1 //===-- Single-precision general exp/log functions ------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8
9 #ifndef LLVM_LIBC_SRC_MATH_GENERIC_EXPLOGXF_H
10 #define LLVM_LIBC_SRC_MATH_GENERIC_EXPLOGXF_H
11
12 #include "common_constants.h"
13 #include "src/__support/CPP/bit.h"
14 #include "src/__support/CPP/optional.h"
15 #include "src/__support/FPUtil/FEnvImpl.h"
16 #include "src/__support/FPUtil/FPBits.h"
17 #include "src/__support/FPUtil/PolyEval.h"
18 #include "src/__support/FPUtil/nearest_integer.h"
19 #include "src/__support/common.h"
20 #include "src/__support/macros/config.h"
21 #include "src/__support/macros/properties/cpu_features.h"
22
23 namespace LIBC_NAMESPACE_DECL {
24
25 struct ExpBase {
26 // Base = e
27 static constexpr int MID_BITS = 5;
28 static constexpr int MID_MASK = (1 << MID_BITS) - 1;
29 // log2(e) * 2^5
30 static constexpr double LOG2_B = 0x1.71547652b82fep+0 * (1 << MID_BITS);
31 // High and low parts of -log(2) * 2^(-5)
32 static constexpr double M_LOGB_2_HI = -0x1.62e42fefa0000p-1 / (1 << MID_BITS);
33 static constexpr double M_LOGB_2_LO =
34 -0x1.cf79abc9e3b3ap-40 / (1 << MID_BITS);
35 // Look up table for bit fields of 2^(i/32) for i = 0..31, generated by Sollya
36 // with:
37 // > for i from 0 to 31 do printdouble(round(2^(i/32), D, RN));
38 static constexpr int64_t EXP_2_MID[1 << MID_BITS] = {
39 0x3ff0000000000000, 0x3ff059b0d3158574, 0x3ff0b5586cf9890f,
40 0x3ff11301d0125b51, 0x3ff172b83c7d517b, 0x3ff1d4873168b9aa,
41 0x3ff2387a6e756238, 0x3ff29e9df51fdee1, 0x3ff306fe0a31b715,
42 0x3ff371a7373aa9cb, 0x3ff3dea64c123422, 0x3ff44e086061892d,
43 0x3ff4bfdad5362a27, 0x3ff5342b569d4f82, 0x3ff5ab07dd485429,
44 0x3ff6247eb03a5585, 0x3ff6a09e667f3bcd, 0x3ff71f75e8ec5f74,
45 0x3ff7a11473eb0187, 0x3ff82589994cce13, 0x3ff8ace5422aa0db,
46 0x3ff93737b0cdc5e5, 0x3ff9c49182a3f090, 0x3ffa5503b23e255d,
47 0x3ffae89f995ad3ad, 0x3ffb7f76f2fb5e47, 0x3ffc199bdd85529c,
48 0x3ffcb720dcef9069, 0x3ffd5818dcfba487, 0x3ffdfc97337b9b5f,
49 0x3ffea4afa2a490da, 0x3fff50765b6e4540,
50 };
51
52 // Approximating e^dx with degree-5 minimax polynomial generated by Sollya:
53 // > Q = fpminimax(expm1(x)/x, 4, [|1, D...|], [-log(2)/64, log(2)/64]);
54 // Then:
55 // e^dx ~ P(dx) = 1 + dx + COEFFS[0] * dx^2 + ... + COEFFS[3] * dx^5.
56 static constexpr double COEFFS[4] = {
57 0x1.ffffffffe5bc8p-2, 0x1.555555555cd67p-3, 0x1.5555c2a9b48b4p-5,
58 0x1.11112a0e34bdbp-7};
59
powb_loExpBase60 LIBC_INLINE static double powb_lo(double dx) {
61 using fputil::multiply_add;
62 double dx2 = dx * dx;
63 double c0 = 1.0 + dx;
64 // c1 = COEFFS[0] + COEFFS[1] * dx
65 double c1 = multiply_add(dx, ExpBase::COEFFS[1], ExpBase::COEFFS[0]);
66 // c2 = COEFFS[2] + COEFFS[3] * dx
67 double c2 = multiply_add(dx, ExpBase::COEFFS[3], ExpBase::COEFFS[2]);
68 // r = c4 + c5 * dx^4
69 // = 1 + dx + COEFFS[0] * dx^2 + ... + COEFFS[5] * dx^7
70 return fputil::polyeval(dx2, c0, c1, c2);
71 }
72 };
73
74 struct Exp10Base : public ExpBase {
75 // log2(10) * 2^5
76 static constexpr double LOG2_B = 0x1.a934f0979a371p1 * (1 << MID_BITS);
77 // High and low parts of -log10(2) * 2^(-5).
