2 * SPDX-License-Identifier: BSD-3-Clause
4 * Copyright (c) 1992, 1993
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8 * modification, are permitted provided that the following conditions
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32 /* @(#)log.c 8.2 (Berkeley) 11/30/93 */
33 #include <sys/cdefs.h>
34 __FBSDID("$FreeBSD$");
40 /* Table-driven natural logarithm.
42 * This code was derived, with minor modifications, from:
43 * Peter Tang, "Table-Driven Implementation of the
44 * Logarithm in IEEE Floating-Point arithmetic." ACM Trans.
45 * Math Software, vol 16. no 4, pp 378-400, Dec 1990).
47 * Calculates log(2^m*F*(1+f/F)), |f/j| <= 1/256,
48 * where F = j/128 for j an integer in [0, 128].
50 * log(2^m) = log2_hi*m + log2_tail*m
51 * since m is an integer, the dominant term is exact.
52 * m has at most 10 digits (for subnormal numbers),
53 * and log2_hi has 11 trailing zero bits.
55 * log(F) = logF_hi[j] + logF_lo[j] is in tabular form in log_table.h
56 * logF_hi[] + 512 is exact.
58 * log(1+f/F) = 2*f/(2*F + f) + 1/12 * (2*f/(2*F + f))**3 + ...
59 * the leading term is calculated to extra precision in two
60 * parts, the larger of which adds exactly to the dominant
62 * There are two cases:
63 * 1. when m, j are non-zero (m | j), use absolute
64 * precision for the leading term.
65 * 2. when m = j = 0, |1-x| < 1/256, and log(x) ~= (x-1).
66 * In this case, use a relative precision of 24 bits.
67 * (This is done differently in the original paper)
70 * 0 return signalling -Inf
71 * neg return signalling NaN
77 /* Table of log(Fj) = logF_head[j] + logF_tail[j], for Fj = 1+j/128.
78 * Used for generation of extend precision logarithms.
79 * The constant 35184372088832 is 2^45, so the divide is exact.
80 * It ensures correct reading of logF_head, even for inaccurate
81 * decimal-to-binary conversion routines. (Everybody gets the
82 * right answer for integers less than 2^53.)
83 * Values for log(F) were generated using error < 10^-57 absolute
84 * with the bc -l package.
86 static double A1 = .08333333333333178827;
87 static double A2 = .01250000000377174923;
88 static double A3 = .002232139987919447809;
89 static double A4 = .0004348877777076145742;
91 static double logF_head[N+1] = {
93 .007782140442060381246,
94 .015504186535963526694,
95 .023167059281547608406,
96 .030771658666765233647,
97 .038318864302141264488,
98 .045809536031242714670,
99 .053244514518837604555,
100 .060624621816486978786,
101 .067950661908525944454,
102 .075223421237524235039,
103 .082443669210988446138,
104 .089612158689760690322,
105 .096729626458454731618,
106 .103796793681567578460,
107 .110814366340264314203,
108 .117783035656430001836,
109 .124703478501032805070,
110 .131576357788617315236,
111 .138402322859292326029,
112 .145182009844575077295,
113 .151916042025732167530,
114 .158605030176659056451,
115 .165249572895390883786,
116 .171850256926518341060,
117 .178407657472689606947,
118 .184922338493834104156,
119 .191394852999565046047,
120 .197825743329758552135,
121 .204215541428766300668,
122 .210564769107350002741,
123 .216873938300523150246,
124 .223143551314024080056,
125 .229374101064877322642,
126 .235566071312860003672,
127 .241719936886966024758,
128 .247836163904594286577,
129 .253915209980732470285,
130 .259957524436686071567,
131 .265963548496984003577,
132 .271933715484010463114,
133 .277868451003087102435,
134 .283768173130738432519,
135 .289633292582948342896,
136 .295464212893421063199,
137 .301261330578199704177,
138 .307025035294827830512,
139 .312755710004239517729,
140 .318453731118097493890,
141 .324119468654316733591,
142 .329753286372579168528,
143 .335355541920762334484,
144 .340926586970454081892,
145 .346466767346100823488,
146 .351976423156884266063,
147 .357455888922231679316,
148 .362905493689140712376,
149 .368325561158599157352,
150 .373716409793814818840,
151 .379078352934811846353,
152 .384411698910298582632,
153 .389716751140440464951,
154 .394993808240542421117,
155 .400243164127459749579,
156 .405465108107819105498,
157 .410659924985338875558,
