xref: /minix3/lib/libm/src/s_scalbn.c (revision 84d9c625bfea59e274550651111ae9edfdc40fbd)
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 
46 static const double
47 two54   =  0x1.0p54,	/* 0x43500000, 0x00000000 */
48 twom54  =  0x1.0p-54,	/* 0x3C900000, 0x00000000 */
49 huge   = 1.0e+300,
50 tiny   = 1.0e-300;
51 
52 #ifdef _LP64
53 double
scalbn(double x,int n)54 scalbn(double x, int n)
55 {
56 	return scalbln(x, n);
57 }
58 #endif
59 
60 double
scalbln(double x,long n)61 scalbln(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