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77 lines
3.0 KiB
C
77 lines
3.0 KiB
C
/* Double-precision vector (SVE) log function.
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Copyright (C) 2023-2024 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 "sv_math.h"
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#define P(i) sv_f64 (__v_log_data.poly[i])
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#define N (1 << V_LOG_TABLE_BITS)
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#define Off (0x3fe6900900000000)
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#define MaxTop (0x7ff)
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#define MinTop (0x001)
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#define ThreshTop (0x7fe) /* MaxTop - MinTop. */
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static svfloat64_t NOINLINE
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special_case (svfloat64_t x, svfloat64_t y, svbool_t cmp)
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{
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return sv_call_f64 (log, x, y, cmp);
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}
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/* SVE port of AdvSIMD log algorithm.
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Maximum measured error is 2.17 ulp:
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SV_NAME_D1 (log)(0x1.a6129884398a3p+0) got 0x1.ffffff1cca043p-2
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want 0x1.ffffff1cca045p-2. */
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svfloat64_t SV_NAME_D1 (log) (svfloat64_t x, const svbool_t pg)
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{
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svuint64_t ix = svreinterpret_u64 (x);
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svuint64_t top = svlsr_x (pg, ix, 52);
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svbool_t cmp = svcmpge (pg, svsub_x (pg, top, MinTop), sv_u64 (ThreshTop));
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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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svuint64_t tmp = svsub_x (pg, ix, Off);
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/* Calculate table index = (tmp >> (52 - V_LOG_TABLE_BITS)) % N.
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The actual value of i is double this due to table layout. */
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svuint64_t i
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= svand_x (pg, svlsr_x (pg, tmp, (51 - V_LOG_TABLE_BITS)), (N - 1) << 1);
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svint64_t k
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= svasr_x (pg, svreinterpret_s64 (tmp), 52); /* Arithmetic shift. */
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svuint64_t iz = svsub_x (pg, ix, svand_x (pg, tmp, 0xfffULL << 52));
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svfloat64_t z = svreinterpret_f64 (iz);
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/* Lookup in 2 global lists (length N). */
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svfloat64_t invc = svld1_gather_index (pg, &__v_log_data.table[0].invc, i);
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svfloat64_t logc = svld1_gather_index (pg, &__v_log_data.table[0].logc, i);
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/* log(x) = log1p(z/c-1) + log(c) + k*Ln2. */
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svfloat64_t r = svmad_x (pg, invc, z, -1);
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svfloat64_t kd = svcvt_f64_x (pg, k);
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/* hi = r + log(c) + k*Ln2. */
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svfloat64_t hi = svmla_x (pg, svadd_x (pg, logc, r), kd, __v_log_data.ln2);
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/* y = r2*(A0 + r*A1 + r2*(A2 + r*A3 + r2*A4)) + hi. */
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svfloat64_t r2 = svmul_x (pg, r, r);
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svfloat64_t y = svmla_x (pg, P (2), r, P (3));
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svfloat64_t p = svmla_x (pg, P (0), r, P (1));
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y = svmla_x (pg, y, r2, P (4));
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y = svmla_x (pg, p, r2, y);
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if (__glibc_unlikely (svptest_any (pg, cmp)))
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return special_case (x, svmla_x (svnot_z (pg, cmp), hi, r2, y), cmp);
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return svmla_x (pg, hi, r2, y);
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}
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