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/* Helper for single-precision Advanced SIMD routines which depend on log1p
Copyright (C) 2024 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/>. */
#ifndef AARCH64_FPU_V_LOG1PF_INLINE_H
#define AARCH64_FPU_V_LOG1PF_INLINE_H
#include "v_math.h"
#include "poly_advsimd_f32.h"
struct v_log1pf_data
{
float32x4_t poly[8], ln2;
uint32x4_t four;
int32x4_t three_quarters;
};
/* Polynomial generated using FPMinimax in [-0.25, 0.5]. First two coefficients
(1, -0.5) are not stored as they can be generated more efficiently. */
#define V_LOG1PF_CONSTANTS_TABLE \
{ \
.poly \
= { V4 (0x1.5555aap-2f), V4 (-0x1.000038p-2f), V4 (0x1.99675cp-3f), \
V4 (-0x1.54ef78p-3f), V4 (0x1.28a1f4p-3f), V4 (-0x1.0da91p-3f), \
V4 (0x1.abcb6p-4f), V4 (-0x1.6f0d5ep-5f) }, \
.ln2 = V4 (0x1.62e43p-1f), .four = V4 (0x40800000), \
.three_quarters = V4 (0x3f400000) \
}
static inline float32x4_t
eval_poly (float32x4_t m, const float32x4_t *c)
{
/* Approximate log(1+m) on [-0.25, 0.5] using pairwise Horner (main routine
uses split Estrin, but this way reduces register pressure in the calling
routine). */
float32x4_t q = vfmaq_f32 (v_f32 (-0.5), m, c[0]);
float32x4_t m2 = vmulq_f32 (m, m);
q = vfmaq_f32 (m, m2, q);
float32x4_t p = v_pw_horner_6_f32 (m, m2, c + 1);
p = vmulq_f32 (m2, p);
return vfmaq_f32 (q, m2, p);
}
static inline float32x4_t
log1pf_inline (float32x4_t x, const struct v_log1pf_data d)
{
/* Helper for calculating log(x + 1). Copied from log1pf_2u1.c, with no
special-case handling. See that file for details of the algorithm. */
float32x4_t m = vaddq_f32 (x, v_f32 (1.0f));
int32x4_t k
= vandq_s32 (vsubq_s32 (vreinterpretq_s32_f32 (m), d.three_quarters),
v_s32 (0xff800000));
uint32x4_t ku = vreinterpretq_u32_s32 (k);
float32x4_t s = vreinterpretq_f32_u32 (vsubq_u32 (d.four, ku));
float32x4_t m_scale
= vreinterpretq_f32_u32 (vsubq_u32 (vreinterpretq_u32_f32 (x), ku));
m_scale = vaddq_f32 (m_scale, vfmaq_f32 (v_f32 (-1.0f), v_f32 (0.25f), s));
float32x4_t p = eval_poly (m_scale, d.poly);
float32x4_t scale_back = vmulq_f32 (vcvtq_f32_s32 (k), v_f32 (0x1.0p-23f));
return vfmaq_f32 (p, scale_back, d.ln2);
}
#endif
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