Marat Dukhan | 5739f70 | 2019-12-22 19:45:09 -0800 | [diff] [blame] | 1 | // Copyright 2019 Google LLC |
| 2 | // |
| 3 | // This source code is licensed under the BSD-style license found in the |
| 4 | // LICENSE file in the root directory of this source tree. |
| 5 | |
| 6 | #include <assert.h> |
| 7 | #include <stddef.h> |
| 8 | |
| 9 | #include <math.h> |
| 10 | |
| 11 | #include <xnnpack/common.h> |
| 12 | #include <xnnpack/math-stubs.h> |
| 13 | |
| 14 | #include <fp16/bitcasts.h> |
| 15 | |
| 16 | |
| 17 | // Table of exp2(k / 64) values, k = 0..63 |
| 18 | static const uint32_t exp2_k_over_64_table[64] = { |
| 19 | 0x3F800000, 0x3F8164D2, 0x3F82CD87, 0x3F843A29, |
| 20 | 0x3F85AAC3, 0x3F871F62, 0x3F88980F, 0x3F8A14D5, |
| 21 | 0x3F8B95C2, 0x3F8D1ADF, 0x3F8EA43A, 0x3F9031DC, |
| 22 | 0x3F91C3D3, 0x3F935A2B, 0x3F94F4F0, 0x3F96942D, |
| 23 | 0x3F9837F0, 0x3F99E046, 0x3F9B8D3A, 0x3F9D3EDA, |
| 24 | 0x3F9EF532, 0x3FA0B051, 0x3FA27043, 0x3FA43516, |
| 25 | 0x3FA5FED7, 0x3FA7CD94, 0x3FA9A15B, 0x3FAB7A3A, |
| 26 | 0x3FAD583F, 0x3FAF3B79, 0x3FB123F6, 0x3FB311C4, |
| 27 | 0x3FB504F3, 0x3FB6FD92, 0x3FB8FBAF, 0x3FBAFF5B, |
| 28 | 0x3FBD08A4, 0x3FBF179A, 0x3FC12C4D, 0x3FC346CD, |
| 29 | 0x3FC5672A, 0x3FC78D75, 0x3FC9B9BE, 0x3FCBEC15, |
| 30 | 0x3FCE248C, 0x3FD06334, 0x3FD2A81E, 0x3FD4F35B, |
| 31 | 0x3FD744FD, 0x3FD99D16, 0x3FDBFBB8, 0x3FDE60F5, |
| 32 | 0x3FE0CCDF, 0x3FE33F89, 0x3FE5B907, 0x3FE8396A, |
| 33 | 0x3FEAC0C7, 0x3FED4F30, 0x3FEFE4BA, 0x3FF28177, |
| 34 | 0x3FF5257D, 0x3FF7D0DF, 0x3FFA83B3, 0x3FFD3E0C, |
| 35 | }; |
| 36 | |
| 37 | void xnn_math_f32_sigmoid__scalar_lut64_p2_div( |
| 38 | size_t n, |
| 39 | const float* input, |
| 40 | float* output) |
| 41 | { |
| 42 | assert(n % sizeof(float) == 0); |
| 43 | |
| 44 | const float vmagic_bias = 0x1.800000p23f; |
Marat Dukhan | 8d3c07e | 2020-01-02 01:20:59 -0800 | [diff] [blame] | 45 | // The largest z for which sigmoidf(-z) is normalized. |
| 46 | // This number is also the largest z for which expf(-z) is normalized. |
| 47 | const float vdenorm_cutoff = 0x1.5D589Ep+6f; |
Marat Dukhan | 5739f70 | 2019-12-22 19:45:09 -0800 | [diff] [blame] | 48 | const float vminus_log2e_x64 = -0x1.715476p6f; |
| 49 | // Last 13 bits are zeroes |
| 50 | const float vln2_o64_hi = 0x1.630000p-7f; |
| 51 | const float vln2_o64_lo = -0x1.BD0106p-19f; |
| 52 | const float vone = 1.0f; |
| 53 | |
| 54 | const float vc2 = 0x1.FFFF0Ap-2f; |
| 55 | |
| 56 | const uint32_t vindex_mask = UINT32_C(0x3F); |
| 57 | |
| 58 | for (; n != 0; n -= sizeof(float)) { |
| 59 | const float vx = *input++; |
| 60 | |
| 61 | // General structure of the algorithm: |
| 62 | // / exp(x) / (1 + exp(x)) if x <= 0 |
| 63 | // f[x] := |
| 64 | // \ 1 - f[-x] if x >= 0 |
| 65 | // |
| 66 | // First we compute f[-z] := exp(-z) / (1 + exp(-z)) where z = abs(x), |
| 67 | // then replace result with 1 - f[-z] if x >= 0. |
| 68 | const float vz = fabsf(vx); |
| 69 | |
| 70 | // Compute reduced argument n := round(-z * 64 / log(2)). |
| 71 | // We do it by adding a large number (magic bias), which cause rounding of the result to an integer, then subtracing |
| 72 | // the large number back. The first addition is combined with multiplication by log2e into a single FMA instruction. |
| 73 | // The trick with adding large number is valid only within certain bounds (|z * 2048 / log(2)| <= 2**22, i.e. |
| 74 | // |z| <= 0x1.62E43p+15 = 45426.09375), but that is acceptable, because inputs x outside of |
