s_scalbn.c

/* @(#)s_scalbn.c 1.3 95/01/18 */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunSoft, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice 
 * is preserved.
 * ====================================================
 */

/* 
 * scalbn (double x, int n)
 * scalbn(x,n) returns x* 2**n  computed by  exponent  
 * manipulation rather than by actually performing an 
 * exponentiation or a multiplication.
 */

#include "fdlibm.h"

static const double
two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
huge   = 1.0e+300,
tiny   = 1.0e-300;

double scalbn (double x, int n)
{
        int  k,hx,lx;
        __getHI(hx,x);
        __getLO(lx,x);
        k = (hx&0x7ff00000)>>20;                /* extract exponent */
        if (k==0) {                             /* 0 or subnormal x */
            if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
            x *= two54; 
            __getHI(hx,x);
            k = ((hx&0x7ff00000)>>20) - 54; 
            if (n< -50000) return tiny*x;       /*underflow*/
            }
        if (k==0x7ff) return x+x;               /* NaN or Inf */
        k = k+n; 
        if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
        if (k > 0)                              /* normal result */
            {__setHI(x, (hx&0x800fffff)|(k<<20)); return x;}
        if (k <= -54) {
            if (n > 50000)      /* in case integer overflow in n+k */
                return huge*copysign(huge,x);   /*overflow*/
            else return tiny*copysign(tiny,x);  /*underflow*/
        }
        k += 54;                                /* subnormal result */
        __setHI(x, (hx&0x800fffff)|(k<<20));
        return x*twom54;
}