1 /* Double-precision floating point 2^x.
2 Copyright (C) 1997, 1998 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4 Contributed by Geoffrey Keating <geoffk@ozemail.com.au>
6 The GNU C Library is free software; you can redistribute it and/or
7 modify it under the terms of the GNU Library General Public License as
8 published by the Free Software Foundation; either version 2 of the
9 License, or (at your option) any later version.
11 The GNU C Library is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 Library General Public License for more details.
16 You should have received a copy of the GNU Library General Public
17 License along with the GNU C Library; see the file COPYING.LIB. If not,
18 write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
19 Boston, MA 02111-1307, USA. */
21 /* The basic design here is from
22 Shmuel Gal and Boris Bachelis, "An Accurate Elementary Mathematical
23 Library for the IEEE Floating Point Standard", ACM Trans. Math. Soft.,
24 17 (1), March 1991, pp. 26-45.
25 It has been slightly modified to compute 2^x instead of e^x.
35 #include <math_private.h>
39 static const volatile double TWO1023
= 8.988465674311579539e+307;
40 static const volatile double TWOM1000
= 9.3326361850321887899e-302;
43 __ieee754_exp2 (double x
)
45 static const double himark
= (double) DBL_MAX_EXP
;
46 static const double lomark
= (double) (DBL_MIN_EXP
- DBL_MANT_DIG
- 1) - 1.0;
48 /* Check for usual case. */
49 if (isless (x
, himark
) && isgreater (x
, lomark
))
51 static const double THREEp42
= 13194139533312.0;
53 double rx
, x22
, result
;
54 union ieee754_double ex2_u
, scale_u
;
57 feholdexcept (&oldenv
);
59 /* If we don't have this, it's too bad. */
60 fesetround (FE_TONEAREST
);
63 /* 1. Argument reduction.
64 Choose integers ex, -256 <= t < 256, and some real
65 -1/1024 <= x1 <= 1024 so that
68 First, calculate rx = ex + t/512. */
71 x
-= rx
; /* Compute x=x1. */
72 /* Compute tval = (ex*512 + t)+256.
73 Now, t = (tval mod 512)-256 and ex=tval/512 [that's mod, NOT %; and
74 /-round-to-nearest not the usual c integer /]. */
75 tval
= (int) (rx
* 512.0 + 256.0);
77 /* 2. Adjust for accurate table entry.
79 x = ex + t/512 + e + x2
80 where -1e6 < e < 1e6, and
82 is accurate to one part in 2^-64. */
84 /* 'tval & 511' is the same as 'tval%512' except that it's always
87 x
-= exp2_deltatable
[tval
& 511];
89 /* 3. Compute ex2 = 2^(t/512+e+ex). */
90 ex2_u
.d
= exp2_accuratetable
[tval
& 511];
92 unsafe
= abs(tval
) >= -DBL_MIN_EXP
- 1;
93 ex2_u
.ieee
.exponent
+= tval
>> unsafe
;
95 scale_u
.ieee
.exponent
+= tval
- (tval
>> unsafe
);
97 /* 4. Approximate 2^x2 - 1, using a fourth-degree polynomial,
98 with maximum error in [-2^-10-2^-30,2^-10+2^-30]
101 x22
= (((.0096181293647031180
102 * x
+ .055504110254308625)
103 * x
+ .240226506959100583)
104 * x
+ .69314718055994495) * ex2_u
.d
;
106 /* 5. Return (2^x2-1) * 2^(t/512+e+ex) + 2^(t/512+e+ex). */
109 result
= x22
* x
+ ex2_u
.d
;
114 return result
* scale_u
.d
;
116 /* Exceptional cases: */
117 else if (isless (x
, himark
))
120 /* e^-inf == 0, with no error. */
124 return TWOM1000
* TWOM1000
;
127 /* Return x, if x is a NaN or Inf; or overflow, otherwise. */