mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-22 21:10:07 +00:00
255 lines
6.9 KiB
C
255 lines
6.9 KiB
C
/* Quad-precision floating point e^x.
|
|
Copyright (C) 1999-2023 Free Software Foundation, Inc.
|
|
This file is part of the GNU C Library.
|
|
|
|
The GNU C Library is free software; you can redistribute it and/or
|
|
modify it under the terms of the GNU Lesser General Public
|
|
License as published by the Free Software Foundation; either
|
|
version 2.1 of the License, or (at your option) any later version.
|
|
|
|
The GNU C Library is distributed in the hope that it will be useful,
|
|
but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
|
|
Lesser General Public License for more details.
|
|
|
|
You should have received a copy of the GNU Lesser General Public
|
|
License along with the GNU C Library; if not, see
|
|
<https://www.gnu.org/licenses/>. */
|
|
|
|
/* The basic design here is from
|
|
Abraham Ziv, "Fast Evaluation of Elementary Mathematical Functions with
|
|
Correctly Rounded Last Bit", ACM Trans. Math. Soft., 17 (3), September 1991,
|
|
pp. 410-423.
|
|
|
|
We work with number pairs where the first number is the high part and
|
|
the second one is the low part. Arithmetic with the high part numbers must
|
|
be exact, without any roundoff errors.
|
|
|
|
The input value, X, is written as
|
|
X = n * ln(2)_0 + arg1[t1]_0 + arg2[t2]_0 + x
|
|
- n * ln(2)_1 + arg1[t1]_1 + arg2[t2]_1 + xl
|
|
|
|
where:
|
|
- n is an integer, 16384 >= n >= -16495;
|
|
- ln(2)_0 is the first 93 bits of ln(2), and |ln(2)_0-ln(2)-ln(2)_1| < 2^-205
|
|
- t1 is an integer, 89 >= t1 >= -89
|
|
- t2 is an integer, 65 >= t2 >= -65
|
|
- |arg1[t1]-t1/256.0| < 2^-53
|
|
- |arg2[t2]-t2/32768.0| < 2^-53
|
|
- x + xl is whatever is left, |x + xl| < 2^-16 + 2^-53
|
|
|
|
Then e^x is approximated as
|
|
|
|
e^x = 2^n_1 ( 2^n_0 e^(arg1[t1]_0 + arg1[t1]_1) e^(arg2[t2]_0 + arg2[t2]_1)
|
|
+ 2^n_0 e^(arg1[t1]_0 + arg1[t1]_1) e^(arg2[t2]_0 + arg2[t2]_1)
|
|
* p (x + xl + n * ln(2)_1))
|
|
where:
|
|
- p(x) is a polynomial approximating e(x)-1
|
|
- e^(arg1[t1]_0 + arg1[t1]_1) is obtained from a table
|
|
- e^(arg2[t2]_0 + arg2[t2]_1) likewise
|
|
- n_1 + n_0 = n, so that |n_0| < -LDBL_MIN_EXP-1.
|
|
|
|
If it happens that n_1 == 0 (this is the usual case), that multiplication
|
|
is omitted.
|
|
*/
|
|
|
|
#ifndef _GNU_SOURCE
|
|
#define _GNU_SOURCE
|
|
#endif
|
|
#include <float.h>
|
|
#include <ieee754.h>
|
|
#include <math.h>
|
|
#include <fenv.h>
|
|
#include <inttypes.h>
|
|
#include <math-barriers.h>
|
|
#include <math_private.h>
|
|
#include <math-underflow.h>
|
|
#include <stdlib.h>
|
|
#include "t_expl.h"
|
|
#include <libm-alias-finite.h>
|
|
|
|
static const _Float128 C[] = {
|
|
/* Smallest integer x for which e^x overflows. */
|
|
#define himark C[0]
|
|
L(11356.523406294143949491931077970765),
|
|
|
|
/* Largest integer x for which e^x underflows. */
|
|
#define lomark C[1]
|
|
L(-11433.4627433362978788372438434526231),
|
|
|
|
/* 3x2^96 */
|
|
#define THREEp96 C[2]
|
|
L(59421121885698253195157962752.0),
|
|
|
|
/* 3x2^103 */
|
|
#define THREEp103 C[3]
|
|
L(30423614405477505635920876929024.0),
|
|
|
|
/* 3x2^111 */
|
|
#define THREEp111 C[4]
|
|
L(7788445287802241442795744493830144.0),
|
|
|
|
/* 1/ln(2) */
|
|
#define M_1_LN2 C[5]
|
|
L(1.44269504088896340735992468100189204),
|
|
|
|
/* first 93 bits of ln(2) */
|
|
#define M_LN2_0 C[6]
|
|
L(0.693147180559945309417232121457981864),
|
|
|
|
/* ln2_0 - ln(2) */
|
|
#define M_LN2_1 C[7]
|
|
L(-1.94704509238074995158795957333327386E-31),
|
|
|
|
/* very small number */
|
|
#define TINY C[8]
|
|
L(1.0e-4900),
|
|
|
|
/* 2^16383 */
|
|
#define TWO16383 C[9]
|
|
L(5.94865747678615882542879663314003565E+4931),
|
|
