diff options
Diffstat (limited to 'sysdeps/aarch64/fpu/erfc_advsimd.c')
| -rw-r--r-- | sysdeps/aarch64/fpu/erfc_advsimd.c | 201 |
1 files changed, 201 insertions, 0 deletions
diff --git a/sysdeps/aarch64/fpu/erfc_advsimd.c b/sysdeps/aarch64/fpu/erfc_advsimd.c new file mode 100644 index 0000000000..548f21a3d6 --- /dev/null +++ b/sysdeps/aarch64/fpu/erfc_advsimd.c @@ -0,0 +1,201 @@ +/* Double-precision vector (Advanced SIMD) erfc function + + 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/>. */ + +#include "v_math.h" +#include "vecmath_config.h" + +static const struct data +{ + uint64x2_t offset, table_scale; + float64x2_t max, shift; + float64x2_t p20, p40, p41, p42; + float64x2_t p51, p52; + float64x2_t qr5, qr6, qr7, qr8, qr9; +#if WANT_SIMD_EXCEPT + float64x2_t uflow_bound; +#endif +} data = { + /* Set an offset so the range of the index used for lookup is 3487, and it + can be clamped using a saturated add on an offset index. + Index offset is 0xffffffffffffffff - asuint64(shift) - 3487. */ + .offset = V2 (0xbd3ffffffffff260), + .table_scale = V2 (0x37f0000000000000 << 1), /* asuint64 (2^-128) << 1. */ + .max = V2 (0x1.b3ep+4), /* 3487/128. */ + .shift = V2 (0x1p45), + .p20 = V2 (0x1.5555555555555p-2), /* 1/3, used to compute 2/3 and 1/6. */ + .p40 = V2 (-0x1.999999999999ap-4), /* 1/10. */ + .p41 = V2 (-0x1.999999999999ap-2), /* 2/5. */ + .p42 = V2 (0x1.1111111111111p-3), /* 2/15. */ + .p51 = V2 (-0x1.c71c71c71c71cp-3), /* 2/9. */ + .p52 = V2 (0x1.6c16c16c16c17p-5), /* 2/45. */ + /* Qi = (i+1) / i, Ri = -2 * i / ((i+1)*(i+2)), for i = 5, ..., 9. */ + .qr5 = { 0x1.3333333333333p0, -0x1.e79e79e79e79ep-3 }, + .qr6 = { 0x1.2aaaaaaaaaaabp0, -0x1.b6db6db6db6dbp-3 }, + .qr7 = { 0x1.2492492492492p0, -0x1.8e38e38e38e39p-3 }, + .qr8 = { 0x1.2p0, -0x1.6c16c16c16c17p-3 }, + .qr9 = { 0x1.1c71c71c71c72p0, -0x1.4f2094f2094f2p-3 }, +#if WANT_SIMD_EXCEPT + .uflow_bound = V2 (0x1.a8b12fc6e4892p+4), +#endif +}; + +#define TinyBound 0x4000000000000000 /* 0x1p-511 << 1. */ +#define Off 0xfffffffffffff260 /* 0xffffffffffffffff - 3487. */ + +struct entry +{ + float64x2_t erfc; + float64x2_t scale; +}; + +static inline struct entry +lookup (uint64x2_t i) +{ + struct entry e; + float64x2_t e1 = vld1q_f64 ((float64_t *) (__erfc_data.tab - Off + i[0])), + e2 = vld1q_f64 ((float64_t *) (__erfc_data.tab - Off + i[1])); + e.erfc = vuzp1q_f64 (e1, e2); + e.scale = vuzp2q_f64 (e1, e2); + return e; +} + +#if WANT_SIMD_EXCEPT +static float64x2_t VPCS_ATTR NOINLINE +special_case (float64x2_t x, float64x2_t y, uint64x2_t cmp) +{ + return v_call_f64 (erfc, x, y, cmp); +} +#endif + +/* Optimized double-precision vector erfc(x). + Approximation based on series expansion near x rounded to + nearest multiple of 1/128. + + Let d = x - r, and scale = 2 / sqrt(pi) * exp(-r^2). For x near r, + + erfc(x) ~ erfc(r) - scale * d * poly(r, d), with + + poly(r, d) = 1 - r d + (2/3 r^2 - 1/3) d^2 - r (1/3 r^2 - 1/2) d^3 + + (2/15 r^4 - 2/5 r^2 + 1/10) d^4 + - r * (2/45 r^4 - 2/9 r^2 + 1/6) d^5 + + p6(r) d^6 + ... + p10(r) d^10 + + Polynomials p6(r) to p10(r) are computed using recurrence relation + + 2(i+1)p_i + 2r(i+2)p_{i+1} + (i+2)(i+3)p_{i+2} = 0, + with p0 = 1, and p1(r) = -r. + + Values of erfc(r) and scale are read from lookup tables. Stored values + are scaled to avoid hitting the subnormal range. + + Note that for x < 0, erfc(x) = 2.0 - erfc(-x). + + Maximum measured error: 1.71 ULP + V_NAME_D1 (erfc)(0x1.46cfe976733p+4) got 0x1.e15fcbea3e7afp-608 + want 0x1.e15fcbea3e7adp-608. */ +VPCS_ATTR +float64x2_t V_NAME_D1 (erfc) (float64x2_t x) +{ + const struct data *dat = ptr_barrier (&data); + +#if WANT_SIMD_EXCEPT + /* |x| < 2^-511. Avoid fabs by left-shifting by 1. */ + uint64x2_t ix = vreinterpretq_u64_f64 (x); + uint64x2_t cmp = vcltq_u64 (vaddq_u64 (ix, ix), v_u64 (TinyBound)); + /* x >= ~26.54 (into subnormal case and uflow case). Comparison is done in + integer domain to avoid raising exceptions in presence of nans. */ + uint64x2_t uflow = vcgeq_s64 (vreinterpretq_s64_f64 (x), + vreinterpretq_s64_f64 (dat->uflow_bound)); + cmp = vorrq_u64 (cmp, uflow); + float64x2_t xm = x; + /* If any lanes are special, mask them with 0 and retain a copy of x to allow + special case handler to fix special lanes later. This is only necessary if + fenv exceptions are to be triggered correctly. */ + if (__glibc_unlikely (v_any_u64 (cmp))) + x = v_zerofy_f64 (x, cmp); +#endif + + float64x2_t a = vabsq_f64 (x); + a = vminq_f64 (a, dat->max); + + /* Lookup erfc(r) and scale(r) in tables, e.g. set erfc(r) to 0 and scale to + 2/sqrt(pi), when x reduced to r = 0. */ + float64x2_t shift = dat->shift; + float64x2_t z = vaddq_f64 (a, shift); + + /* Clamp index to a range of 3487. A naive approach would use a subtract and + min. Instead we offset the table address and the index, then use a + saturating add. */ + uint64x2_t i = vqaddq_u64 (vreinterpretq_u64_f64 (z), dat->offset); + + struct entry e = lookup (i); + + /* erfc(x) ~ erfc(r) - scale * d * poly(r, d). */ + float64x2_t r = vsubq_f64 (z, shift); + float64x2_t d = vsubq_f64 (a, r); + float64x2_t d2 = vmulq_f64 (d, d); + float64x2_t r2 = vmulq_f64 (r, r); + + float64x2_t p1 = r; + float64x2_t p2 = vfmsq_f64 (dat->p20, r2, vaddq_f64 (dat->p20, dat->p20)); + float64x2_t p3 = vmulq_f64 (r, vfmaq_f64 (v_f64 (-0.5), r2, dat->p20)); + float64x2_t p4 = vfmaq_f64 (dat->p41, r2, dat->p42); + p4 = vfmsq_f64 (dat->p40, r2, p4); + float64x2_t p5 = vfmaq_f64 (dat->p51, r2, dat->p52); + p5 = vmulq_f64 (r, vfmaq_f64 (vmulq_f64 (v_f64 (0.5), dat->p20), r2, p5)); + /* Compute p_i using recurrence relation: + p_{i+2} = (p_i + r * Q_{i+1} * p_{i+1}) * R_{i+1}. */ + float64x2_t p6 = vfmaq_f64 (p4, p5, vmulq_laneq_f64 (r, dat->qr5, 0)); + p6 = vmulq_laneq_f64 (p6, dat->qr5, 1); + float64x2_t p7 = vfmaq_f64 (p5, p6, vmulq_laneq_f64 (r, dat->qr6, 0)); + p7 = vmulq_laneq_f64 (p7, dat->qr6, 1); + float64x2_t p8 = vfmaq_f64 (p6, p7, vmulq_laneq_f64 (r, dat->qr7, 0)); + p8 = vmulq_laneq_f64 (p8, dat->qr7, 1); + float64x2_t p9 = vfmaq_f64 (p7, p8, vmulq_laneq_f64 (r, dat->qr8, 0)); + p9 = vmulq_laneq_f64 (p9, dat->qr8, 1); + float64x2_t p10 = vfmaq_f64 (p8, p9, vmulq_laneq_f64 (r, dat->qr9, 0)); + p10 = vmulq_laneq_f64 (p10, dat->qr9, 1); + /* Compute polynomial in d using pairwise Horner scheme. */ + float64x2_t p90 = vfmaq_f64 (p9, d, p10); + float64x2_t p78 = vfmaq_f64 (p7, d, p8); + float64x2_t p56 = vfmaq_f64 (p5, d, p6); + float64x2_t p34 = vfmaq_f64 (p3, d, p4); + float64x2_t p12 = vfmaq_f64 (p1, d, p2); + float64x2_t y = vfmaq_f64 (p78, d2, p90); + y = vfmaq_f64 (p56, d2, y); + y = vfmaq_f64 (p34, d2, y); + y = vfmaq_f64 (p12, d2, y); + + y = vfmsq_f64 (e.erfc, e.scale, vfmsq_f64 (d, d2, y)); + + /* Offset equals 2.0 if sign, else 0.0. */ + uint64x2_t sign = vshrq_n_u64 (vreinterpretq_u64_f64 (x), 63); + float64x2_t off = vreinterpretq_f64_u64 (vshlq_n_u64 (sign, 62)); + /* Copy sign and scale back in a single fma. Since the bit patterns do not + overlap, then logical or and addition are equivalent here. */ + float64x2_t fac = vreinterpretq_f64_u64 ( + vsraq_n_u64 (vshlq_n_u64 (sign, 63), dat->table_scale, 1)); + +#if WANT_SIMD_EXCEPT + if (__glibc_unlikely (v_any_u64 (cmp))) + return special_case (xm, vfmaq_f64 (off, fac, y), cmp); +#endif + + return vfmaq_f64 (off, fac, y); +} |