78 // Notice that since |x * log2(10)| < 150:
79 // |k| = |round(x * log2(10) * 2^5)| < 2^8 * 2^5 = 2^13
80 // So when the FMA instructions are not available, in order for the product
81 // k * M_LOGB_2_HI
82 // to be exact, we only store the high part of log10(2) up to 38 bits
83 // (= 53 - 15) of precision.
84 // It is generated by Sollya with:
85 // > round(log10(2), 44, RN);
86 static constexpr double M_LOGB_2_HI = -0x1.34413509f8p-2 / (1 << MID_BITS);
87 // > round(log10(2) - 0x1.34413509f8p-2, D, RN);
88 static constexpr double M_LOGB_2_LO = 0x1.80433b83b532ap-44 / (1 << MID_BITS);
89
90 // Approximating 10^dx with degree-5 minimax polynomial generated by Sollya:
91 // > Q = fpminimax((10^x - 1)/x, 4, [|D...|], [-log10(2)/2^6, log10(2)/2^6]);
92 // Then:
93 // 10^dx ~ P(dx) = 1 + COEFFS[0] * dx + ... + COEFFS[4] * dx^5.
94 static constexpr double COEFFS[5] = {0x1.26bb1bbb55515p1, 0x1.53524c73bd3eap1,
95 0x1.0470591dff149p1, 0x1.2bd7c0a9fbc4dp0,
96 0x1.1429e74a98f43p-1};
97
powb_loExp10Base98 static double powb_lo(double dx) {
99 using fputil::multiply_add;
100 double dx2 = dx * dx;
101 // c0 = 1 + COEFFS[0] * dx
102 double c0 = multiply_add(dx, Exp10Base::COEFFS[0], 1.0);
103 // c1 = COEFFS[1] + COEFFS[2] * dx
104 double c1 = multiply_add(dx, Exp10Base::COEFFS[2], Exp10Base::COEFFS[1]);
105 // c2 = COEFFS[3] + COEFFS[4] * dx
106 double c2 = multiply_add(dx, Exp10Base::COEFFS[4], Exp10Base::COEFFS[3]);
107 // r = c0 + dx^2 * (c1 + c2 * dx^2)
108 // = c0 + c1 * dx^2 + c2 * dx^4
109 // = 1 + COEFFS[0] * dx + ... + COEFFS[4] * dx^5.
110 return fputil::polyeval(dx2, c0, c1, c2);
111 }
112 };
113
114 constexpr int LOG_P1_BITS = 6;
115 constexpr int LOG_P1_SIZE = 1 << LOG_P1_BITS;
116
117 // N[Table[Log[2, 1 + x], {x, 0/64, 63/64, 1/64}], 40]
118 extern const double LOG_P1_LOG2[LOG_P1_SIZE];
119
120 // N[Table[1/(1 + x), {x, 0/64, 63/64, 1/64}], 40]
121 extern const double LOG_P1_1_OVER[LOG_P1_SIZE];
122
123 // Taylor series expansion for Log[2, 1 + x] splitted to EVEN AND ODD numbers
124 // K_LOG2_ODD starts from x^3
125 extern const double K_LOG2_ODD[4];
126 extern const double K_LOG2_EVEN[4];
127
128 // Output of range reduction for exp_b: (2^(mid + hi), lo)
129 // where:
130 // b^x = 2^(mid + hi) * b^lo
131 struct exp_b_reduc_t {
132 double mh; // 2^(mid + hi)
133 double lo;
134 };
135
136 // The function correctly calculates b^x value with at least float precision
137 // in a limited range.
138 // Range reduction:
139 // b^x = 2^(hi + mid) * b^lo
140 // where:
141 // x = (hi + mid) * log_b(2) + lo
142 // hi is an integer,
143 // 0 <= mid * 2^MID_BITS < 2^MID_BITS is an integer
144 // -2^(-MID_BITS - 1) <= lo * log2(b) <= 2^(-MID_BITS - 1)
145 // Base class needs to provide the following constants:
146 // - MID_BITS : number of bits after decimal points used for mid
147 // - MID_MASK : 2^MID_BITS - 1, mask to extract mid bits
148 // - LOG2_B : log2(b) * 2^MID_BITS for scaling
149 // - M_LOGB_2_HI : high part of -log_b(2) * 2^(-MID_BITS)
150 // - M_LOGB_2_LO : low part of -log_b(2) * 2^(-MID_BITS)
151 // - EXP_2_MID : look up table for bit fields of 2^mid
152 // Return:
153 // { 2^(hi + mid), lo }
exp_b_range_reduc(float x)154 template <class Base> LIBC_INLINE exp_b_reduc_t exp_b_range_reduc(float x) {
155 double xd = static_cast<double>(x);
156 // kd = round((hi + mid) * log2(b) * 2^MID_BITS)
157 double kd = fputil::nearest_integer(Base::LOG2_B * xd);
158 // k = round((hi + mid) * log2(b) * 2^MID_BITS)
159 int k = static_cast<int>(kd);
160 // hi = floor(kd * 2^(-MID_BITS))
161 // exp_hi = shift hi to the exponent field of double precision.