|
|
/* 256 */
|
|
#define TWO8 C[10]
|
|
256,
|
|
|
|
/* 32768 */
|
|
#define TWO15 C[11]
|
|
32768,
|
|
|
|
/* Chebyshev polynom coefficients for (exp(x)-1)/x */
|
|
#define P1 C[12]
|
|
#define P2 C[13]
|
|
#define P3 C[14]
|
|
#define P4 C[15]
|
|
#define P5 C[16]
|
|
#define P6 C[17]
|
|
L(0.5),
|
|
L(1.66666666666666666666666666666666683E-01),
|
|
L(4.16666666666666666666654902320001674E-02),
|
|
L(8.33333333333333333333314659767198461E-03),
|
|
L(1.38888888889899438565058018857254025E-03),
|
|
L(1.98412698413981650382436541785404286E-04),
|
|
};
|
|
|
|
_Float128
|
|
__ieee754_expl (_Float128 x)
|
|
{
|
|
/* Check for usual case. */
|
|
if (isless (x, himark) && isgreater (x, lomark))
|
|
{
|
|
int tval1, tval2, unsafe, n_i;
|
|
_Float128 x22, n, t, result, xl;
|
|
union ieee854_long_double ex2_u, scale_u;
|
|
fenv_t oldenv;
|
|
|
|
feholdexcept (&oldenv);
|
|
#ifdef FE_TONEAREST
|
|
fesetround (FE_TONEAREST);
|
|
#endif
|
|
|
|
/* Calculate n. */
|
|
n = x * M_1_LN2 + THREEp111;
|
|
n -= THREEp111;
|
|
x = x - n * M_LN2_0;
|
|
xl = n * M_LN2_1;
|
|
|
|
/* Calculate t/256. */
|
|
t = x + THREEp103;
|
|
t -= THREEp103;
|
|
|
|
/* Compute tval1 = t. */
|
|
tval1 = (int) (t * TWO8);
|
|
|
|
x -= __expl_table[T_EXPL_ARG1+2*tval1];
|
|
xl -= __expl_table[T_EXPL_ARG1+2*tval1+1];
|
|
|
|
/* Calculate t/32768. */
|
|
t = x + THREEp96;
|
|
t -= THREEp96;
|
|
|
|
/* Compute tval2 = t. */
|
|
tval2 = (int) (t * TWO15);
|
|
|
|
x -= __expl_table[T_EXPL_ARG2+2*tval2];
|
|
xl -= __expl_table[T_EXPL_ARG2+2*tval2+1];
|
|
|
|
x = x + xl;
|
|
|
|
/* Compute ex2 = 2^n_0 e^(argtable[tval1]) e^(argtable[tval2]). */
|
|
ex2_u.d = __expl_table[T_EXPL_RES1 + tval1]
|
|
* __expl_table[T_EXPL_RES2 + tval2];
|
|
n_i = (int)n;
|
|
/* 'unsafe' is 1 iff n_1 != 0. */
|
|
unsafe = abs(n_i) >= 15000;
|
|
ex2_u.ieee.exponent += n_i >> unsafe;
|
|
|
|
/* Compute scale = 2^n_1. */
|
|
scale_u.d = 1;
|
|
scale_u.ieee.exponent += n_i - (n_i >> unsafe);
|
|
|
|
/* Approximate e^x2 - 1, using a seventh-degree polynomial,
|
|
with maximum error in [-2^-16-2^-53,2^-16+2^-53]
|
|
less than 4.8e-39. */
|
|
x22 = x + x*x*(P1+x*(P2+x*(P3+x*(P4+x*(P5+x*P6)))));
|
|
math_force_eval (x22);
|
|
|
|
/* Return result. */
|
|
fesetenv (&oldenv);
|
|
|
|
result = x22 * ex2_u.d + ex2_u.d;
|
|
|
|
/* Now we can test whether the result is ultimate or if we are unsure.
|
|
In the later case we should probably call a mpn based routine to give
|
|
the ultimate result.
|
|
Empirically, this routine is already ultimate in about 99.9986% of
|
|
cases, the test below for the round to nearest case will be false
|
|
in ~ 99.9963% of cases.
|
|
Without proc2 routine maximum error which has been seen is
|
|
0.5000262 ulp.
|
|
|
|
union ieee854_long_double ex3_u;
|
|
|
|
#ifdef FE_TONEAREST
|
|
fesetround (FE_TONEAREST);
|
|
#endif
|
|
ex3_u.d = (result - ex2_u.d) - x22 * ex2_u.d;
|
|
ex2_u.d = result;
|
|
ex3_u.ieee.exponent += LDBL_MANT_DIG + 15 + IEEE854_LONG_DOUBLE_BIAS
|
|
- ex2_u.ieee.exponent;
|
|
n_i = abs (ex3_u.d);
|
|
n_i = (n_i + 1) / 2;
|
|
fesetenv (&oldenv);
|
|
#ifdef FE_TONEAREST
|
|
if (fegetround () == FE_TONEAREST)
|
|
n_i -= 0x4000;
|
|
#endif
|
|
if (!n_i) {
|
|
return __ieee754_expl_proc2 (origx);
|
|
}
|
|
*/
|
|
if (!unsafe)
|
|
return result;
|
|
else
|
|
{
|
|
result *= scale_u.d;
|
|
math_check_force_underflow_nonneg (result);
|
|
return result;
|
|
}
|
|
}
|
|
/* Exceptional cases: */
|
|
else if (isless (x, himark))
|
|
{
|
|
if (isinf (x))
|
|
/* e^-inf == 0, with no error. */
|
|
return 0;
|
|
else
|
|
/* Underflow */
|
|
return TINY * TINY;
|
|
}
|
|
else
|
|
/* Return x, if x is a NaN or Inf; or overflow, otherwise. */
|
|
return TWO16383*x;
|
|
}
|
|
libm_alias_finite (__ieee754_expl, __expl)
|