mirror of git://sourceware.org/git/glibc.git
97 lines
3.5 KiB
C
97 lines
3.5 KiB
C
/* Double-precision vector (SVE) log2 function
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Copyright (C) 2023-2025 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 N (1 << V_LOG2_TABLE_BITS)
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#define Max (0x7ff0000000000000)
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#define Min (0x0010000000000000)
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#define Thresh (0x7fe0000000000000) /* Max - Min. */
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static const struct data
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{
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double c0, c2;
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double c1, c3;
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double invln2, c4;
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uint64_t off;
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} data = {
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.c0 = -0x1.71547652b83p-1,
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.c1 = 0x1.ec709dc340953p-2,
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.c2 = -0x1.71547651c8f35p-2,
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.c3 = 0x1.2777ebe12dda5p-2,
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.c4 = -0x1.ec738d616fe26p-3,
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.invln2 = 0x1.71547652b82fep0,
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.off = 0x3fe6900900000000,
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};
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static svfloat64_t NOINLINE
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special_case (svfloat64_t w, svuint64_t tmp, svfloat64_t y, svfloat64_t r2,
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svbool_t special, const struct data *d)
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{
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svfloat64_t x = svreinterpret_f64 (svadd_x (svptrue_b64 (), tmp, d->off));
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return sv_call_f64 (log2, x, svmla_x (svptrue_b64 (), w, r2, y), special);
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}
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/* Double-precision SVE log2 routine.
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Implements the same algorithm as AdvSIMD log10, with coefficients and table
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entries scaled in extended precision.
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The maximum observed error is 2.58 ULP:
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SV_NAME_D1 (log2)(0x1.0b556b093869bp+0) got 0x1.fffb34198d9dap-5
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want 0x1.fffb34198d9ddp-5. */
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svfloat64_t SV_NAME_D1 (log2) (svfloat64_t x, const svbool_t pg)
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{
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const struct data *d = ptr_barrier (&data);
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svuint64_t ix = svreinterpret_u64 (x);
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svbool_t special = svcmpge (pg, svsub_x (pg, ix, Min), Thresh);
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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, d->off);
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svuint64_t i = svlsr_x (pg, tmp, 51 - V_LOG2_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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svfloat64_t invc = svld1_gather_index (pg, &__v_log2_data.table[0].invc, i);
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svfloat64_t log2c
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= svld1_gather_index (pg, &__v_log2_data.table[0].log2c, i);
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/* log2(x) = log1p(z/c-1)/log(2) + log2(c) + k. */
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svfloat64_t invln2_and_c4 = svld1rq_f64 (svptrue_b64 (), &d->invln2);
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svfloat64_t r = svmad_x (pg, invc, z, -1.0);
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svfloat64_t w = svmla_lane_f64 (log2c, r, invln2_and_c4, 0);
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w = svadd_x (pg, k, w);
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svfloat64_t odd_coeffs = svld1rq_f64 (svptrue_b64 (), &d->c1);
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svfloat64_t r2 = svmul_x (svptrue_b64 (), r, r);
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svfloat64_t y = svmla_lane_f64 (sv_f64 (d->c2), r, odd_coeffs, 1);
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svfloat64_t p = svmla_lane_f64 (sv_f64 (d->c0), r, odd_coeffs, 0);
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y = svmla_lane_f64 (y, r2, invln2_and_c4, 1);
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y = svmla_x (pg, p, r2, y);
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if (__glibc_unlikely (svptest_any (pg, special)))
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return special_case (w, tmp, y, r2, special, d);
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return svmla_x (pg, w, r2, y);
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}
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