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git.ipfire.org Git - thirdparty/glibc.git/blob - sysdeps/aarch64/fpu/log2f_sve.c
1 /* Single-precision vector (SVE) log2 function
3 Copyright (C) 2023 Free Software Foundation, Inc.
4 This file is part of the GNU C Library.
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.
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.
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/>. */
22 static const struct data
28 /* Coefficients copied from the AdvSIMD routine, then rearranged so that coeffs
29 1, 3, 5 and 7 can be loaded as a single quad-word, hence used with _lane
30 variant of MLA intrinsic. */
31 -0x1.715458p
-1f
, -0x1.7171a4p
-2f
, -0x1.e5143ep
-3f
, -0x1.c675bp
-3f
33 .poly_02468
= { 0x1.715476p0f
, 0x1.ec701cp
-2f
, 0x1.27a0b8p
-2f
,
34 0x1.9d8ecap
-3f
, 0x1.9e495p
-3f
},
37 #define Min (0x00800000)
38 #define Max (0x7f800000)
39 #define Thres (0x7f000000) /* Max - Min. */
40 #define MantissaMask (0x007fffff)
41 #define Off (0x3f2aaaab) /* 0.666667. */
43 static svfloat32_t NOINLINE
44 special_case (svfloat32_t x
, svfloat32_t y
, svbool_t cmp
)
46 return sv_call_f32 (log2f
, x
, y
, cmp
);
49 /* Optimised implementation of SVE log2f, using the same algorithm
50 and polynomial as AdvSIMD log2f.
51 Maximum error is 2.48 ULPs:
52 SV_NAME_F1 (log2)(0x1.558174p+0) got 0x1.a9be84p-2
54 svfloat32_t
SV_NAME_F1 (log2
) (svfloat32_t x
, const svbool_t pg
)
56 const struct data
*d
= ptr_barrier (&data
);
58 svuint32_t u
= svreinterpret_u32 (x
);
59 svbool_t special
= svcmpge (pg
, svsub_x (pg
, u
, Min
), Thres
);
61 /* x = 2^n * (1+r), where 2/3 < 1+r < 4/3. */
62 u
= svsub_x (pg
, u
, Off
);
63 svfloat32_t n
= svcvt_f32_x (
64 pg
, svasr_x (pg
, svreinterpret_s32 (u
), 23)); /* Sign-extend. */
65 u
= svand_x (pg
, u
, MantissaMask
);
66 u
= svadd_x (pg
, u
, Off
);
67 svfloat32_t r
= svsub_x (pg
, svreinterpret_f32 (u
), 1.0f
);
69 /* y = log2(1+r) + n. */
70 svfloat32_t r2
= svmul_x (pg
, r
, r
);
72 /* Evaluate polynomial using pairwise Horner scheme. */
73 svfloat32_t p_1357
= svld1rq (svptrue_b32 (), &d
->poly_1357
[0]);
74 svfloat32_t q_01
= svmla_lane (sv_f32 (d
->poly_02468
[0]), r
, p_1357
, 0);
75 svfloat32_t q_23
= svmla_lane (sv_f32 (d
->poly_02468
[1]), r
, p_1357
, 1);
76 svfloat32_t q_45
= svmla_lane (sv_f32 (d
->poly_02468
[2]), r
, p_1357
, 2);
77 svfloat32_t q_67
= svmla_lane (sv_f32 (d
->poly_02468
[3]), r
, p_1357
, 3);
78 svfloat32_t y
= svmla_x (pg
, q_67
, r2
, sv_f32 (d
->poly_02468
[4]));
79 y
= svmla_x (pg
, q_45
, r2
, y
);
80 y
= svmla_x (pg
, q_23
, r2
, y
);
81 y
= svmla_x (pg
, q_01
, r2
, y
);
83 if (__glibc_unlikely (svptest_any (pg
, special
)))
84 return special_case (x
, svmla_x (svnot_z (pg
, special
), n
, r
, y
), special
);
85 return svmla_x (pg
, n
, r
, y
);