162 uint64_t exp_hi = static_cast<uint64_t>(k >> Base::MID_BITS)
163 << fputil::FPBits<double>::FRACTION_LEN;
164 // mh = 2^hi * 2^mid
165 // mh_bits = bit field of mh
166 uint64_t mh_bits = Base::EXP_2_MID[k & Base::MID_MASK] + exp_hi;
167 double mh = fputil::FPBits<double>(mh_bits).get_val();
168 // dx = lo = x - (hi + mid) * log(2)
169 double dx = fputil::multiply_add(
170 kd, Base::M_LOGB_2_LO, fputil::multiply_add(kd, Base::M_LOGB_2_HI, xd));
171 return {mh, dx};
172 }
173
174 // The function correctly calculates sinh(x) and cosh(x) by calculating exp(x)
175 // and exp(-x) simultaneously.
176 // To compute e^x, we perform the following range
177 // reduction: find hi, mid, lo such that:
178 // x = (hi + mid) * log(2) + lo, in which
179 // hi is an integer,
180 // 0 <= mid * 2^5 < 32 is an integer
181 // -2^(-6) <= lo * log2(e) <= 2^-6.
182 // In particular,
183 // hi + mid = round(x * log2(e) * 2^5) * 2^(-5).
184 // Then,
185 // e^x = 2^(hi + mid) * e^lo = 2^hi * 2^mid * e^lo.
186 // 2^mid is stored in the lookup table of 32 elements.
187 // e^lo is computed using a degree-5 minimax polynomial
188 // generated by Sollya:
189 // e^lo ~ P(lo) = 1 + lo + c2 * lo^2 + ... + c5 * lo^5
190 // = (1 + c2*lo^2 + c4*lo^4) + lo * (1 + c3*lo^2 + c5*lo^4)
191 // = P_even + lo * P_odd
192 // We perform 2^hi * 2^mid by simply add hi to the exponent field
193 // of 2^mid.
194 // To compute e^(-x), notice that:
195 // e^(-x) = 2^(-(hi + mid)) * e^(-lo)
196 // ~ 2^(-(hi + mid)) * P(-lo)
197 // = 2^(-(hi + mid)) * (P_even - lo * P_odd)
198 // So:
199 // sinh(x) = (e^x - e^(-x)) / 2
200 // ~ 0.5 * (2^(hi + mid) * (P_even + lo * P_odd) -
201 // 2^(-(hi + mid)) * (P_even - lo * P_odd))
202 // = 0.5 * (P_even * (2^(hi + mid) - 2^(-(hi + mid))) +
203 // lo * P_odd * (2^(hi + mid) + 2^(-(hi + mid))))
204 // And similarly:
205 // cosh(x) = (e^x + e^(-x)) / 2
206 // ~ 0.5 * (P_even * (2^(hi + mid) + 2^(-(hi + mid))) +
207 // lo * P_odd * (2^(hi + mid) - 2^(-(hi + mid))))
208 // The main point of these formulas is that the expensive part of calculating
209 // the polynomials approximating lower parts of e^(x) and e^(-x) are shared
210 // and only done once.
exp_pm_eval(float x)211 template <bool is_sinh> LIBC_INLINE double exp_pm_eval(float x) {
212 double xd = static_cast<double>(x);
213
214 // kd = round(x * log2(e) * 2^5)
215 // k_p = round(x * log2(e) * 2^5)
216 // k_m = round(-x * log2(e) * 2^5)
217 double kd;
218 int k_p, k_m;
219
220 #ifdef LIBC_TARGET_CPU_HAS_NEAREST_INT
221 kd = fputil::nearest_integer(ExpBase::LOG2_B * xd);
222 k_p = static_cast<int>(kd);
223 k_m = -k_p;
224 #else
225 constexpr double HALF_WAY[2] = {0.5, -0.5};
226
227 k_p = static_cast<int>(
228 fputil::multiply_add(xd, ExpBase::LOG2_B, HALF_WAY[x < 0.0f]));
229 k_m = -k_p;
230 kd = static_cast<double>(k_p);
231 #endif // LIBC_TARGET_CPU_HAS_NEAREST_INT
232
233 // hi = floor(kf * 2^(-5))
234 // exp_hi = shift hi to the exponent field of double precision.
