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168 lines
6.3 KiB
C
168 lines
6.3 KiB
C
/* Double-precision vector (SVE) erfc function
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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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#include "sv_math.h"
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#include "vecmath_config.h"
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static const struct data
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{
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uint64_t off_idx, off_arr;
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double max, shift;
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double p20, p40, p41, p42;
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double p51, p52;
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double q5, r5;
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double q6, r6;
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double q7, r7;
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double q8, r8;
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double q9, r9;
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uint64_t table_scale;
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} data = {
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/* Set an offset so the range of the index used for lookup is 3487, and it
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can be clamped using a saturated add on an offset index.
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Index offset is 0xffffffffffffffff - asuint64(shift) - 3487. */
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.off_idx = 0xbd3ffffffffff260,
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.off_arr = 0xfffffffffffff260, /* 0xffffffffffffffff - 3487. */
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.max = 0x1.b3ep+4, /* 3487/128. */
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.shift = 0x1p45,
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.table_scale = 0x37f0000000000000, /* asuint64(0x1p-128). */
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.p20 = 0x1.5555555555555p-2, /* 1/3, used to compute 2/3 and 1/6. */
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.p40 = -0x1.999999999999ap-4, /* 1/10. */
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.p41 = -0x1.999999999999ap-2, /* 2/5. */
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.p42 = 0x1.1111111111111p-3, /* 2/15. */
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.p51 = -0x1.c71c71c71c71cp-3, /* 2/9. */
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.p52 = 0x1.6c16c16c16c17p-5, /* 2/45. */
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/* Qi = (i+1) / i, for i = 5, ..., 9. */
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.q5 = 0x1.3333333333333p0,
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.q6 = 0x1.2aaaaaaaaaaabp0,
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.q7 = 0x1.2492492492492p0,
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.q8 = 0x1.2p0,
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.q9 = 0x1.1c71c71c71c72p0,
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/* Ri = -2 * i / ((i+1)*(i+2)), for i = 5, ..., 9. */
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.r5 = -0x1.e79e79e79e79ep-3,
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.r6 = -0x1.b6db6db6db6dbp-3,
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.r7 = -0x1.8e38e38e38e39p-3,
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.r8 = -0x1.6c16c16c16c17p-3,
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.r9 = -0x1.4f2094f2094f2p-3,
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};
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/* Optimized double-precision vector erfc(x).
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Approximation based on series expansion near x rounded to
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nearest multiple of 1/128.
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Let d = x - r, and scale = 2 / sqrt(pi) * exp(-r^2). For x near r,
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erfc(x) ~ erfc(r) - scale * d * poly(r, d), with
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poly(r, d) = 1 - r d + (2/3 r^2 - 1/3) d^2 - r (1/3 r^2 - 1/2) d^3
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+ (2/15 r^4 - 2/5 r^2 + 1/10) d^4
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- r * (2/45 r^4 - 2/9 r^2 + 1/6) d^5
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+ p6(r) d^6 + ... + p10(r) d^10
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Polynomials p6(r) to p10(r) are computed using recurrence relation
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2(i+1)p_i + 2r(i+2)p_{i+1} + (i+2)(i+3)p_{i+2} = 0,
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with p0 = 1, and p1(r) = -r.
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Values of erfc(r) and scale are read from lookup tables. Stored values
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are scaled to avoid hitting the subnormal range.
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Note that for x < 0, erfc(x) = 2.0 - erfc(-x).
