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110 lines
4.2 KiB
C
110 lines
4.2 KiB
C
/* Helper for double-precision SVE routines which depend on log1p
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Copyright (C) 2024 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_LOG1P_INLINE_H
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#define AARCH64_FPU_SV_LOG1P_INLINE_H
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#include "sv_math.h"
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#include "poly_sve_f64.h"
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static const struct sv_log1p_data
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{
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double poly[19], ln2[2];
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uint64_t hf_rt2_top;
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uint64_t one_m_hf_rt2_top;
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uint32_t bottom_mask;
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int64_t one_top;
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} sv_log1p_data = {
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/* Coefficients generated using Remez, deg=20, in [sqrt(2)/2-1, sqrt(2)-1].
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*/
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.poly = { -0x1.ffffffffffffbp-2, 0x1.55555555551a9p-2, -0x1.00000000008e3p-2,
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0x1.9999999a32797p-3, -0x1.555555552fecfp-3, 0x1.249248e071e5ap-3,
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-0x1.ffffff8bf8482p-4, 0x1.c71c8f07da57ap-4, -0x1.9999ca4ccb617p-4,
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0x1.7459ad2e1dfa3p-4, -0x1.554d2680a3ff2p-4, 0x1.3b4c54d487455p-4,
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-0x1.2548a9ffe80e6p-4, 0x1.0f389a24b2e07p-4, -0x1.eee4db15db335p-5,
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0x1.e95b494d4a5ddp-5, -0x1.15fdf07cb7c73p-4, 0x1.0310b70800fcfp-4,
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-0x1.cfa7385bdb37ep-6 },
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.ln2 = { 0x1.62e42fefa3800p-1, 0x1.ef35793c76730p-45 },
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.hf_rt2_top = 0x3fe6a09e00000000,
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.one_m_hf_rt2_top = 0x00095f6200000000,
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.bottom_mask = 0xffffffff,
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.one_top = 0x3ff
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};
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static inline svfloat64_t
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sv_log1p_inline (svfloat64_t x, const svbool_t pg)
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{
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/* Helper for calculating log(x + 1). Adapted from v_log1p_inline.h, which
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differs from v_log1p_2u5.c by:
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- No special-case handling - this should be dealt with by the caller.
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- Pairwise Horner polynomial evaluation for improved accuracy.
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- Optionally simulate the shortcut for k=0, used in the scalar routine,
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using svsel, for improved accuracy when the argument to log1p is close
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to 0. This feature is enabled by defining WANT_SV_LOG1P_K0_SHORTCUT as 1
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in the source of the caller before including this file.
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See sv_log1p_2u1.c for details of the algorithm. */
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const struct sv_log1p_data *d = ptr_barrier (&sv_log1p_data);
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svfloat64_t m = svadd_x (pg, x, 1);
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svuint64_t mi = svreinterpret_u64 (m);
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svuint64_t u = svadd_x (pg, mi, d->one_m_hf_rt2_top);
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svint64_t ki
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= svsub_x (pg, svreinterpret_s64 (svlsr_x (pg, u, 52)), d->one_top);
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svfloat64_t k = svcvt_f64_x (pg, ki);
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/* Reduce x to f in [sqrt(2)/2, sqrt(2)]. */
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svuint64_t utop
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= svadd_x (pg, svand_x (pg, u, 0x000fffff00000000), d->hf_rt2_top);
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svuint64_t u_red = svorr_x (pg, utop, svand_x (pg, mi, d->bottom_mask));
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svfloat64_t f = svsub_x (pg, svreinterpret_f64 (u_red), 1);
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/* Correction term c/m. */
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svfloat64_t c = svsub_x (pg, x, svsub_x (pg, m, 1));
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svfloat64_t cm;
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#ifndef WANT_SV_LOG1P_K0_SHORTCUT
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#error \
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"Cannot use sv_log1p_inline.h without specifying whether you need the k0 shortcut for greater accuracy close to 0"
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#elif WANT_SV_LOG1P_K0_SHORTCUT
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/* Shortcut if k is 0 - set correction term to 0 and f to x. The result is
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that the approximation is solely the polynomial. */
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svbool_t knot0 = svcmpne (pg, k, 0);
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cm = svdiv_z (knot0, c, m);
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if (__glibc_likely (!svptest_any (pg, knot0)))
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{
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f = svsel (knot0, f, x);
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}
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#else
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/* No shortcut. */
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cm = svdiv_x (pg, c, m);
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#endif
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/* Approximate log1p(f) on the reduced input using a polynomial. */
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svfloat64_t f2 = svmul_x (pg, f, f);
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svfloat64_t p = sv_pw_horner_18_f64_x (pg, f, f2, d->poly);
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/* Assemble log1p(x) = k * log2 + log1p(f) + c/m. */
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svfloat64_t ylo = svmla_x (pg, cm, k, d->ln2[0]);
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svfloat64_t yhi = svmla_x (pg, f, k, d->ln2[1]);
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return svmla_x (pg, svadd_x (pg, ylo, yhi), f2, p);
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}
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#endif
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