (i0&0x0fffff);
                i0 &= 0xfffe0000;
                i0 |= ((i1|-i1)>>12)&0x80000;
                SET_HIGH_WORD(x,i0);
                w = TWO52[sx]+x;
                t =  w-TWO52[sx];
                GET_HIGH_WORD(i0,t);
                SET_HIGH_WORD(t,(i0&0x7fffffff)|(sx<<31));
                return t;
            } else {
                i = (0x000fffff)>>j0;
                if(((i0&i)|i1)==0) return x; /* x is integral */
                i>>=1;
                if(((i0&i)|i1)!=0) {
                    if(j0==19) i1 = 0x40000000; else
                    i0 = (i0&(~i))|((0x20000)>>j0);
                }
            }
        } else if (j0>51) {
            if(j0==0x400) return x+x;   /* inf or NaN */
            else return x;              /* x is integral */
        } else {
            i = ((uint32_t)(0xffffffff))>>(j0-20);
            if((i1&i)==0) return x;     /* x is integral */
            i>>=1;
            if((i1&i)!=0) i1 = (i1&(~i))|((0x40000000)>>(j0-20));
        }
        INSERT_WORDS(x,i0,i1);
        w = TWO52[sx]+x;
        return w-TWO52[sx];
}