1 // Copyright (c) the JPEG XL Project Authors. All rights reserved.
3 // Use of this source code is governed by a BSD-style
4 // license that can be found in the LICENSE file.
6 // Fast SIMD math ops (log2, encoder only, cos, erf for splines)
8 #if defined(LIB_JXL_FAST_MATH_INL_H_) == defined(HWY_TARGET_TOGGLE)
9 #ifdef LIB_JXL_FAST_MATH_INL_H_
10 #undef LIB_JXL_FAST_MATH_INL_H_
12 #define LIB_JXL_FAST_MATH_INL_H_
15 #include <hwy/highway.h>
17 #include "lib/jxl/common.h"
18 #include "lib/jxl/rational_polynomial-inl.h"
19 HWY_BEFORE_NAMESPACE();
21 namespace HWY_NAMESPACE {
23 // These templates are not found via ADL.
24 using hwy::HWY_NAMESPACE::Abs;
25 using hwy::HWY_NAMESPACE::Add;
26 using hwy::HWY_NAMESPACE::Eq;
27 using hwy::HWY_NAMESPACE::Floor;
28 using hwy::HWY_NAMESPACE::Ge;
29 using hwy::HWY_NAMESPACE::GetLane;
30 using hwy::HWY_NAMESPACE::IfThenElse;
31 using hwy::HWY_NAMESPACE::IfThenZeroElse;
32 using hwy::HWY_NAMESPACE::Le;
33 using hwy::HWY_NAMESPACE::Min;
34 using hwy::HWY_NAMESPACE::Mul;
35 using hwy::HWY_NAMESPACE::MulAdd;
36 using hwy::HWY_NAMESPACE::NegMulAdd;
37 using hwy::HWY_NAMESPACE::Rebind;
38 using hwy::HWY_NAMESPACE::ShiftLeft;
39 using hwy::HWY_NAMESPACE::ShiftRight;
40 using hwy::HWY_NAMESPACE::Sub;
41 using hwy::HWY_NAMESPACE::Xor;
43 // Computes base-2 logarithm like std::log2. Undefined if negative / NaN.
45 template <class DF, class V>
46 V FastLog2f(const DF df, V x) {
47 // 2,2 rational polynomial approximation of std::log1p(x) / std::log(2).
48 HWY_ALIGN const float p[4 * (2 + 1)] = {HWY_REP4(-1.8503833400518310E-06f),
49 HWY_REP4(1.4287160470083755E+00f),
50 HWY_REP4(7.4245873327820566E-01f)};
51 HWY_ALIGN const float q[4 * (2 + 1)] = {HWY_REP4(9.9032814277590719E-01f),
52 HWY_REP4(1.0096718572241148E+00f),
53 HWY_REP4(1.7409343003366853E-01f)};
55 const Rebind<int32_t, DF> di;
56 const auto x_bits = BitCast(di, x);
58 // Range reduction to [-1/3, 1/3] - 3 integer, 2 float ops
59 const auto exp_bits = Sub(x_bits, Set(di, 0x3f2aaaab)); // = 2/3
60 // Shifted exponent = log2; also used to clear mantissa.
61 const auto exp_shifted = ShiftRight<23>(exp_bits);
62 const auto mantissa = BitCast(df, Sub(x_bits, ShiftLeft<23>(exp_shifted)));
63 const auto exp_val = ConvertTo(df, exp_shifted);
64 return Add(EvalRationalPolynomial(df, Sub(mantissa, Set(df, 1.0f)), p, q),
68 // max relative error ~3e-7
69 template <class DF, class V>
70 V FastPow2f(const DF df, V x) {
71 const Rebind<int32_t, DF> di;
72 auto floorx = Floor(x);
74 BitCast(df, ShiftLeft<23>(Add(ConvertTo(di, floorx), Set(di, 127))));
75 auto frac = Sub(x, floorx);
76 auto num = Add(frac, Set(df, 1.01749063e+01));
77 num = MulAdd(num, frac, Set(df, 4.88687798e+01));
78 num = MulAdd(num, frac, Set(df, 9.85506591e+01));
80 auto den = MulAdd(frac, Set(df, 2.10242958e-01), Set(df, -2.22328856e-02));
81 den = MulAdd(den, frac, Set(df, -1.94414990e+01));
82 den = MulAdd(den, frac, Set(df, 9.85506633e+01));
86 // max relative error ~3e-5
87 template <class DF, class V>
88 V FastPowf(const DF df, V base, V exponent) {
89 return FastPow2f(df, Mul(FastLog2f(df, base), exponent));
92 // Computes cosine like std::cos.
94 template <class DF, class V>
95 V FastCosf(const DF df, V x) {
96 // Step 1: range reduction to [0, 2pi)
97 const auto pi2 = Set(df, kPi * 2.0f);
98 const auto pi2_inv = Set(df, 0.5f / kPi);
99 const auto npi2 = Mul(Floor(Mul(x, pi2_inv)), pi2);
100 const auto xmodpi2 = Sub(x, npi2);
101 // Step 2: range reduction to [0, pi]
102 const auto x_pi = Min(xmodpi2, Sub(pi2, xmodpi2));
103 // Step 3: range reduction to [0, pi/2]
104 const auto above_pihalf = Ge(x_pi, Set(df, kPi / 2.0f));
105 const auto x_pihalf = IfThenElse(above_pihalf, Sub(Set(df, kPi), x_pi), x_pi);
106 // Step 4: Taylor-like approximation, scaled by 2**0.75 to make angle
107 // duplication steps faster, on x/4.
