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 #ifndef lint
14 static char rcsid[] = "$FreeBSD$";
15 #endif
16 
17 /*
18  * scalbn (double x, int n)
19  * scalbn(x,n) returns x* 2**n  computed by  exponent
20  * manipulation rather than by actually performing an
21  * exponentiation or a multiplication.
22  */
23 
24 #include <sys/cdefs.h>
25 #include <float.h>
26 
27 #include "math.h"
28 #include "math_private.h"
29 
30 static const double
31 two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
32 twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
33 huge   = 1.0e+300,
34 tiny   = 1.0e-300;
35 
36 double
scalbn(double x,int n)37 scalbn (double x, int n)
38 {
39 	int32_t k,hx,lx;
40 	EXTRACT_WORDS(hx,lx,x);
41         k = (hx&0x7ff00000)>>20;		/* extract exponent */
42         if (k==0) {				/* 0 or subnormal x */
43             if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
44 	    x *= two54;
45 	    GET_HIGH_WORD(hx,x);
46 	    k = ((hx&0x7ff00000)>>20) - 54;
47             if (n< -50000) return tiny*x; 	/*underflow*/
48 	    }
49         if (k==0x7ff) return x+x;		/* NaN or Inf */
50         k = k+n;
51         if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
52         if (k > 0) 				/* normal result */
53 	    {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
54         if (k <= -54)
55             if (n > 50000) 	/* in case integer overflow in n+k */
56 		return huge*copysign(huge,x);	/*overflow*/
57 	    else return tiny*copysign(tiny,x); 	/*underflow*/
58         k += 54;				/* subnormal result */
59 	SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
60         return x*twom54;
61 }
62 
63 #if (LDBL_MANT_DIG == 53)
64 __weak_reference(scalbn, ldexpl);
65 __weak_reference(scalbn, scalbnl);
66 #endif
67