mirror of git://sourceware.org/git/glibc.git
98 lines
3.8 KiB
C
98 lines
3.8 KiB
C
/* Helper for single-precision SVE routines which depend on log1p
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Copyright (C) 2024-2025 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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#ifndef AARCH64_FPU_SV_LOG1PF_INLINE_H
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#define AARCH64_FPU_SV_LOG1PF_INLINE_H
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#include "sv_math.h"
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#include "vecmath_config.h"
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#define SignExponentMask 0xff800000
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static const struct sv_log1pf_data
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{
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float c0, c2, c4, c6;
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float c1, c3, c5, c7;
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float ln2, exp_bias, quarter;
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uint32_t four, three_quarters;
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} sv_log1pf_data = {
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/* Do not store first term of polynomial, which is -0.5, as
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this can be fmov-ed directly instead of including it in
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the main load-and-mla polynomial schedule. */
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.c0 = 0x1.5555aap-2f, .c1 = -0x1.000038p-2f, .c2 = 0x1.99675cp-3f,
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.c3 = -0x1.54ef78p-3f, .c4 = 0x1.28a1f4p-3f, .c5 = -0x1.0da91p-3f,
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.c6 = 0x1.abcb6p-4f, .c7 = -0x1.6f0d5ep-5f, .ln2 = 0x1.62e43p-1f,
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.exp_bias = 0x1p-23f, .quarter = 0x1p-2f, .four = 0x40800000,
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.three_quarters = 0x3f400000,
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};
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static inline svfloat32_t
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sv_log1pf_inline (svfloat32_t x, svbool_t pg)
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{
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const struct sv_log1pf_data *d = ptr_barrier (&sv_log1pf_data);
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/* With x + 1 = t * 2^k (where t = m + 1 and k is chosen such that m
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is in [-0.25, 0.5]):
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log1p(x) = log(t) + log(2^k) = log1p(m) + k*log(2).
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We approximate log1p(m) with a polynomial, then scale by
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k*log(2). Instead of doing this directly, we use an intermediate
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scale factor s = 4*k*log(2) to ensure the scale is representable
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as a normalised fp32 number. */
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svfloat32_t m = svadd_x (pg, x, 1);
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/* Choose k to scale x to the range [-1/4, 1/2]. */
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svint32_t k
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= svand_x (pg, svsub_x (pg, svreinterpret_s32 (m), d->three_quarters),
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sv_s32 (SignExponentMask));
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/* Scale x by exponent manipulation. */
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svfloat32_t m_scale = svreinterpret_f32 (
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svsub_x (pg, svreinterpret_u32 (x), svreinterpret_u32 (k)));
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/* Scale up to ensure that the scale factor is representable as normalised
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fp32 number, and scale m down accordingly. */
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svfloat32_t s = svreinterpret_f32 (svsubr_x (pg, k, d->four));
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svfloat32_t fconst = svld1rq_f32 (svptrue_b32 (), &d->ln2);
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m_scale = svadd_x (pg, m_scale, svmla_lane_f32 (sv_f32 (-1), s, fconst, 2));
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/* Evaluate polynomial on reduced interval. */
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svfloat32_t ms2 = svmul_x (svptrue_b32 (), m_scale, m_scale);
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svfloat32_t c1357 = svld1rq_f32 (svptrue_b32 (), &d->c1);
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svfloat32_t p01 = svmla_lane_f32 (sv_f32 (d->c0), m_scale, c1357, 0);
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svfloat32_t p23 = svmla_lane_f32 (sv_f32 (d->c2), m_scale, c1357, 1);
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svfloat32_t p45 = svmla_lane_f32 (sv_f32 (d->c4), m_scale, c1357, 2);
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svfloat32_t p67 = svmla_lane_f32 (sv_f32 (d->c6), m_scale, c1357, 3);
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svfloat32_t p = svmla_x (pg, p45, p67, ms2);
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p = svmla_x (pg, p23, p, ms2);
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p = svmla_x (pg, p01, p, ms2);
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p = svmad_x (pg, m_scale, p, -0.5);
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p = svmla_x (pg, m_scale, m_scale, svmul_x (pg, m_scale, p));
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/* The scale factor to be applied back at the end - by multiplying float(k)
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by 2^-23 we get the unbiased exponent of k. */
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svfloat32_t scale_back = svmul_lane_f32 (svcvt_f32_x (pg, k), fconst, 1);
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return svmla_lane_f32 (p, scale_back, fconst, 0);
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}
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#endif
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