235 int64_t exp_hi_p = static_cast<int64_t>((k_p >> ExpBase::MID_BITS))
236 << fputil::FPBits<double>::FRACTION_LEN;
237 int64_t exp_hi_m = static_cast<int64_t>((k_m >> ExpBase::MID_BITS))
238 << fputil::FPBits<double>::FRACTION_LEN;
239 // mh_p = 2^(hi + mid)
240 // mh_m = 2^(-(hi + mid))
241 // mh_bits_* = bit field of mh_*
242 int64_t mh_bits_p = ExpBase::EXP_2_MID[k_p & ExpBase::MID_MASK] + exp_hi_p;
243 int64_t mh_bits_m = ExpBase::EXP_2_MID[k_m & ExpBase::MID_MASK] + exp_hi_m;
244 double mh_p = fputil::FPBits<double>(uint64_t(mh_bits_p)).get_val();
245 double mh_m = fputil::FPBits<double>(uint64_t(mh_bits_m)).get_val();
246 // mh_sum = 2^(hi + mid) + 2^(-(hi + mid))
247 double mh_sum = mh_p + mh_m;
248 // mh_diff = 2^(hi + mid) - 2^(-(hi + mid))
249 double mh_diff = mh_p - mh_m;
250
251 // dx = lo = x - (hi + mid) * log(2)
252 double dx =
253 fputil::multiply_add(kd, ExpBase::M_LOGB_2_LO,
254 fputil::multiply_add(kd, ExpBase::M_LOGB_2_HI, xd));
255 double dx2 = dx * dx;
256
257 // c0 = 1 + COEFFS[0] * lo^2
258 // P_even = (1 + COEFFS[0] * lo^2 + COEFFS[2] * lo^4) / 2
259 double p_even = fputil::polyeval(dx2, 0.5, ExpBase::COEFFS[0] * 0.5,
260 ExpBase::COEFFS[2] * 0.5);
261 // P_odd = (1 + COEFFS[1] * lo^2 + COEFFS[3] * lo^4) / 2
262 double p_odd = fputil::polyeval(dx2, 0.5, ExpBase::COEFFS[1] * 0.5,
263 ExpBase::COEFFS[3] * 0.5);
264
265 double r;
266 if constexpr (is_sinh)
267 r = fputil::multiply_add(dx * mh_sum, p_odd, p_even * mh_diff);
268 else
269 r = fputil::multiply_add(dx * mh_diff, p_odd, p_even * mh_sum);
270 return r;
271 }
272
273 // x should be positive, normal finite value
log2_eval(double x)274 LIBC_INLINE static double log2_eval(double x) {
275 using FPB = fputil::FPBits<double>;
276 FPB bs(x);
277
278 double result = 0;
279 result += bs.get_exponent();
280
281 int p1 = (bs.get_mantissa() >> (FPB::FRACTION_LEN - LOG_P1_BITS)) &
282 (LOG_P1_SIZE - 1);
283
284 bs.set_uintval(bs.uintval() & (FPB::FRACTION_MASK >> LOG_P1_BITS));
285 bs.set_biased_exponent(FPB::EXP_BIAS);
286 double dx = (bs.get_val() - 1.0) * LOG_P1_1_OVER[p1];
287
288 // Taylor series for log(2,1+x)
289 double c1 = fputil::multiply_add(dx, K_LOG2_ODD[0], K_LOG2_EVEN[0]);
290 double c2 = fputil::multiply_add(dx, K_LOG2_ODD[1], K_LOG2_EVEN[1]);
291 double c3 = fputil::multiply_add(dx, K_LOG2_ODD[2], K_LOG2_EVEN[2]);
292 double c4 = fputil::multiply_add(dx, K_LOG2_ODD[3], K_LOG2_EVEN[3]);
293
294 // c0 = dx * (1.0 / ln(2)) + LOG_P1_LOG2[p1]
295 double c0 = fputil::multiply_add(dx, 0x1.71547652b82fep+0, LOG_P1_LOG2[p1]);
296 result += LIBC_NAMESPACE::fputil::polyeval(dx * dx, c0, c1, c2, c3, c4);
297 return result;
298 }
299
300 // x should be positive, normal finite value
log_eval(double x)301 LIBC_INLINE static double log_eval(double x) {
302 // For x = 2^ex * (1 + mx)
303 // log(x) = ex * log(2) + log(1 + mx)
304 using FPB = fputil::FPBits<double>;
305 FPB bs(x);
306
307 double ex = static_cast<double>(bs.get_exponent());
308
309 // p1 is the leading 7 bits of mx, i.e.
