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---
s_scalbn.c (1887B)
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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: src/lib/msun/src/s_scalbn.c,v 1.9 2003/07/23 04:53:47 peter Exp $";
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 "math.h"
25 #include "math_private.h"
26
27 static const double
28 two54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
29 twom54 = 5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
30 huge = 1.0e+300,
31 tiny = 1.0e-300;
32
33 double
34 scalbn (double x, int n)
35 {
36 int32_t k,hx,lx;
37 EXTRACT_WORDS(hx,lx,x);
38 k = (hx&0x7ff00000)>>20; /* extract exponent */
39 if (k==0) { /* 0 or subnormal x */
40 if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
41 x *= two54;
42 GET_HIGH_WORD(hx,x);
43 k = ((hx&0x7ff00000)>>20) - 54;
44 if (n< -50000) return tiny*x; /*underflow*/
45 }
46 if (k==0x7ff) return x+x; /* NaN or Inf */
47 k = k+n;
48 if (k > 0x7fe) return huge*copysign(huge,x); /* overflow */
49 if (k > 0) /* normal result */
50 {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
51 if (k <= -54)
52 if (n > 50000) /* in case integer overflow in n+k */
53 return huge*copysign(huge,x); /*overflow*/
54 else return tiny*copysign(tiny,x); /*underflow*/
55 k += 54; /* subnormal result */
56 SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
57 return x*twom54;
58 }