158 .415827895143593195825,
159 .420969294644237379543,
160 .426084395310681429691,
161 .431173464818130014464,
162 .436236766774527495726,
163 .441274560805140936281,
164 .446287102628048160113,
165 .451274644139630254358,
166 .456237433481874177232,
167 .461175715122408291790,
168 .466089729924533457960,
169 .470979715219073113985,
170 .475845904869856894947,
171 .480688529345570714212,
172 .485507815781602403149,
173 .490303988045525329653,
174 .495077266798034543171,
175 .499827869556611403822,
176 .504556010751912253908,
177 .509261901790523552335,
178 .513945751101346104405,
179 .518607764208354637958,
180 .523248143765158602036,
181 .527867089620485785417,
182 .532464798869114019908,
183 .537041465897345915436,
184 .541597282432121573947,
185 .546132437597407260909,
186 .550647117952394182793,
187 .555141507540611200965,
188 .559615787935399566777,
189 .564070138285387656651,
190 .568504735352689749561,
191 .572919753562018740922,
192 .577315365035246941260,
193 .581691739635061821900,
194 .586049045003164792433,
195 .590387446602107957005,
196 .594707107746216934174,
197 .599008189645246602594,
198 .603290851438941899687,
199 .607555250224322662688,
200 .611801541106615331955,
201 .616029877215623855590,
202 .620240409751204424537,
203 .624433288012369303032,
204 .628608659422752680256,
205 .632766669570628437213,
206 .636907462236194987781,
207 .641031179420679109171,
208 .645137961373620782978,
209 .649227946625615004450,
210 .653301272011958644725,
211 .657358072709030238911,
212 .661398482245203922502,
213 .665422632544505177065,
214 .669430653942981734871,
215 .673422675212350441142,
216 .677398823590920073911,
217 .681359224807238206267,
218 .685304003098281100392,
219 .689233281238557538017,
220 .693147180560117703862
223 static double logF_tail[N+1] = {
225 -.00000000000000543229938420049,
226 .00000000000000172745674997061,
227 -.00000000000001323017818229233,
228 -.00000000000001154527628289872,
229 -.00000000000000466529469958300,
230 .00000000000005148849572685810,
231 -.00000000000002532168943117445,
232 -.00000000000005213620639136504,
233 -.00000000000001819506003016881,
234 .00000000000006329065958724544,
235 .00000000000008614512936087814,
236 -.00000000000007355770219435028,
237 .00000000000009638067658552277,
238 .00000000000007598636597194141,
239 .00000000000002579999128306990,
240 -.00000000000004654729747598444,
241 -.00000000000007556920687451336,
242 .00000000000010195735223708472,
243 -.00000000000017319034406422306,
244 -.00000000000007718001336828098,
245 .00000000000010980754099855238,
246 -.00000000000002047235780046195,
247 -.00000000000008372091099235912,
248 .00000000000014088127937111135,
249 .00000000000012869017157588257,
250 .00000000000017788850778198106,
251 .00000000000006440856150696891,
252 .00000000000016132822667240822,
253 -.00000000000007540916511956188,
254 -.00000000000000036507188831790,
255 .00000000000009120937249914984,
256 .00000000000018567570959796010,
257 -.00000000000003149265065191483,
258 -.00000000000009309459495196889,
259 .00000000000017914338601329117,
260 -.00000000000001302979717330866,
261 .00000000000023097385217586939,
262 .00000000000023999540484211737,
263 .00000000000015393776174455408,
264 -.00000000000036870428315837678,
265 .00000000000036920375082080089,
266 -.00000000000009383417223663699,
267 .00000000000009433398189512690,
268 .00000000000041481318704258568,
269 -.00000000000003792316480209314,
270 .00000000000008403156304792424,
271 -.00000000000034262934348285429,
272 .00000000000043712191957429145,
273 -.00000000000010475750058776541,
274 -.00000000000011118671389559323,
275 .00000000000037549577257259853,
276 .00000000000013912841212197565,
277 .00000000000010775743037572640,
278 .00000000000029391859187648000,
279 -.00000000000042790509060060774,
280 .00000000000022774076114039555,
281 .00000000000010849569622967912,
282 -.00000000000023073801945705758,
283 .00000000000015761203773969435,
284 .00000000000003345710269544082,
285 -.00000000000041525158063436123,
286 .00000000000032655698896907146,