310 // p1 * 2^(-7) <= m_x < (p1 + 1) * 2^(-7).
311 int p1 = static_cast<int>(bs.get_mantissa() >> (FPB::FRACTION_LEN - 7));
312
313 // Set bs to (1 + (mx - p1*2^(-7))
314 bs.set_uintval(bs.uintval() & (FPB::FRACTION_MASK >> 7));
315 bs.set_biased_exponent(FPB::EXP_BIAS);
316 // dx = (mx - p1*2^(-7)) / (1 + p1*2^(-7)).
317 double dx = (bs.get_val() - 1.0) * ONE_OVER_F[p1];
318
319 // Minimax polynomial of log(1 + dx) generated by Sollya with:
320 // > P = fpminimax(log(1 + x)/x, 6, [|D...|], [0, 2^-7]);
321 const double COEFFS[6] = {-0x1.ffffffffffffcp-2, 0x1.5555555552ddep-2,
322 -0x1.ffffffefe562dp-3, 0x1.9999817d3a50fp-3,
323 -0x1.554317b3f67a5p-3, 0x1.1dc5c45e09c18p-3};
324 double dx2 = dx * dx;
325 double c1 = fputil::multiply_add(dx, COEFFS[1], COEFFS[0]);
326 double c2 = fputil::multiply_add(dx, COEFFS[3], COEFFS[2]);
327 double c3 = fputil::multiply_add(dx, COEFFS[5], COEFFS[4]);
328
329 double p = fputil::polyeval(dx2, dx, c1, c2, c3);
330 double result =
331 fputil::multiply_add(ex, /*log(2)*/ 0x1.62e42fefa39efp-1, LOG_F[p1] + p);
332 return result;
333 }
334
335 // Rounding tests for 2^hi * (mid + lo) when the output might be denormal. We
336 // assume further that 1 <= mid < 2, mid + lo < 2, and |lo| << mid.
337 // Notice that, if 0 < x < 2^-1022,
338 // double(2^-1022 + x) - 2^-1022 = double(x).
339 // So if we scale x up by 2^1022, we can use
340 // double(1.0 + 2^1022 * x) - 1.0 to test how x is rounded in denormal range.
ziv_test_denorm(int hi,double mid,double lo,double err)341 LIBC_INLINE cpp::optional<double> ziv_test_denorm(int hi, double mid, double lo,
342 double err) {
343 using FPBits = typename fputil::FPBits<double>;
344
345 // Scaling factor = 1/(min normal number) = 2^1022
346 int64_t exp_hi = static_cast<int64_t>(hi + 1022) << FPBits::FRACTION_LEN;
347 double mid_hi = cpp::bit_cast<double>(exp_hi + cpp::bit_cast<int64_t>(mid));
348 double lo_scaled =
349 (lo != 0.0) ? cpp::bit_cast<double>(exp_hi + cpp::bit_cast<int64_t>(lo))
350 : 0.0;
351
352 double extra_factor = 0.0;
353 uint64_t scale_down = 0x3FE0'0000'0000'0000; // 1022 in the exponent field.
354
355 // Result is denormal if (mid_hi + lo_scale < 1.0).
356 if ((1.0 - mid_hi) > lo_scaled) {
357 // Extra rounding step is needed, which adds more rounding errors.
358 err += 0x1.0p-52;
359 extra_factor = 1.0;
360 scale_down = 0x3FF0'0000'0000'0000; // 1023 in the exponent field.
361 }
362
363 double err_scaled =
364 cpp::bit_cast<double>(exp_hi + cpp::bit_cast<int64_t>(err));
365
366 double lo_u = lo_scaled + err_scaled;
367 double lo_l = lo_scaled - err_scaled;
368
369 // By adding 1.0, the results will have similar rounding points as denormal
370 // outputs.
371 double upper = extra_factor + (mid_hi + lo_u);
372 double lower = extra_factor + (mid_hi + lo_l);
373
374 if (LIBC_LIKELY(upper == lower)) {
375 return cpp::bit_cast<double>(cpp::bit_cast<uint64_t>(upper) - scale_down);
376 }
377
378 return cpp::nullopt;
379 }
380
381 } // namespace LIBC_NAMESPACE_DECL
382
383 #endif // LLVM_LIBC_SRC_MATH_GENERIC_EXPLOGXF_H
384