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Diffstat (limited to 'pl/math/sv_log1p_inline.h')
| -rw-r--r-- | pl/math/sv_log1p_inline.h | 96 |
1 files changed, 96 insertions, 0 deletions
diff --git a/pl/math/sv_log1p_inline.h b/pl/math/sv_log1p_inline.h new file mode 100644 index 000000000000..983f8e1b0413 --- /dev/null +++ b/pl/math/sv_log1p_inline.h @@ -0,0 +1,96 @@ +/* + * Helper for SVE double-precision routines which calculate log(1 + x) and do + * not need special-case handling + * + * Copyright (c) 2022-2023, Arm Limited. + * SPDX-License-Identifier: MIT OR Apache-2.0 WITH LLVM-exception + */ +#ifndef PL_MATH_SV_LOG1P_INLINE_H +#define PL_MATH_SV_LOG1P_INLINE_H + +#include "sv_math.h" +#include "poly_sve_f64.h" + +static const struct sv_log1p_data +{ + double poly[19], ln2[2]; + uint64_t hf_rt2_top; + uint64_t one_m_hf_rt2_top; + uint32_t bottom_mask; + int64_t one_top; +} sv_log1p_data = { + /* Coefficients generated using Remez, deg=20, in [sqrt(2)/2-1, sqrt(2)-1]. + */ + .poly = { -0x1.ffffffffffffbp-2, 0x1.55555555551a9p-2, -0x1.00000000008e3p-2, + 0x1.9999999a32797p-3, -0x1.555555552fecfp-3, 0x1.249248e071e5ap-3, + -0x1.ffffff8bf8482p-4, 0x1.c71c8f07da57ap-4, -0x1.9999ca4ccb617p-4, + 0x1.7459ad2e1dfa3p-4, -0x1.554d2680a3ff2p-4, 0x1.3b4c54d487455p-4, + -0x1.2548a9ffe80e6p-4, 0x1.0f389a24b2e07p-4, -0x1.eee4db15db335p-5, + 0x1.e95b494d4a5ddp-5, -0x1.15fdf07cb7c73p-4, 0x1.0310b70800fcfp-4, + -0x1.cfa7385bdb37ep-6 }, + .ln2 = { 0x1.62e42fefa3800p-1, 0x1.ef35793c76730p-45 }, + .hf_rt2_top = 0x3fe6a09e00000000, + .one_m_hf_rt2_top = 0x00095f6200000000, + .bottom_mask = 0xffffffff, + .one_top = 0x3ff +}; + +static inline svfloat64_t +sv_log1p_inline (svfloat64_t x, const svbool_t pg) +{ + /* Helper for calculating log(x + 1). Adapted from v_log1p_inline.h, which + differs from v_log1p_2u5.c by: + - No special-case handling - this should be dealt with by the caller. + - Pairwise Horner polynomial evaluation for improved accuracy. + - Optionally simulate the shortcut for k=0, used in the scalar routine, + using svsel, for improved accuracy when the argument to log1p is close + to 0. This feature is enabled by defining WANT_SV_LOG1P_K0_SHORTCUT as 1 + in the source of the caller before including this file. + See sv_log1p_2u1.c for details of the algorithm. */ + const struct sv_log1p_data *d = ptr_barrier (&sv_log1p_data); + svfloat64_t m = svadd_x (pg, x, 1); + svuint64_t mi = svreinterpret_u64 (m); + svuint64_t u = svadd_x (pg, mi, d->one_m_hf_rt2_top); + + svint64_t ki + = svsub_x (pg, svreinterpret_s64 (svlsr_x (pg, u, 52)), d->one_top); + svfloat64_t k = svcvt_f64_x (pg, ki); + + /* Reduce x to f in [sqrt(2)/2, sqrt(2)]. */ + svuint64_t utop + = svadd_x (pg, svand_x (pg, u, 0x000fffff00000000), d->hf_rt2_top); + svuint64_t u_red = svorr_x (pg, utop, svand_x (pg, mi, d->bottom_mask)); + svfloat64_t f = svsub_x (pg, svreinterpret_f64 (u_red), 1); + + /* Correction term c/m. */ + svfloat64_t c = svsub_x (pg, x, svsub_x (pg, m, 1)); + svfloat64_t cm; + +#ifndef WANT_SV_LOG1P_K0_SHORTCUT +#error \ + "Cannot use sv_log1p_inline.h without specifying whether you need the k0 shortcut for greater accuracy close to 0" +#elif WANT_SV_LOG1P_K0_SHORTCUT + /* Shortcut if k is 0 - set correction term to 0 and f to x. The result is + that the approximation is solely the polynomial. */ + svbool_t knot0 = svcmpne (pg, k, 0); + cm = svdiv_z (knot0, c, m); + if (likely (!svptest_any (pg, knot0))) + { + f = svsel (knot0, f, x); + } +#else + /* No shortcut. */ + cm = svdiv_x (pg, c, m); +#endif + + /* Approximate log1p(f) on the reduced input using a polynomial. */ + svfloat64_t f2 = svmul_x (pg, f, f); + svfloat64_t p = sv_pw_horner_18_f64_x (pg, f, f2, d->poly); + + /* Assemble log1p(x) = k * log2 + log1p(f) + c/m. */ + svfloat64_t ylo = svmla_x (pg, cm, k, d->ln2[0]); + svfloat64_t yhi = svmla_x (pg, f, k, d->ln2[1]); + + return svmla_x (pg, svadd_x (pg, ylo, yhi), f2, p); +} +#endif // PL_MATH_SV_LOG1P_INLINE_H |