s_scalbn.c revision 97413 
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
14static char rcsid[] = "$FreeBSD: head/lib/msun/src/s_scalbn.c 97413 2002-05-28 18:15:04Z alfred $";
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
27static const double
28two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
29twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
30huge   = 1.0e+300,
31tiny   = 1.0e-300;
32
33double
34__generic_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}
59