diff options
Diffstat (limited to 'sysdeps/aarch64/fpu/v_log1p_inline.h')
| -rw-r--r-- | sysdeps/aarch64/fpu/v_log1p_inline.h | 103 |
1 files changed, 103 insertions, 0 deletions
diff --git a/sysdeps/aarch64/fpu/v_log1p_inline.h b/sysdeps/aarch64/fpu/v_log1p_inline.h new file mode 100644 index 0000000000..242e43b6ee --- /dev/null +++ b/sysdeps/aarch64/fpu/v_log1p_inline.h @@ -0,0 +1,103 @@ +/* Helper for double-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_LOG1P_INLINE_H +#define AARCH64_FPU_V_LOG1P_INLINE_H + +#include "v_math.h" +#include "poly_advsimd_f64.h" + +struct v_log1p_data +{ + float64x2_t poly[19], ln2[2]; + uint64x2_t hf_rt2_top, one_m_hf_rt2_top, umask; + int64x2_t one_top; +}; + +/* Coefficients generated using Remez, deg=20, in [sqrt(2)/2-1, sqrt(2)-1]. */ +#define V_LOG1P_CONSTANTS_TABLE \ + { \ + .poly = { V2 (-0x1.ffffffffffffbp-2), V2 (0x1.55555555551a9p-2), \ + V2 (-0x1.00000000008e3p-2), V2 (0x1.9999999a32797p-3), \ + V2 (-0x1.555555552fecfp-3), V2 (0x1.249248e071e5ap-3), \ + V2 (-0x1.ffffff8bf8482p-4), V2 (0x1.c71c8f07da57ap-4), \ + V2 (-0x1.9999ca4ccb617p-4), V2 (0x1.7459ad2e1dfa3p-4), \ + V2 (-0x1.554d2680a3ff2p-4), V2 (0x1.3b4c54d487455p-4), \ + V2 (-0x1.2548a9ffe80e6p-4), V2 (0x1.0f389a24b2e07p-4), \ + V2 (-0x1.eee4db15db335p-5), V2 (0x1.e95b494d4a5ddp-5), \ + V2 (-0x1.15fdf07cb7c73p-4), V2 (0x1.0310b70800fcfp-4), \ + V2 (-0x1.cfa7385bdb37ep-6) }, \ + .ln2 = { V2 (0x1.62e42fefa3800p-1), V2 (0x1.ef35793c76730p-45) }, \ + .hf_rt2_top = V2 (0x3fe6a09e00000000), \ + .one_m_hf_rt2_top = V2 (0x00095f6200000000), \ + .umask = V2 (0x000fffff00000000), .one_top = V2 (0x3ff) \ + } + +#define BottomMask v_u64 (0xffffffff) + +static inline float64x2_t +log1p_inline (float64x2_t x, const struct v_log1p_data *d) +{ + /* Helper for calculating log(x + 1). Copied from v_log1p_2u5.c, with several + modifications: + - No special-case handling - this should be dealt with by the caller. + - Pairwise Horner polynomial evaluation for improved accuracy. + - Optionally simulate the shortcut for k=0, used in the scalar routine, + using v_sel, for improved accuracy when the argument to log1p is close to + 0. This feature is enabled by defining WANT_V_LOG1P_K0_SHORTCUT as 1 in + the source of the caller before including this file. + See v_log1pf_2u1.c for details of the algorithm. */ + float64x2_t m = vaddq_f64 (x, v_f64 (1)); + uint64x2_t mi = vreinterpretq_u64_f64 (m); + uint64x2_t u = vaddq_u64 (mi, d->one_m_hf_rt2_top); + + int64x2_t ki + = vsubq_s64 (vreinterpretq_s64_u64 (vshrq_n_u64 (u, 52)), d->one_top); + float64x2_t k = vcvtq_f64_s64 (ki); + + /* Reduce x to f in [sqrt(2)/2, sqrt(2)]. */ + uint64x2_t utop = vaddq_u64 (vandq_u64 (u, d->umask), d->hf_rt2_top); + uint64x2_t u_red = vorrq_u64 (utop, vandq_u64 (mi, BottomMask)); + float64x2_t f = vsubq_f64 (vreinterpretq_f64_u64 (u_red), v_f64 (1)); + + /* Correction term c/m. */ + float64x2_t cm = vdivq_f64 (vsubq_f64 (x, vsubq_f64 (m, v_f64 (1))), m); + +#ifndef WANT_V_LOG1P_K0_SHORTCUT +#error \ + "Cannot use v_log1p_inline.h without specifying whether you need the k0 shortcut for greater accuracy close to 0" +#elif WANT_V_LOG1P_K0_SHORTCUT + /* Shortcut if k is 0 - set correction term to 0 and f to x. The result is + that the approximation is solely the polynomial. */ + uint64x2_t k0 = vceqzq_f64 (k); + cm = v_zerofy_f64 (cm, k0); + f = vbslq_f64 (k0, x, f); +#endif + + /* Approximate log1p(f) on the reduced input using a polynomial. */ + float64x2_t f2 = vmulq_f64 (f, f); + float64x2_t p = v_pw_horner_18_f64 (f, f2, d->poly); + + /* Assemble log1p(x) = k * log2 + log1p(f) + c/m. */ + float64x2_t ylo = vfmaq_f64 (cm, k, d->ln2[1]); + float64x2_t yhi = vfmaq_f64 (f, k, d->ln2[0]); + return vfmaq_f64 (vaddq_f64 (ylo, yhi), f2, p); +} + +#endif |