108 const auto xs = Mul(x_pihalf, Set(df, 0.25f));
109 const auto x2 = Mul(xs, xs);
110 const auto x4 = Mul(x2, x2);
111 const auto cosx_prescaling =
112 MulAdd(x4, Set(df, 0.06960438),
113 MulAdd(x2, Set(df, -0.84087373), Set(df, 1.68179268)));
114 // Step 5: angle duplication.
115 const auto cosx_scale1 =
116 MulAdd(cosx_prescaling, cosx_prescaling, Set(df, -1.414213562));
117 const auto cosx_scale2 = MulAdd(cosx_scale1, cosx_scale1, Set(df, -1));
118 // Step 6: change sign if needed.
119 const Rebind<uint32_t, DF> du;
120 auto signbit = ShiftLeft<31>(BitCast(du, VecFromMask(df, above_pihalf)));
121 return BitCast(df, Xor(signbit, BitCast(du, cosx_scale2)));
124 // Computes the error function like std::erf.
126 template <class DF, class V>
127 V FastErff(const DF df, V x) {
129 // https://en.wikipedia.org/wiki/Error_function#Numerical_approximations
130 // but constants have been recomputed.
131 const auto xle0 = Le(x, Zero(df));
132 const auto absx = Abs(x);
133 // Compute 1 - 1 / ((((x * a + b) * x + c) * x + d) * x + 1)**4
135 MulAdd(absx, Set(df, 7.77394369e-02), Set(df, 2.05260015e-04));
136 const auto denom2 = MulAdd(denom1, absx, Set(df, 2.32120216e-01));
137 const auto denom3 = MulAdd(denom2, absx, Set(df, 2.77820801e-01));
138 const auto denom4 = MulAdd(denom3, absx, Set(df, 1.0f));
139 const auto denom5 = Mul(denom4, denom4);
140 const auto inv_denom5 = Div(Set(df, 1.0f), denom5);
141 const auto result = NegMulAdd(inv_denom5, inv_denom5, Set(df, 1.0f));
142 // Change sign if needed.
143 const Rebind<uint32_t, DF> du;
144 auto signbit = ShiftLeft<31>(BitCast(du, VecFromMask(df, xle0)));
145 return BitCast(df, Xor(signbit, BitCast(du, result)));
148 inline float FastLog2f(float f) {
149 HWY_CAPPED(float, 1) D;
150 return GetLane(FastLog2f(D, Set(D, f)));
153 inline float FastPow2f(float f) {
154 HWY_CAPPED(float, 1) D;
155 return GetLane(FastPow2f(D, Set(D, f)));
158 inline float FastPowf(float b, float e) {
159 HWY_CAPPED(float, 1) D;
160 return GetLane(FastPowf(D, Set(D, b), Set(D, e)));
163 inline float FastCosf(float f) {
164 HWY_CAPPED(float, 1) D;
165 return GetLane(FastCosf(D, Set(D, f)));
168 inline float FastErff(float f) {
169 HWY_CAPPED(float, 1) D;
170 return GetLane(FastErff(D, Set(D, f)));
173 // Returns cbrt(x) + add with 6 ulp max error.
174 // Modified from vectormath_exp.h, Apache 2 license.
175 // https://www.agner.org/optimize/vectorclass.zip
177 V CubeRootAndAdd(const V x, const V add) {
178 const HWY_FULL(float) df;
179 const HWY_FULL(int32_t) di;
181 const auto kExpBias = Set(di, 0x54800000); // cast(1.) + cast(1.) / 3
182 const auto kExpMul = Set(di, 0x002AAAAA); // shifted 1/3
183 const auto k1_3 = Set(df, 1.0f / 3);
184 const auto k4_3 = Set(df, 4.0f / 3);
186 const auto xa = x; // assume inputs never negative
187 const auto xa_3 = Mul(k1_3, xa);
189 // Multiply exponent by -1/3
190 const auto m1 = BitCast(di, xa);
191 // Special case for 0. 0 is represented with an exponent of 0, so the
192 // "kExpBias - 1/3 * exp" below gives the wrong result. The IfThenZeroElse()
193 // sets those values as 0, which prevents having NaNs in the computations
195 // TODO(eustas): use fused op
196 const auto m2 = IfThenZeroElse(
197 Eq(m1, Zero(di)), Sub(kExpBias, Mul((ShiftRight<23>(m1)), kExpMul)));
198 auto r = BitCast(df, m2);
200 // Newton-Raphson iterations
201 for (int i = 0; i < 3; i++) {
202 const auto r2 = Mul(r, r);
203 r = NegMulAdd(xa_3, Mul(r2, r2), Mul(k4_3, r));
207 r = MulAdd(k1_3, NegMulAdd(xa, Mul(r2, r2), r), r);
209 r = MulAdd(r2, x, add);
214 // NOLINTNEXTLINE(google-readability-namespace-comments)
215 } // namespace HWY_NAMESPACE
217 HWY_AFTER_NAMESPACE();
219 #endif // LIB_JXL_FAST_MATH_INL_H_
222 #ifndef FAST_MATH_ONCE
223 #define FAST_MATH_ONCE
226 inline float FastLog2f(float f) { return HWY_STATIC_DISPATCH(FastLog2f)(f); }
227 inline float FastPow2f(float f) { return HWY_STATIC_DISPATCH(FastPow2f)(f); }
228 inline float FastPowf(float b, float e) {
229 return HWY_STATIC_DISPATCH(FastPowf)(b, e);
231 inline float FastCosf(float f) { return HWY_STATIC_DISPATCH(FastCosf)(f); }
232 inline float FastErff(float f) { return HWY_STATIC_DISPATCH(FastErff)(f); }
235 #endif // FAST_MATH_ONCE