s_scalbn.c revision 1.1 
1
2/* @(#)s_scalbn.c 5.1 93/09/24 */
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 * scalbn (double x, int n)
16 * scalbn(x,n) returns x* 2**n  computed by  exponent
17 * manipulation rather than by actually performing an
18 * exponentiation or a multiplication.
19 */
20
21#include "fdlibm.h"
22
23#ifdef __STDC__
24static const double
25#else
26static double
27#endif
28two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
29twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
30huge   = 1.0e+300,
31tiny   = 1.0e-300;
32
33#ifdef __STDC__
34	double scalbn (double x, int n)
35#else
36	double scalbn (x,n)
37	double x; int n;
38#endif
39{
40	int  k,n0,hx,lx;
41	n0 = ((*(int*)&two54)>>30)^1;		/* high word index */
42	hx = *(n0+(int*)&x);
43	lx = *(1-n0+(int*)&x);
44        k = (hx&0x7ff00000)>>20;		/* extract exponent */
45        if (k==0) {				/* 0 or subnormal x */
46            if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
47	    x *= two54;
48	    hx = *(n0+(int*)&x);
49	    k = ((hx&0x7ff00000)>>20) - 54;
50            if (n< -50000) return tiny*x; 	/*underflow*/
51	    }
52        if (k==0x7ff) return x+x;		/* NaN or Inf */
53        k = k+n;
54        if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
55        if (k > 0) 				/* normal result */
56	    {*(n0+(int*)&x) = (hx&0x800fffff)|(k<<20); return x;}
57        if (k <= -54)
58            if (n > 50000) 	/* in case integer overflow in n+k */
59		return huge*copysign(huge,x);	/*overflow*/
60	    else return tiny*copysign(tiny,x); 	/*underflow*/
61        k += 54;				/* subnormal result */
62        *(n0+(int*)&x) = (hx&0x800fffff)|(k<<20);
63        return x*twom54;
64}
65