s_scalbn.c revision 1.18 
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 1.18 2013/05/20 19:40:09 joerg 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__weak_alias(ldexp, _scalbn)
44#endif
45
46static const double
47two54   =  0x1.0p54,	/* 0x43500000, 0x00000000 */
48twom54  =  0x1.0p-54,	/* 0x3C900000, 0x00000000 */
49huge   = 1.0e+300,
50tiny   = 1.0e-300;
51
52#ifdef _LP64
53double
54scalbn(double x, int n)
55{
56	return scalbln(x, n);
57}
58#endif
59
60double
61scalbln(double x, long n)
62{
63	int32_t k,hx,lx;
64	EXTRACT_WORDS(hx,lx,x);
65        k = ((uint32_t)hx&0x7ff00000)>>20;		/* extract exponent */
66        if (k==0) {				/* 0 or subnormal x */
67            if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
68	    x *= two54;
69	    GET_HIGH_WORD(hx,x);
70	    k = (((uint32_t)hx&0x7ff00000)>>20) - 54;
71            if (n< -50000) return tiny*x; 	/*underflow*/
72	    }
73        if (k==0x7ff) return x+x;		/* NaN or Inf */
74        k = k+n;
75        if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
76        if (k > 0) 				/* normal result */
77	    {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
78        if (k <= -54) {
79            if (n > 50000) 	/* in case integer overflow in n+k */
80		return huge*copysign(huge,x);	/*overflow*/
81	    else return tiny*copysign(tiny,x); 	/*underflow*/
82	}
83        k += 54;				/* subnormal result */
84	SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
85        return x*twom54;
86}
87