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124 lines
3.6 KiB
C
124 lines
3.6 KiB
C
/* Single-precision floating point 2^x.
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Copyright (C) 1997-2013 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Contributed by Geoffrey Keating <geoffk@ozemail.com.au>
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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/* The basic design here is from
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Shmuel Gal and Boris Bachelis, "An Accurate Elementary Mathematical
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Library for the IEEE Floating Point Standard", ACM Trans. Math. Soft.,
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17 (1), March 1991, pp. 26-45.
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It has been slightly modified to compute 2^x instead of e^x, and for
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single-precision.
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*/
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#ifndef _GNU_SOURCE
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# define _GNU_SOURCE
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#endif
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#include <stdlib.h>
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#include <float.h>
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#include <ieee754.h>
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#include <math.h>
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#include <fenv.h>
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#include <inttypes.h>
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#include <math_private.h>
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#include "t_exp2f.h"
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static const volatile float TWOM100 = 7.88860905e-31;
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static const volatile float TWO127 = 1.7014118346e+38;
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float
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__ieee754_exp2f (float x)
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{
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static const float himark = (float) FLT_MAX_EXP;
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static const float lomark = (float) (FLT_MIN_EXP - FLT_MANT_DIG - 1);
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/* Check for usual case. */
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if (isless (x, himark) && isgreaterequal (x, lomark))
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{
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static const float THREEp14 = 49152.0;
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int tval, unsafe;
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float rx, x22, result;
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union ieee754_float ex2_u, scale_u;
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{
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SET_RESTORE_ROUND_NOEXF (FE_TONEAREST);
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/* 1. Argument reduction.
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Choose integers ex, -128 <= t < 128, and some real
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-1/512 <= x1 <= 1/512 so that
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x = ex + t/512 + x1.
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First, calculate rx = ex + t/256. */
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rx = x + THREEp14;
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rx -= THREEp14;
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x -= rx; /* Compute x=x1. */
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/* Compute tval = (ex*256 + t)+128.
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Now, t = (tval mod 256)-128 and ex=tval/256 [that's mod, NOT %;
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and /-round-to-nearest not the usual c integer /]. */
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tval = (int) (rx * 256.0f + 128.0f);
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/* 2. Adjust for accurate table entry.
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Find e so that
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x = ex + t/256 + e + x2
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where -7e-4 < e < 7e-4, and
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(float)(2^(t/256+e))
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is accurate to one part in 2^-64. */
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/* 'tval & 255' is the same as 'tval%256' except that it's always
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positive.
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Compute x = x2. */
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x -= __exp2f_deltatable[tval & 255];
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/* 3. Compute ex2 = 2^(t/255+e+ex). */
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ex2_u.f = __exp2f_atable[tval & 255];
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tval >>= 8;
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unsafe = abs(tval) >= -FLT_MIN_EXP - 1;
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ex2_u.ieee.exponent += tval >> unsafe;
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scale_u.f = 1.0;
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scale_u.ieee.exponent += tval - (tval >> unsafe);
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/* 4. Approximate 2^x2 - 1, using a second-degree polynomial,
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with maximum error in [-2^-9 - 2^-14, 2^-9 + 2^-14]
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less than 1.3e-10. */
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x22 = (.24022656679f * x + .69314736128f) * ex2_u.f;
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}
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/* 5. Return (2^x2-1) * 2^(t/512+e+ex) + 2^(t/512+e+ex). */
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result = x22 * x + ex2_u.f;
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if (!unsafe)
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return result;
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else
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return result * scale_u.f;
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}
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/* Exceptional cases: */
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else if (isless (x, himark))
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{
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if (__isinf_nsf (x))
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/* e^-inf == 0, with no error. */
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return 0;
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else
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/* Underflow */
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return TWOM100 * TWOM100;
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}
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else
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/* Return x, if x is a NaN or Inf; or overflow, otherwise. */
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return TWO127*x;
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}
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strong_alias (__ieee754_exp2f, __exp2f_finite)
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