287 -.00000000000044704265010452446,
288 .00000000000034527647952039772,
289 -.00000000000007048962392109746,
290 .00000000000011776978751369214,
291 -.00000000000010774341461609578,
292 .00000000000021863343293215910,
293 .00000000000024132639491333131,
294 .00000000000039057462209830700,
295 -.00000000000026570679203560751,
296 .00000000000037135141919592021,
297 -.00000000000017166921336082431,
298 -.00000000000028658285157914353,
299 -.00000000000023812542263446809,
300 .00000000000006576659768580062,
301 -.00000000000028210143846181267,
302 .00000000000010701931762114254,
303 .00000000000018119346366441110,
304 .00000000000009840465278232627,
305 -.00000000000033149150282752542,
306 -.00000000000018302857356041668,
307 -.00000000000016207400156744949,
308 .00000000000048303314949553201,
309 -.00000000000071560553172382115,
310 .00000000000088821239518571855,
311 -.00000000000030900580513238244,
312 -.00000000000061076551972851496,
313 .00000000000035659969663347830,
314 .00000000000035782396591276383,
315 -.00000000000046226087001544578,
316 .00000000000062279762917225156,
317 .00000000000072838947272065741,
318 .00000000000026809646615211673,
319 -.00000000000010960825046059278,
320 .00000000000002311949383800537,
321 -.00000000000058469058005299247,
322 -.00000000000002103748251144494,
323 -.00000000000023323182945587408,
324 -.00000000000042333694288141916,
325 -.00000000000043933937969737844,
326 .00000000000041341647073835565,
327 .00000000000006841763641591466,
328 .00000000000047585534004430641,
329 .00000000000083679678674757695,
330 -.00000000000085763734646658640,
331 .00000000000021913281229340092,
332 -.00000000000062242842536431148,
333 -.00000000000010983594325438430,
334 .00000000000065310431377633651,
335 -.00000000000047580199021710769,
336 -.00000000000037854251265457040,
337 .00000000000040939233218678664,
338 .00000000000087424383914858291,
339 .00000000000025218188456842882,
340 -.00000000000003608131360422557,
341 -.00000000000050518555924280902,
342 .00000000000078699403323355317,
343 -.00000000000067020876961949060,
344 .00000000000016108575753932458,
345 .00000000000058527188436251509,
346 -.00000000000035246757297904791,
347 -.00000000000018372084495629058,
348 .00000000000088606689813494916,
349 .00000000000066486268071468700,
350 .00000000000063831615170646519,
351 .00000000000025144230728376072,
352 -.00000000000017239444525614834
364 double F, f, g, q, u, u2, v, zero = 0.0, one = 1.0;
367 /* Catch special cases */
369 if (x == zero) /* log(0) = -Inf */
371 else /* log(neg) = NaN */
374 return (x+x); /* x = NaN, Inf */
376 /* Argument reduction: 1 <= g < 2; x/2^m = g; */
377 /* y = F*(1 + f/F) for |f| <= 2^-8 */
386 F = (1.0/N) * j + 1; /* F*128 is an integer in [128, 512] */
389 /* Approximate expansion for log(1+f/F) ~= u + q */
393 q = u*v*(A1 + v*(A2 + v*(A3 + v*A4)));
395 /* case 1: u1 = u rounded to 2^-43 absolute. Since u < 2^-8,
396 * u1 has at most 35 bits, and F*u1 is exact, as F has < 8 bits.
397 * It also adds exactly to |m*log2_hi + log_F_head[j] | < 750
400 u1 = u + 513, u1 -= 513;
402 /* case 2: |1-x| < 1/256. The m- and j- dependent terms are zero;
407 u2 = (2.0*(f - F*u1) - u1*f) * g;
408 /* u1 + u2 = 2f/(2F+f) to extra precision. */
410 /* log(x) = log(2^m*F*(1+f/F)) = */
411 /* (m*log2_hi+logF_head[j]+u1) + (m*log2_lo+logF_tail[j]+q); */
412 /* (exact) + (tiny) */
414 u1 += m*logF_head[N] + logF_head[j]; /* exact */
415 u2 = (u2 + logF_tail[j]) + q; /* tiny */
416 u2 += logF_tail[N]*m;
422 * Extra precision variant, returning struct {double a, b;};
423 * log(x) = a+b to 63 bits, with a rounded to 26 bits.
429 __log__D(x) double x;
433 double F, f, g, q, u, v, u2;
437 /* Argument reduction: 1 <= g < 2; x/2^m = g; */
438 /* y = F*(1 + f/F) for |f| <= 2^-8 */
453 q = u*v*(A1 + v*(A2 + v*(A3 + v*A4)));
455 u1 = u + 513, u1 -= 513;
458 u2 = (2.0*(f - F*u1) - u1*f) * g;
460 u1 += m*logF_head[N] + logF_head[j];
462 u2 += logF_tail[j]; u2 += q;
463 u2 += logF_tail[N]*m;
464 r.a = u1 + u2; /* Only difference is here */
466 r.b = (u1 - r.a) + u2;