source: trunk/sys/libm/s_scalbn.c @ 1

Last change on this file since 1 was 1, checked in by alain, 5 years ago

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1
2/* @(#)s_scalbn.c 5.1 93/09/24 */
3/*
4 * ====================================================
5 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
6 *
7 * Developed at SunPro, a Sun Microsystems, Inc. business.
8 * Permission to use, copy, modify, and distribute this
9 * software is freely granted, provided that this notice
10 * is preserved.
11 * ====================================================
12 */
13
14/*
15 * scalbn (double x, int n)
16 * scalbn(x,n) returns x* 2**n  computed by  exponent 
17 * manipulation rather than by actually performing an
18 * exponentiation or a multiplication.
19 */
20
21#include <libm/fdlibm.h>
22
23#ifdef __STDC__
24static const double
25#else
26static double
27#endif
28two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
29twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
30huge   = 1.0e+300,
31tiny   = 1.0e-300;
32
33#ifdef __STDC__
34        double scalbn (double x, int n)
35#else
36        double scalbn (x,n)
37        double x; int n;
38#endif
39{
40        int  k,n0,hx,lx;
41        n0 = ((*(int*)&two54)>>30)^1;           /* high word index */
42        hx = *(n0+(int*)&x);
43        lx = *(1-n0+(int*)&x);
44        k = (hx&0x7ff00000)>>20;                /* extract exponent */
45        if (k==0) {                             /* 0 or subnormal x */
46            if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
47            x *= two54; 
48            hx = *(n0+(int*)&x);
49            k = ((hx&0x7ff00000)>>20) - 54; 
50            if (n< -50000) return tiny*x;       /*underflow*/
51            }
52        if (k==0x7ff) return x+x;               /* NaN or Inf */
53        k = k+n; 
54        if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
55        if (k > 0)                              /* normal result */
56            {*(n0+(int*)&x) = (hx&0x800fffff)|(k<<20); return x;}
57        if (k <= -54)
58        {
59            if (n > 50000)      /* in case integer overflow in n+k */
60                return huge*copysign(huge,x);   /*overflow*/
61            else return tiny*copysign(tiny,x);  /*underflow*/
62        }
63        k += 54;                                /* subnormal result */
64        *(n0+(int*)&x) = (hx&0x800fffff)|(k<<20);
65        return x*twom54;
66}
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