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/* Double-precision vector (Advanced SIMD) exp 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"
#define N (1 << V_EXP_TABLE_BITS)
#define IndexMask (N - 1)
const static volatile struct
{
float64x2_t poly[3];
float64x2_t inv_ln2, ln2_hi, ln2_lo, shift;
float64x2_t special_bound, scale_thresh;
} data = {
/* maxerr: 1.88 +0.5 ulp
rel error: 1.4337*2^-53
abs error: 1.4299*2^-53 in [ -ln2/256, ln2/256 ]. */
.poly = { V2 (0x1.ffffffffffd43p-2), V2 (0x1.55555c75adbb2p-3),
V2 (0x1.55555da646206p-5) },
.scale_thresh = V2 (163840.0), /* 1280.0 * N. */
.special_bound = V2 (704.0),
.inv_ln2 = V2 (0x1.71547652b82fep7), /* N/ln2. */
.ln2_hi = V2 (0x1.62e42fefa39efp-8), /* ln2/N. */
.ln2_lo = V2 (0x1.abc9e3b39803f3p-63),
.shift = V2 (0x1.8p+52)
};
#define C(i) data.poly[i]
#define Tab __v_exp_data
# define SpecialOffset v_u64 (0x6000000000000000) /* 0x1p513. */
/* SpecialBias1 + SpecialBias1 = asuint(1.0). */
# define SpecialBias1 v_u64 (0x7000000000000000) /* 0x1p769. */
# define SpecialBias2 v_u64 (0x3010000000000000) /* 0x1p-254. */
static inline float64x2_t VPCS_ATTR
special_case (float64x2_t s, float64x2_t y, float64x2_t n)
{
/* 2^(n/N) may overflow, break it up into s1*s2. */
uint64x2_t b = vandq_u64 (vcltzq_f64 (n), SpecialOffset);
float64x2_t s1 = vreinterpretq_f64_u64 (vsubq_u64 (SpecialBias1, b));
float64x2_t s2 = vreinterpretq_f64_u64 (
vaddq_u64 (vsubq_u64 (vreinterpretq_u64_f64 (s), SpecialBias2), b));
uint64x2_t cmp = vcagtq_f64 (n, data.scale_thresh);
float64x2_t r1 = vmulq_f64 (s1, s1);
float64x2_t r0 = vmulq_f64 (vfmaq_f64 (s2, y, s2), s1);
return vbslq_f64 (cmp, r1, r0);
}
float64x2_t VPCS_ATTR V_NAME_D1 (exp) (float64x2_t x)
{
float64x2_t n, r, r2, s, y, z;
uint64x2_t cmp, u, e;
cmp = vcagtq_f64 (x, data.special_bound);
/* n = round(x/(ln2/N)). */
z = vfmaq_f64 (data.shift, x, data.inv_ln2);
u = vreinterpretq_u64_f64 (z);
n = vsubq_f64 (z, data.shift);
/* r = x - n*ln2/N. */
r = vfmsq_f64 (x, data.ln2_hi, n);
r = vfmsq_f64 (r, data.ln2_lo, n);
e = vshlq_n_u64 (u, 52 - V_EXP_TABLE_BITS);
/* y = exp(r) - 1 ~= r + C0 r^2 + C1 r^3 + C2 r^4. */
r2 = vmulq_f64 (r, r);
y = vfmaq_f64 (C (0), C (1), r);
y = vfmaq_f64 (y, C (2), r2);
y = vfmaq_f64 (r, y, r2);
/* s = 2^(n/N). */
u = (uint64x2_t){ Tab[u[0] & IndexMask], Tab[u[1] & IndexMask] };
s = vreinterpretq_f64_u64 (vaddq_u64 (u, e));
if (__glibc_unlikely (v_any_u64 (cmp)))
return special_case (s, y, n);
return vfmaq_f64 (s, y, s);
}
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