diff options
Diffstat (limited to 'sysdeps/aarch64/fpu/expm1f_advsimd.c')
| -rw-r--r-- | sysdeps/aarch64/fpu/expm1f_advsimd.c | 117 |
1 files changed, 117 insertions, 0 deletions
diff --git a/sysdeps/aarch64/fpu/expm1f_advsimd.c b/sysdeps/aarch64/fpu/expm1f_advsimd.c new file mode 100644 index 0000000000..b27b75068a --- /dev/null +++ b/sysdeps/aarch64/fpu/expm1f_advsimd.c @@ -0,0 +1,117 @@ +/* Single-precision AdvSIMD expm1 + + Copyright (C) 2023 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 "poly_advsimd_f32.h" + +static const struct data +{ + float32x4_t poly[5]; + float32x4_t invln2, ln2_lo, ln2_hi, shift; + int32x4_t exponent_bias; +#if WANT_SIMD_EXCEPT + uint32x4_t thresh; +#else + float32x4_t oflow_bound; +#endif +} data = { + /* Generated using fpminimax with degree=5 in [-log(2)/2, log(2)/2]. */ + .poly = { V4 (0x1.fffffep-2), V4 (0x1.5554aep-3), V4 (0x1.555736p-5), + V4 (0x1.12287cp-7), V4 (0x1.6b55a2p-10) }, + .invln2 = V4 (0x1.715476p+0f), + .ln2_hi = V4 (0x1.62e4p-1f), + .ln2_lo = V4 (0x1.7f7d1cp-20f), + .shift = V4 (0x1.8p23f), + .exponent_bias = V4 (0x3f800000), +#if !WANT_SIMD_EXCEPT + /* Value above which expm1f(x) should overflow. Absolute value of the + underflow bound is greater than this, so it catches both cases - there is + a small window where fallbacks are triggered unnecessarily. */ + .oflow_bound = V4 (0x1.5ebc4p+6), +#else + /* asuint(oflow_bound) - asuint(0x1p-23), shifted left by 1 for absolute + compare. */ + .thresh = V4 (0x1d5ebc40), +#endif +}; + +/* asuint(0x1p-23), shifted by 1 for abs compare. */ +#define TinyBound v_u32 (0x34000000 << 1) + +static float32x4_t VPCS_ATTR NOINLINE +special_case (float32x4_t x, float32x4_t y, uint32x4_t special) +{ + return v_call_f32 (expm1f, x, y, special); +} + +/* Single-precision vector exp(x) - 1 function. + The maximum error is 1.51 ULP: + _ZGVnN4v_expm1f (0x1.8baa96p-2) got 0x1.e2fb9p-2 + want 0x1.e2fb94p-2. */ +float32x4_t VPCS_ATTR V_NAME_F1 (expm1) (float32x4_t x) +{ + const struct data *d = ptr_barrier (&data); + uint32x4_t ix = vreinterpretq_u32_f32 (x); + +#if WANT_SIMD_EXCEPT + /* If fp exceptions are to be triggered correctly, fall back to scalar for + |x| < 2^-23, |x| > oflow_bound, Inf & NaN. Add ix to itself for + shift-left by 1, and compare with thresh which was left-shifted offline - + this is effectively an absolute compare. */ + uint32x4_t special + = vcgeq_u32 (vsubq_u32 (vaddq_u32 (ix, ix), TinyBound), d->thresh); + if (__glibc_unlikely (v_any_u32 (special))) + x = v_zerofy_f32 (x, special); +#else + /* Handles very large values (+ve and -ve), +/-NaN, +/-Inf. */ + uint32x4_t special = vceqzq_u32 (vcaltq_f32 (x, d->oflow_bound)); +#endif + + /* Reduce argument to smaller range: + Let i = round(x / ln2) + and f = x - i * ln2, then f is in [-ln2/2, ln2/2]. + exp(x) - 1 = 2^i * (expm1(f) + 1) - 1 + where 2^i is exact because i is an integer. */ + float32x4_t j = vsubq_f32 (vfmaq_f32 (d->shift, d->invln2, x), d->shift); + int32x4_t i = vcvtq_s32_f32 (j); + float32x4_t f = vfmsq_f32 (x, j, d->ln2_hi); + f = vfmsq_f32 (f, j, d->ln2_lo); + + /* Approximate expm1(f) using polynomial. + Taylor expansion for expm1(x) has the form: + x + ax^2 + bx^3 + cx^4 .... + So we calculate the polynomial P(f) = a + bf + cf^2 + ... + and assemble the approximation expm1(f) ~= f + f^2 * P(f). */ + float32x4_t p = v_horner_4_f32 (f, d->poly); + p = vfmaq_f32 (f, vmulq_f32 (f, f), p); + + /* Assemble the result. + expm1(x) ~= 2^i * (p + 1) - 1 + Let t = 2^i. */ + int32x4_t u = vaddq_s32 (vshlq_n_s32 (i, 23), d->exponent_bias); + float32x4_t t = vreinterpretq_f32_s32 (u); + + if (__glibc_unlikely (v_any_u32 (special))) + return special_case (vreinterpretq_f32_u32 (ix), + vfmaq_f32 (vsubq_f32 (t, v_f32 (1.0f)), p, t), + special); + + /* expm1(x) ~= p * t + (t - 1). */ + return vfmaq_f32 (vsubq_f32 (t, v_f32 (1.0f)), p, t); +} |