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Maximum measured error: 1.71 ULP
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_ZGVsMxv_erfc(0x1.46cfe976733p+4) got 0x1.e15fcbea3e7afp-608
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want 0x1.e15fcbea3e7adp-608. */
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svfloat64_t SV_NAME_D1 (erfc) (svfloat64_t x, const svbool_t pg)
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{
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const struct data *dat = ptr_barrier (&data);
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svfloat64_t a = svabs_x (pg, x);
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/* Clamp input at |x| <= 3487/128. */
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a = svmin_x (pg, a, dat->max);
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/* Reduce x to the nearest multiple of 1/128. */
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svfloat64_t shift = sv_f64 (dat->shift);
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svfloat64_t z = svadd_x (pg, a, shift);
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/* Saturate index for the NaN case. */
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svuint64_t i = svqadd (svreinterpret_u64 (z), dat->off_idx);
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/* Lookup erfc(r) and 2/sqrt(pi)*exp(-r^2) in tables. */
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i = svadd_x (pg, i, i);
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const float64_t *p = &__erfc_data.tab[0].erfc - 2 * dat->off_arr;
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svfloat64_t erfcr = svld1_gather_index (pg, p, i);
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svfloat64_t scale = svld1_gather_index (pg, p + 1, i);
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/* erfc(x) ~ erfc(r) - scale * d * poly(r, d). */
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svfloat64_t r = svsub_x (pg, z, shift);
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svfloat64_t d = svsub_x (pg, a, r);
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svfloat64_t d2 = svmul_x (pg, d, d);
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svfloat64_t r2 = svmul_x (pg, r, r);
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/* poly (d, r) = 1 + p1(r) * d + p2(r) * d^2 + ... + p9(r) * d^9. */
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svfloat64_t p1 = r;
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svfloat64_t third = sv_f64 (dat->p20);
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svfloat64_t twothird = svmul_x (pg, third, 2.0);
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svfloat64_t sixth = svmul_x (pg, third, 0.5);
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svfloat64_t p2 = svmls_x (pg, third, r2, twothird);
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svfloat64_t p3 = svmad_x (pg, r2, third, -0.5);
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p3 = svmul_x (pg, r, p3);
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svfloat64_t p4 = svmla_x (pg, sv_f64 (dat->p41), r2, dat->p42);
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p4 = svmls_x (pg, sv_f64 (dat->p40), r2, p4);
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svfloat64_t p5 = svmla_x (pg, sv_f64 (dat->p51), r2, dat->p52);
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p5 = svmla_x (pg, sixth, r2, p5);
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p5 = svmul_x (pg, r, p5);
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/* Compute p_i using recurrence relation:
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p_{i+2} = (p_i + r * Q_{i+1} * p_{i+1}) * R_{i+1}. */
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svfloat64_t qr5 = svld1rq (svptrue_b64 (), &dat->q5);
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svfloat64_t qr6 = svld1rq (svptrue_b64 (), &dat->q6);
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svfloat64_t qr7 = svld1rq (svptrue_b64 (), &dat->q7);
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svfloat64_t qr8 = svld1rq (svptrue_b64 (), &dat->q8);
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svfloat64_t qr9 = svld1rq (svptrue_b64 (), &dat->q9);
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svfloat64_t p6 = svmla_x (pg, p4, p5, svmul_lane (r, qr5, 0));
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p6 = svmul_lane (p6, qr5, 1);
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svfloat64_t p7 = svmla_x (pg, p5, p6, svmul_lane (r, qr6, 0));
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p7 = svmul_lane (p7, qr6, 1);
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svfloat64_t p8 = svmla_x (pg, p6, p7, svmul_lane (r, qr7, 0));
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p8 = svmul_lane (p8, qr7, 1);
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svfloat64_t p9 = svmla_x (pg, p7, p8, svmul_lane (r, qr8, 0));
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p9 = svmul_lane (p9, qr8, 1);
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svfloat64_t p10 = svmla_x (pg, p8, p9, svmul_lane (r, qr9, 0));
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p10 = svmul_lane (p10, qr9, 1);
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/* Compute polynomial in d using pairwise Horner scheme. */
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svfloat64_t p90 = svmla_x (pg, p9, d, p10);
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svfloat64_t p78 = svmla_x (pg, p7, d, p8);
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svfloat64_t p56 = svmla_x (pg, p5, d, p6);
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svfloat64_t p34 = svmla_x (pg, p3, d, p4);
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svfloat64_t p12 = svmla_x (pg, p1, d, p2);
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svfloat64_t y = svmla_x (pg, p78, d2, p90);
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y = svmla_x (pg, p56, d2, y);
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y = svmla_x (pg, p34, d2, y);
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y = svmla_x (pg, p12, d2, y);
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y = svmls_x (pg, erfcr, scale, svmls_x (pg, d, d2, y));
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/* Offset equals 2.0 if sign, else 0.0. */
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svuint64_t sign = svand_x (pg, svreinterpret_u64 (x), 0x8000000000000000);
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svfloat64_t off = svreinterpret_f64 (svlsr_x (pg, sign, 1));
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/* Handle sign and scale back in a single fma. */
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svfloat64_t fac = svreinterpret_f64 (svorr_x (pg, sign, dat->table_scale));
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return svmla_x (pg, off, fac, y);
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}
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