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067a3415 JR |
1 | /* Double-precision vector (AdvSIMD) log10 function |
2 | ||
3 | Copyright (C) 2023 Free Software Foundation, Inc. | |
4 | This file is part of the GNU C Library. | |
5 | ||
6 | The GNU C Library is free software; you can redistribute it and/or | |
7 | modify it under the terms of the GNU Lesser General Public | |
8 | License as published by the Free Software Foundation; either | |
9 | version 2.1 of the License, or (at your option) any later version. | |
10 | ||
11 | The GNU C Library is distributed in the hope that it will be useful, | |
12 | but WITHOUT ANY WARRANTY; without even the implied warranty of | |
13 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
14 | Lesser General Public License for more details. | |
15 | ||
16 | You should have received a copy of the GNU Lesser General Public | |
17 | License along with the GNU C Library; if not, see | |
18 | <https://www.gnu.org/licenses/>. */ | |
19 | ||
20 | #include "v_math.h" | |
21 | #include "poly_advsimd_f64.h" | |
22 | ||
23 | #define N (1 << V_LOG10_TABLE_BITS) | |
24 | ||
25 | static const struct data | |
26 | { | |
27 | uint64x2_t min_norm; | |
28 | uint32x4_t special_bound; | |
29 | float64x2_t poly[5]; | |
30 | float64x2_t invln10, log10_2, ln2; | |
31 | uint64x2_t sign_exp_mask; | |
32 | } data = { | |
33 | /* Computed from log coefficients divided by log(10) then rounded to double | |
34 | precision. */ | |
35 | .poly = { V2 (-0x1.bcb7b1526e506p-3), V2 (0x1.287a7636be1d1p-3), | |
36 | V2 (-0x1.bcb7b158af938p-4), V2 (0x1.63c78734e6d07p-4), | |
37 | V2 (-0x1.287461742fee4p-4) }, | |
38 | .ln2 = V2 (0x1.62e42fefa39efp-1), | |
39 | .invln10 = V2 (0x1.bcb7b1526e50ep-2), | |
40 | .log10_2 = V2 (0x1.34413509f79ffp-2), | |
41 | .min_norm = V2 (0x0010000000000000), /* asuint64(0x1p-1022). */ | |
42 | .special_bound = V4 (0x7fe00000), /* asuint64(inf) - min_norm. */ | |
43 | .sign_exp_mask = V2 (0xfff0000000000000), | |
44 | }; | |
45 | ||
46 | #define Off v_u64 (0x3fe6900900000000) | |
47 | #define IndexMask (N - 1) | |
48 | ||
49 | #define T(s, i) __v_log10_data.s[i] | |
50 | ||
51 | struct entry | |
52 | { | |
53 | float64x2_t invc; | |
54 | float64x2_t log10c; | |
55 | }; | |
56 | ||
57 | static inline struct entry | |
58 | lookup (uint64x2_t i) | |
59 | { | |
60 | struct entry e; | |
61 | uint64_t i0 = (i[0] >> (52 - V_LOG10_TABLE_BITS)) & IndexMask; | |
62 | uint64_t i1 = (i[1] >> (52 - V_LOG10_TABLE_BITS)) & IndexMask; | |
63 | float64x2_t e0 = vld1q_f64 (&__v_log10_data.table[i0].invc); | |
64 | float64x2_t e1 = vld1q_f64 (&__v_log10_data.table[i1].invc); | |
65 | e.invc = vuzp1q_f64 (e0, e1); | |
66 | e.log10c = vuzp2q_f64 (e0, e1); | |
67 | return e; | |
68 | } | |
69 | ||
70 | static float64x2_t VPCS_ATTR NOINLINE | |
71 | special_case (float64x2_t x, float64x2_t y, float64x2_t hi, float64x2_t r2, | |
72 | uint32x2_t special) | |
73 | { | |
74 | return v_call_f64 (log10, x, vfmaq_f64 (hi, r2, y), vmovl_u32 (special)); | |
75 | } | |
76 | ||
77 | /* Fast implementation of double-precision vector log10 | |
78 | is a slight modification of double-precision vector log. | |
79 | Max ULP error: < 2.5 ulp (nearest rounding.) | |
80 | Maximum measured at 2.46 ulp for x in [0.96, 0.97] | |
81 | _ZGVnN2v_log10(0x1.13192407fcb46p+0) got 0x1.fff6be3cae4bbp-6 | |
82 | want 0x1.fff6be3cae4b9p-6. */ | |
83 | float64x2_t VPCS_ATTR V_NAME_D1 (log10) (float64x2_t x) | |
84 | { | |
85 | const struct data *d = ptr_barrier (&data); | |
86 | uint64x2_t ix = vreinterpretq_u64_f64 (x); | |
87 | uint32x2_t special = vcge_u32 (vsubhn_u64 (ix, d->min_norm), | |
88 | vget_low_u32 (d->special_bound)); | |
89 | ||
90 | /* x = 2^k z; where z is in range [OFF,2*OFF) and exact. | |
91 | The range is split into N subintervals. | |
92 | The ith subinterval contains z and c is near its center. */ | |
93 | uint64x2_t tmp = vsubq_u64 (ix, Off); | |
94 | int64x2_t k = vshrq_n_s64 (vreinterpretq_s64_u64 (tmp), 52); | |
95 | uint64x2_t iz = vsubq_u64 (ix, vandq_u64 (tmp, d->sign_exp_mask)); | |
96 | float64x2_t z = vreinterpretq_f64_u64 (iz); | |
97 | ||
98 | struct entry e = lookup (tmp); | |
99 | ||
100 | /* log10(x) = log1p(z/c-1)/log(10) + log10(c) + k*log10(2). */ | |
101 | float64x2_t r = vfmaq_f64 (v_f64 (-1.0), z, e.invc); | |
102 | float64x2_t kd = vcvtq_f64_s64 (k); | |
103 | ||
104 | /* hi = r / log(10) + log10(c) + k*log10(2). | |
105 | Constants in v_log10_data.c are computed (in extended precision) as | |
106 | e.log10c := e.logc * ivln10. */ | |
107 | float64x2_t w = vfmaq_f64 (e.log10c, r, d->invln10); | |
108 | ||
109 | /* y = log10(1+r) + n * log10(2). */ | |
110 | float64x2_t hi = vfmaq_f64 (w, kd, d->log10_2); | |
111 | ||
112 | /* y = r2*(A0 + r*A1 + r2*(A2 + r*A3 + r2*A4)) + hi. */ | |
113 | float64x2_t r2 = vmulq_f64 (r, r); | |
114 | float64x2_t y = v_pw_horner_4_f64 (r, r2, d->poly); | |
115 | ||
116 | if (__glibc_unlikely (v_any_u32h (special))) | |
117 | return special_case (x, y, hi, r2, special); | |
118 | return vfmaq_f64 (hi, r2, y); | |
119 | } |