s_scalbn.c revision 1.12 
1/* @(#)s_scalbn.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#include <sys/cdefs.h>
14#if defined(LIBM_SCCS) && !defined(lint)
15__RCSID("$NetBSD: s_scalbn.c,v 1.12 2002/05/26 22:01:58 wiz Exp $");
16#endif
17
18/*
19 * scalbn (double x, int n)
20 * scalbn(x,n) returns x* 2**n  computed by  exponent
21 * manipulation rather than by actually performing an
22 * exponentiation or a multiplication.
23 */
24
25#include "math.h"
26#include "math_private.h"
27
28static const double
29two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
30twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
31huge   = 1.0e+300,
32tiny   = 1.0e-300;
33
34double
35scalbn(double x, int n)
36{
37	int32_t k,hx,lx;
38	EXTRACT_WORDS(hx,lx,x);
39        k = (hx&0x7ff00000)>>20;		/* extract exponent */
40        if (k==0) {				/* 0 or subnormal x */
41            if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
42	    x *= two54;
43	    GET_HIGH_WORD(hx,x);
44	    k = ((hx&0x7ff00000)>>20) - 54;
45            if (n< -50000) return tiny*x; 	/*underflow*/
46	    }
47        if (k==0x7ff) return x+x;		/* NaN or Inf */
48        k = k+n;
49        if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
50        if (k > 0) 				/* normal result */
51	    {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
52        if (k <= -54) {
53            if (n > 50000) 	/* in case integer overflow in n+k */
54		return huge*copysign(huge,x);	/*overflow*/
55	    else return tiny*copysign(tiny,x); 	/*underflow*/
56	}
57        k += 54;				/* subnormal result */
58	SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
59        return x*twom54;
60}
61