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77 lines
2.9 KiB
C
77 lines
2.9 KiB
C
/* Double-precision vector (SVE) log10 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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#include "poly_sve_f64.h"
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#define Min 0x0010000000000000
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#define Max 0x7ff0000000000000
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#define Thres 0x7fe0000000000000 /* Max - Min. */
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#define Off 0x3fe6900900000000
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#define N (1 << V_LOG10_TABLE_BITS)
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static svfloat64_t NOINLINE
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special_case (svfloat64_t x, svfloat64_t y, svbool_t special)
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{
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return sv_call_f64 (log10, x, y, special);
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}
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/* SVE log10 algorithm.
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Maximum measured error is 2.46 ulps.
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SV_NAME_D1 (log10)(0x1.131956cd4b627p+0) got 0x1.fffbdf6eaa669p-6
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want 0x1.fffbdf6eaa667p-6. */
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svfloat64_t SV_NAME_D1 (log10) (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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svbool_t special = svcmpge (pg, svsub_x (pg, ix, Min), Thres);
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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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svuint64_t i = svlsr_x (pg, tmp, 51 - V_LOG10_TABLE_BITS);
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i = svand_x (pg, i, (N - 1) << 1);
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svfloat64_t k = svcvt_f64_x (pg, svasr_x (pg, svreinterpret_s64 (tmp), 52));
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svfloat64_t z = svreinterpret_f64 (
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svsub_x (pg, ix, svand_x (pg, tmp, 0xfffULL << 52)));
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/* log(x) = k*log(2) + log(c) + log(z/c). */
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svfloat64_t invc = svld1_gather_index (pg, &__v_log10_data.table[0].invc, i);
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svfloat64_t logc
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= svld1_gather_index (pg, &__v_log10_data.table[0].log10c, i);
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/* We approximate log(z/c) with a polynomial P(x) ~= log(x + 1):
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r = z/c - 1 (we look up precomputed 1/c)
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log(z/c) ~= P(r). */
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svfloat64_t r = svmad_x (pg, invc, z, -1.0);
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/* hi = log(c) + k*log(2). */
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svfloat64_t w = svmla_x (pg, logc, r, __v_log10_data.invln10);
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svfloat64_t hi = svmla_x (pg, w, k, __v_log10_data.log10_2);
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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 = sv_pw_horner_4_f64_x (pg, r, r2, __v_log10_data.poly);
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if (__glibc_unlikely (svptest_any (pg, special)))
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return special_case (x, svmla_x (svnot_z (pg, special), hi, r2, y),
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special);
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return svmla_x (pg, hi, r2, y);
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}
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