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/* Double-precision vector (Advanced SIMD) log function.
Copyright (C) 2023-2025 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include "v_math.h"
static const struct data
{
uint64x2_t off, sign_exp_mask, offset_lower_bound;
uint32x4_t special_bound;
float64x2_t c0, c2;
double c1, c3, ln2, c4;
} data = {
/* Rel error: 0x1.6272e588p-56 in [ -0x1.fc1p-9 0x1.009p-8 ]. */
.c0 = V2 (-0x1.ffffffffffff7p-2),
.c1 = 0x1.55555555170d4p-2,
.c2 = V2 (-0x1.0000000399c27p-2),
.c3 = 0x1.999b2e90e94cap-3,
.c4 = -0x1.554e550bd501ep-3,
.ln2 = 0x1.62e42fefa39efp-1,
.sign_exp_mask = V2 (0xfff0000000000000),
.off = V2 (0x3fe6900900000000),
/* Lower bound is 0x0010000000000000. For
optimised register use subnormals are detected after offset has been
subtracted, so lower bound - offset (which wraps around). */
.offset_lower_bound = V2 (0x0010000000000000 - 0x3fe6900900000000),
.special_bound = V4 (0x7fe00000), /* asuint64(inf) - asuint64(0x1p-126). */
};
#define N (1 << V_LOG_TABLE_BITS)
#define IndexMask (N - 1)
struct entry
{
float64x2_t invc;
float64x2_t logc;
};
static inline struct entry
lookup (uint64x2_t i)
{
/* Since N is a power of 2, n % N = n & (N - 1). */
struct entry e;
uint64_t i0 = (vgetq_lane_u64 (i, 0) >> (52 - V_LOG_TABLE_BITS)) & IndexMask;
uint64_t i1 = (vgetq_lane_u64 (i, 1) >> (52 - V_LOG_TABLE_BITS)) & IndexMask;
float64x2_t e0 = vld1q_f64 (&__v_log_data.table[i0].invc);
float64x2_t e1 = vld1q_f64 (&__v_log_data.table[i1].invc);
e.invc = vuzp1q_f64 (e0, e1);
e.logc = vuzp2q_f64 (e0, e1);
return e;
}
static float64x2_t VPCS_ATTR NOINLINE
special_case (float64x2_t hi, uint64x2_t u_off, float64x2_t y, float64x2_t r2,
uint32x2_t special, const struct data *d)
{
float64x2_t x = vreinterpretq_f64_u64 (vaddq_u64 (u_off, d->off));
return v_call_f64 (log, x, vfmaq_f64 (hi, y, r2), vmovl_u32 (special));
}
/* Double-precision vector log routine.
The maximum observed error is 2.17 ULP:
_ZGVnN2v_log(0x1.a6129884398a3p+0) got 0x1.ffffff1cca043p-2
want 0x1.ffffff1cca045p-2. */
float64x2_t VPCS_ATTR V_NAME_D1 (log) (float64x2_t x)
{
const struct data *d = ptr_barrier (&data);
/* To avoid having to mov x out of the way, keep u after offset has been
applied, and recover x by adding the offset back in the special-case
handler. */
uint64x2_t u = vreinterpretq_u64_f64 (x);
uint64x2_t u_off = vsubq_u64 (u, d->off);
/* x = 2^k z; where z is in range [Off,2*Off) and exact.
The range is split into N subintervals.
The ith subinterval contains z and c is near its center. */
int64x2_t k = vshrq_n_s64 (vreinterpretq_s64_u64 (u_off), 52);
uint64x2_t iz = vsubq_u64 (u, vandq_u64 (u_off, d->sign_exp_mask));
float64x2_t z = vreinterpretq_f64_u64 (iz);
struct entry e = lookup (u_off);
uint32x2_t special = vcge_u32 (vsubhn_u64 (u_off, d->offset_lower_bound),
vget_low_u32 (d->special_bound));
/* log(x) = log1p(z/c-1) + log(c) + k*Ln2. */
float64x2_t r = vfmaq_f64 (v_f64 (-1.0), z, e.invc);
float64x2_t kd = vcvtq_f64_s64 (k);
/* hi = r + log(c) + k*Ln2. */
float64x2_t ln2_and_c4 = vld1q_f64 (&d->ln2);
float64x2_t hi = vfmaq_laneq_f64 (vaddq_f64 (e.logc, r), kd, ln2_and_c4, 0);
/* y = r2*(A0 + r*A1 + r2*(A2 + r*A3 + r2*A4)) + hi. */
float64x2_t odd_coeffs = vld1q_f64 (&d->c1);
float64x2_t r2 = vmulq_f64 (r, r);
float64x2_t y = vfmaq_laneq_f64 (d->c2, r, odd_coeffs, 1);
float64x2_t p = vfmaq_laneq_f64 (d->c0, r, odd_coeffs, 0);
y = vfmaq_laneq_f64 (y, r2, ln2_and_c4, 1);
y = vfmaq_f64 (p, r2, y);
if (__glibc_unlikely (v_any_u32h (special)))
return special_case (hi, u_off, y, r2, special, d);
return vfmaq_f64 (hi, y, r2);
}
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