s_scalbn.c revision 1.16 
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 1.16 2013/01/28 06:34:09 matt 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 "namespace.h"
26#include "math.h"
27#include "math_private.h"
28
29#ifndef _LP64
30__strong_alias(_scalbn, _scalbln)
31#endif
32
33#ifndef __HAVE_LONG_DOUBLE
34__strong_alias(_scalbnl, _scalbn)
35__strong_alias(_scalblnl, _scalbln)
36__weak_alias(scalbnl, _scalbnl)
37__weak_alias(scalblnl, _scalblnl)
38#endif
39
40#ifdef __weak_alias
41__weak_alias(scalbn, _scalbn)
42__weak_alias(scalbln, _scalbln)
43#endif
44
45static const double
46two54   =  0x1.0p54,	/* 0x43500000, 0x00000000 */
47twom54  =  0x1.0p-54,	/* 0x3C900000, 0x00000000 */
48huge   = 1.0e+300,
49tiny   = 1.0e-300;
50
51#ifdef _LP64
52double
53scalbn(double x, int n)
54{
55	return scalbln(x, n);
56}
57#endif
58
59double
60scalbln(double x, long n)
61{
62	int32_t k,hx,lx;
63	EXTRACT_WORDS(hx,lx,x);
64        k = ((uint32_t)hx&0x7ff00000)>>20;		/* extract exponent */
65        if (k==0) {				/* 0 or subnormal x */
66            if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
67	    x *= two54;
68	    GET_HIGH_WORD(hx,x);
69	    k = (((uint32_t)hx&0x7ff00000)>>20) - 54;
70            if (n< -50000) return tiny*x; 	/*underflow*/
71	    }
72        if (k==0x7ff) return x+x;		/* NaN or Inf */
73        k = k+n;
74        if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
75        if (k > 0) 				/* normal result */
76	    {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
77        if (k <= -54) {
78            if (n > 50000) 	/* in case integer overflow in n+k */
79		return huge*copysign(huge,x);	/*overflow*/
80	    else return tiny*copysign(tiny,x); 	/*underflow*/
81	}
82        k += 54;				/* subnormal result */
83	SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
84        return x*twom54;
85}
86