1 /* 2 * Helper for single-precision routines which calculate log(1 + x) and do not 3 * need special-case handling 4 * 5 * Copyright (c) 2022-2023, Arm Limited. 6 * SPDX-License-Identifier: MIT OR Apache-2.0 WITH LLVM-exception 7 */ 8 9 #ifndef PL_MATH_V_LOG1PF_INLINE_H 10 #define PL_MATH_V_LOG1PF_INLINE_H 11 12 #include "v_math.h" 13 #include "poly_advsimd_f32.h" 14 15 struct v_log1pf_data 16 { 17 float32x4_t poly[8], ln2; 18 uint32x4_t four; 19 int32x4_t three_quarters; 20 }; 21 22 /* Polynomial generated using FPMinimax in [-0.25, 0.5]. First two coefficients 23 (1, -0.5) are not stored as they can be generated more efficiently. */ 24 #define V_LOG1PF_CONSTANTS_TABLE \ 25 { \ 26 .poly \ 27 = { V4 (0x1.5555aap-2f), V4 (-0x1.000038p-2f), V4 (0x1.99675cp-3f), \ 28 V4 (-0x1.54ef78p-3f), V4 (0x1.28a1f4p-3f), V4 (-0x1.0da91p-3f), \ 29 V4 (0x1.abcb6p-4f), V4 (-0x1.6f0d5ep-5f) }, \ 30 .ln2 = V4 (0x1.62e43p-1f), .four = V4 (0x40800000), \ 31 .three_quarters = V4 (0x3f400000) \ 32 } 33 34 static inline float32x4_t 35 eval_poly (float32x4_t m, const float32x4_t *c) 36 { 37 /* Approximate log(1+m) on [-0.25, 0.5] using pairwise Horner (main routine 38 uses split Estrin, but this way reduces register pressure in the calling 39 routine). */ 40 float32x4_t q = vfmaq_f32 (v_f32 (-0.5), m, c[0]); 41 float32x4_t m2 = vmulq_f32 (m, m); 42 q = vfmaq_f32 (m, m2, q); 43 float32x4_t p = v_pw_horner_6_f32 (m, m2, c + 1); 44 p = vmulq_f32 (m2, p); 45 return vfmaq_f32 (q, m2, p); 46 } 47 48 static inline float32x4_t 49 log1pf_inline (float32x4_t x, const struct v_log1pf_data d) 50 { 51 /* Helper for calculating log(x + 1). Copied from log1pf_2u1.c, with no 52 special-case handling. See that file for details of the algorithm. */ 53 float32x4_t m = vaddq_f32 (x, v_f32 (1.0f)); 54 int32x4_t k 55 = vandq_s32 (vsubq_s32 (vreinterpretq_s32_f32 (m), d.three_quarters), 56 v_s32 (0xff800000)); 57 uint32x4_t ku = vreinterpretq_u32_s32 (k); 58 float32x4_t s = vreinterpretq_f32_u32 (vsubq_u32 (d.four, ku)); 59 float32x4_t m_scale 60 = vreinterpretq_f32_u32 (vsubq_u32 (vreinterpretq_u32_f32 (x), ku)); 61 m_scale = vaddq_f32 (m_scale, vfmaq_f32 (v_f32 (-1.0f), v_f32 (0.25f), s)); 62 float32x4_t p = eval_poly (m_scale, d.poly); 63 float32x4_t scale_back = vmulq_f32 (vcvtq_f32_s32 (k), v_f32 (0x1.0p-23f)); 64 return vfmaq_f32 (p, scale_back, d.ln2); 65 } 66 67 #endif // PL_MATH_V_LOG1PF_INLINE_H 68