| 75 | // [-87.336544, 17.328678] (i.e. z outsize [0, 87.336544]) underflow or saturate sigmoidf(x). We fixup the result |
| 76 | // for such inputs at the very end of the algorithm. |
| 77 | float vn = vz * vminus_log2e_x64 + vmagic_bias; |
| 78 | |
| 79 | // Create a floating-point number s (scale) such that s := 2**(n / 64) for such inputs that sigmoidf(-z) is |
| 80 | // normalized, i.e. 0 <= z <= 87.33642. As n has 6 fractional bits, we split s == 2**(n / 64) = |
| 81 | // = 2**e * 2**(n / 64 - e), where e := int(n / 64). We create s in two steps: |
| 82 | // 1. Fetch 2**(n / 64 - e) = 2**(n % 64) from exp2_k_over_64_table using the 6 low bits of n, as integer. Note |
| 83 | // that the fetched values are in the [1.0, 2.0) range, i.e. their floating-point exponent is 0. |
| 84 | // 2. Adjust fecthed value by addition of e to its floating-point exponent. The result is always a normalized |
| 85 | // number, because for 0 <= z <= 87.33642 (inputs for which sigmoidf(-z) is normalized) we have -126 <= e <= 0, |
| 86 | // and thus the adjusted exponent is not lower than -126. |
| 87 | // |
| 88 | // Extract e from bits 6:14 of n and shift it into bits 23:31 (position of floating-point exponent). |
| 89 | const uint32_t ve = (fp32_to_bits(vn) & ~vindex_mask) << 17; |
| 90 | |
| 91 | // Use bits 0:6 bits of n, as integer, as an index for table lookup of l := 2**(n % 64). |
| 92 | const uint32_t vidx = fp32_to_bits(vn) & vindex_mask; |
| 93 | // Adjust exponent of the value l fetched from the exp2_k_over_64_table to get the final s value. |
| 94 | const float vs = fp32_from_bits(exp2_k_over_64_table[vidx] + ve); |
| 95 | |
| 96 | // Subtract the large number back to get the final n := round(-z * 64 / log(2)) as a floating-point number. |
| 97 | vn -= vmagic_bias; |
| 98 | |
| 99 | // Compute reduced argument t := (z + n * log(2) / 64). Note that -t = -z - n * log(2) / 64. |
| 100 | // Use Cody-Waite range reduction method (note two constants to represent log(2) / 64) to improve accuracy. |
| 101 | float vt = vn * vln2_o64_hi + vz; |
| 102 | vt = vn * vln2_o64_lo + vt; |
| 103 | |
| 104 | // Compute degree-2 polynomial approxiatmion for exp(-t) on [-log(2)/128, log(2)/128]. |
| 105 | // P1(t) = 1 + t * (-1 + t * c2) |
| 106 | float vp = vt * vc2; |
| 107 | vp = vt - vp * vt; |
| 108 | |
| 109 | // Reconstruct the final f value: |
| 110 | // f = s * (1 + t * (-1 + t * c2)) |
| 111 | // = s * (1 - t + t * (t * c2)) |
| 112 | // = s - s * (t - t * (t * c2)) |
| 113 | // = s - s * p |
| 114 | const float vy = vs - vs * vp; |
| 115 | |
| 116 | // Reconstruct sigmoid(-z) = exp(-z) / (1.0 + exp(-z)) |
| 117 | float vf = vy / (vy + vone); |
| 118 | |
Marat Dukhan | 8d3c07e | 2020-01-02 01:20:59 -0800 | [diff] [blame] | 119 | // For inputs below denormal cutoff, replace output with +0.0f. |
| 120 | // Note that for NaN inputs, comparison result is false, and outputs are left unchanged. |
| 121 | if XNN_UNPREDICTABLE(vz > vdenorm_cutoff) { |
| 122 | vf = 0.0f; |
| 123 | } |
| 124 | |
Marat Dukhan | 5739f70 | 2019-12-22 19:45:09 -0800 | [diff] [blame] | 125 | // Reconstruct sigmoid(x) = x < 0 ? sigmoid(-z) : 1.0 - sigmoid(-z) |
| 126 | if XNN_UNPREDICTABLE(vx > 0.0f) { |
| 127 | vf = vone - vf; |
| 128 | } |
| 129 | |
Marat Dukhan | 5739f70 | 2019-12-22 19:45:09 -0800 | [diff] [blame] | 130 | *output++ = vf; |
| 131 | } |
| 132 | } |