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104 lines
4.5 KiB
C
104 lines
4.5 KiB
C
/* Helper for double-precision Advanced SIMD 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_V_LOG1P_INLINE_H
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#define AARCH64_FPU_V_LOG1P_INLINE_H
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#include "v_math.h"
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#include "poly_advsimd_f64.h"
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struct v_log1p_data
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{
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float64x2_t poly[19], ln2[2];
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uint64x2_t hf_rt2_top, one_m_hf_rt2_top, umask;
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int64x2_t one_top;
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};
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/* Coefficients generated using Remez, deg=20, in [sqrt(2)/2-1, sqrt(2)-1]. */
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#define V_LOG1P_CONSTANTS_TABLE \
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{ \
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.poly = { V2 (-0x1.ffffffffffffbp-2), V2 (0x1.55555555551a9p-2), \
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V2 (-0x1.00000000008e3p-2), V2 (0x1.9999999a32797p-3), \
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V2 (-0x1.555555552fecfp-3), V2 (0x1.249248e071e5ap-3), \
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V2 (-0x1.ffffff8bf8482p-4), V2 (0x1.c71c8f07da57ap-4), \
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V2 (-0x1.9999ca4ccb617p-4), V2 (0x1.7459ad2e1dfa3p-4), \
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V2 (-0x1.554d2680a3ff2p-4), V2 (0x1.3b4c54d487455p-4), \
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V2 (-0x1.2548a9ffe80e6p-4), V2 (0x1.0f389a24b2e07p-4), \
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V2 (-0x1.eee4db15db335p-5), V2 (0x1.e95b494d4a5ddp-5), \
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V2 (-0x1.15fdf07cb7c73p-4), V2 (0x1.0310b70800fcfp-4), \
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V2 (-0x1.cfa7385bdb37ep-6) }, \
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.ln2 = { V2 (0x1.62e42fefa3800p-1), V2 (0x1.ef35793c76730p-45) }, \
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.hf_rt2_top = V2 (0x3fe6a09e00000000), \
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.one_m_hf_rt2_top = V2 (0x00095f6200000000), \
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.umask = V2 (0x000fffff00000000), .one_top = V2 (0x3ff) \
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}
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#define BottomMask v_u64 (0xffffffff)
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static inline float64x2_t
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log1p_inline (float64x2_t x, const struct v_log1p_data *d)
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{
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/* Helper for calculating log(x + 1). Copied from v_log1p_2u5.c, with several
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modifications:
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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 v_sel, for improved accuracy when the argument to log1p is close to
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0. This feature is enabled by defining WANT_V_LOG1P_K0_SHORTCUT as 1 in
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the source of the caller before including this file.
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See v_log1pf_2u1.c for details of the algorithm. */
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float64x2_t m = vaddq_f64 (x, v_f64 (1));
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uint64x2_t mi = vreinterpretq_u64_f64 (m);
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uint64x2_t u = vaddq_u64 (mi, d->one_m_hf_rt2_top);
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int64x2_t ki
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= vsubq_s64 (vreinterpretq_s64_u64 (vshrq_n_u64 (u, 52)), d->one_top);
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float64x2_t k = vcvtq_f64_s64 (ki);
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/* Reduce x to f in [sqrt(2)/2, sqrt(2)]. */
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uint64x2_t utop = vaddq_u64 (vandq_u64 (u, d->umask), d->hf_rt2_top);
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uint64x2_t u_red = vorrq_u64 (utop, vandq_u64 (mi, BottomMask));
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float64x2_t f = vsubq_f64 (vreinterpretq_f64_u64 (u_red), v_f64 (1));
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/* Correction term c/m. */
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float64x2_t cm = vdivq_f64 (vsubq_f64 (x, vsubq_f64 (m, v_f64 (1))), m);
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#ifndef WANT_V_LOG1P_K0_SHORTCUT
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#error \
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"Cannot use v_log1p_inline.h without specifying whether you need the k0 shortcut for greater accuracy close to 0"
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#elif WANT_V_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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uint64x2_t k0 = vceqzq_f64 (k);
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cm = v_zerofy_f64 (cm, k0);
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f = vbslq_f64 (k0, x, f);
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#endif
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/* Approximate log1p(f) on the reduced input using a polynomial. */
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float64x2_t f2 = vmulq_f64 (f, f);
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float64x2_t p = v_pw_horner_18_f64 (f, f2, d->poly);
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/* Assemble log1p(x) = k * log2 + log1p(f) + c/m. */
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float64x2_t ylo = vfmaq_f64 (cm, k, d->ln2[1]);
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float64x2_t yhi = vfmaq_f64 (f, k, d->ln2[0]);
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return vfmaq_f64 (vaddq_f64 (ylo, yhi), f2, p);
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}
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#endif
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