xref: /freebsd/lib/libc/gen/ldexp.c (revision 62cfcf62f627e5093fb37026a6d8c98e4d2ef04c)
1 /* @(#)s_scalbn.c 5.1 93/09/24 */
2 /* @(#)fdlibm.h 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 #include <sys/cdefs.h>
15 __FBSDID("$FreeBSD$");
16 
17 #include <sys/types.h>
18 #include <machine/endian.h>
19 #include <math.h>
20 
21 /* Bit fiddling routines copied from msun/src/math_private.h,v 1.15 */
22 
23 #if BYTE_ORDER == BIG_ENDIAN
24 
25 typedef union
26 {
27   double value;
28   struct
29   {
30     u_int32_t msw;
31     u_int32_t lsw;
32   } parts;
33 } ieee_double_shape_type;
34 
35 #endif
36 
37 #if BYTE_ORDER == LITTLE_ENDIAN
38 
39 typedef union
40 {
41   double value;
42   struct
43   {
44     u_int32_t lsw;
45     u_int32_t msw;
46   } parts;
47 } ieee_double_shape_type;
48 
49 #endif
50 
51 /* Get two 32 bit ints from a double.  */
52 
53 #define EXTRACT_WORDS(ix0,ix1,d)				\
54 do {								\
55   ieee_double_shape_type ew_u;					\
56   ew_u.value = (d);						\
57   (ix0) = ew_u.parts.msw;					\
58   (ix1) = ew_u.parts.lsw;					\
59 } while (0)
60 
61 /* Get the more significant 32 bit int from a double.  */
62 
63 #define GET_HIGH_WORD(i,d)					\
64 do {								\
65   ieee_double_shape_type gh_u;					\
66   gh_u.value = (d);						\
67   (i) = gh_u.parts.msw;						\
68 } while (0)
69 
70 /* Set the more significant 32 bits of a double from an int.  */
71 
72 #define SET_HIGH_WORD(d,v)					\
73 do {								\
74   ieee_double_shape_type sh_u;					\
75   sh_u.value = (d);						\
76   sh_u.parts.msw = (v);						\
77   (d) = sh_u.value;						\
78 } while (0)
79 
80 
81 static const double
82 two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
83 twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
84 huge   = 1.0e+300,
85 tiny   = 1.0e-300;
86 
87 static double
88 _copysign(double x, double y)
89 {
90 	u_int32_t hx,hy;
91 	GET_HIGH_WORD(hx,x);
92 	GET_HIGH_WORD(hy,y);
93 	SET_HIGH_WORD(x,(hx&0x7fffffff)|(hy&0x80000000));
94 	return x;
95 }
96 
97 double
98 ldexp(double x, int n)
99 {
100 	int32_t k,hx,lx;
101 	EXTRACT_WORDS(hx,lx,x);
102         k = (hx&0x7ff00000)>>20;		/* extract exponent */
103         if (k==0) {				/* 0 or subnormal x */
104             if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
105 	    x *= two54;
106 	    GET_HIGH_WORD(hx,x);
107 	    k = ((hx&0x7ff00000)>>20) - 54;
108             if (n< -50000) return tiny*x; 	/*underflow*/
109 	    }
110         if (k==0x7ff) return x+x;		/* NaN or Inf */
111         k = k+n;
112         if (k >  0x7fe) return huge*_copysign(huge,x); /* overflow  */
113         if (k > 0) 				/* normal result */
114 	    {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
115         if (k <= -54) {
116             if (n > 50000) 	/* in case integer overflow in n+k */
117 		return huge*_copysign(huge,x);	/*overflow*/
118 	    else return tiny*_copysign(tiny,x); 	/*underflow*/
119 	}
120         k += 54;				/* subnormal result */
121 	SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
122         return x*twom54;
123 }
124