95 lines
2.8 KiB
C
95 lines
2.8 KiB
C
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/* Single-precision log function.
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Copyright (C) 2017-2022 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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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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<https://www.gnu.org/licenses/>. */
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#include <math.h>
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#include <stdint.h>
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#include <libm-alias-finite.h>
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#include <libm-alias-float.h>
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#include "math_config.h"
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/*
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LOGF_TABLE_BITS = 4
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LOGF_POLY_ORDER = 4
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ULP error: 0.818 (nearest rounding.)
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Relative error: 1.957 * 2^-26 (before rounding.)
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*/
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#define T __logf_data.tab
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#define A __logf_data.poly
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#define Ln2 __logf_data.ln2
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#define N (1 << LOGF_TABLE_BITS)
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#define OFF 0x3f330000
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float
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__logf (float x)
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{
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/* double_t for better performance on targets with FLT_EVAL_METHOD==2. */
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double_t z, r, r2, y, y0, invc, logc;
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uint32_t ix, iz, tmp;
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int k, i;
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ix = asuint (x);
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#if WANT_ROUNDING
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/* Fix sign of zero with downward rounding when x==1. */
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if (__glibc_unlikely (ix == 0x3f800000))
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return 0;
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#endif
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if (__glibc_unlikely (ix - 0x00800000 >= 0x7f800000 - 0x00800000))
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{
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/* x < 0x1p-126 or inf or nan. */
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if (ix * 2 == 0)
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return __math_divzerof (1);
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if (ix == 0x7f800000) /* log(inf) == inf. */
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return x;
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if ((ix & 0x80000000) || ix * 2 >= 0xff000000)
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return __math_invalidf (x);
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/* x is subnormal, normalize it. */
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ix = asuint (x * 0x1p23f);
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ix -= 23 << 23;
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}
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/* x = 2^k z; where z is in range [OFF,2*OFF] and exact.
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The range is split into N subintervals.
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The ith subinterval contains z and c is near its center. */
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tmp = ix - OFF;
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i = (tmp >> (23 - LOGF_TABLE_BITS)) % N;
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k = (int32_t) tmp >> 23; /* arithmetic shift */
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iz = ix - (tmp & 0x1ff << 23);
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invc = T[i].invc;
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logc = T[i].logc;
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z = (double_t) asfloat (iz);
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/* log(x) = log1p(z/c-1) + log(c) + k*Ln2 */
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r = z * invc - 1;
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y0 = logc + (double_t) k * Ln2;
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/* Pipelined polynomial evaluation to approximate log1p(r). */
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r2 = r * r;
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y = A[1] * r + A[2];
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y = A[0] * r2 + y;
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y = y * r2 + (y0 + r);
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return (float) y;
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}
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#ifndef __logf
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strong_alias (__logf, __ieee754_logf)
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libm_alias_finite (__ieee754_logf, __logf)
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versioned_symbol (libm, __logf, logf, GLIBC_2_27);
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libm_alias_float_other (__log, log)
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#endif
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