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/*
* Double-precision vector log(x) function.
*
* Copyright (c) 2019, Arm Limited.
* SPDX-License-Identifier: MIT
*/
#include "mathlib.h"
#include "v_math.h"
#include "v_log.h"
#if V_SUPPORTED
/* Worst-case error: 1.17 + 0.5 ulp. */
static const f64_t Poly[] = {
/* rel error: 0x1.6272e588p-56 in [ -0x1.fc1p-9 0x1.009p-8 ]. */
-0x1.ffffffffffff7p-2,
0x1.55555555170d4p-2,
-0x1.0000000399c27p-2,
0x1.999b2e90e94cap-3,
-0x1.554e550bd501ep-3,
};
#define A0 v_f64 (Poly[0])
#define A1 v_f64 (Poly[1])
#define A2 v_f64 (Poly[2])
#define A3 v_f64 (Poly[3])
#define A4 v_f64 (Poly[4])
#define Ln2 v_f64 (0x1.62e42fefa39efp-1)
#define N (1 << V_LOG_TABLE_BITS)
#define OFF v_u64 (0x3fe6900900000000)
struct entry
{
v_f64_t invc;
v_f64_t logc;
};
static inline struct entry
lookup (v_u64_t i)
{
struct entry e;
#ifdef SCALAR
e.invc = __v_log_data[i].invc;
e.logc = __v_log_data[i].logc;
#else
e.invc[0] = __v_log_data[i[0]].invc;
e.logc[0] = __v_log_data[i[0]].logc;
e.invc[1] = __v_log_data[i[1]].invc;
e.logc[1] = __v_log_data[i[1]].logc;
#endif
return e;
}
VPCS_ATTR
__attribute__ ((noinline)) static v_f64_t
specialcase (v_f64_t x, v_f64_t y, v_u64_t cmp)
{
return v_call_f64 (log, x, y, cmp);
}
VPCS_ATTR
v_f64_t
V_NAME(log) (v_f64_t x)
{
v_f64_t z, r, r2, p, y, kd, hi;
v_u64_t ix, iz, tmp, top, i, cmp;
v_s64_t k;
struct entry e;
ix = v_as_u64_f64 (x);
top = ix >> 48;
cmp = v_cond_u64 (top - v_u64 (0x0010) >= v_u64 (0x7ff0 - 0x0010));
/* 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. */
tmp = ix - OFF;
i = (tmp >> (52 - V_LOG_TABLE_BITS)) % N;
k = v_as_s64_u64 (tmp) >> 52; /* arithmetic shift */
iz = ix - (tmp & v_u64 (0xfffULL << 52));
z = v_as_f64_u64 (iz);
e = lookup (i);
/* log(x) = log1p(z/c-1) + log(c) + k*Ln2. */
r = v_fma_f64 (z, e.invc, v_f64 (-1.0));
kd = v_to_f64_s64 (k);
/* hi = r + log(c) + k*Ln2. */
hi = v_fma_f64 (kd, Ln2, e.logc + r);
/* y = r2*(A0 + r*A1 + r2*(A2 + r*A3 + r2*A4)) + hi. */
r2 = r * r;
y = v_fma_f64 (A3, r, A2);
p = v_fma_f64 (A1, r, A0);
y = v_fma_f64 (A4, r2, y);
y = v_fma_f64 (y, r2, p);
y = v_fma_f64 (y, r2, hi);
if (unlikely (v_any_u64 (cmp)))
return specialcase (x, y, cmp);
return y;
}
VPCS_ALIAS
#endif
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