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glibc/sysdeps/generic/log.c
Roland McGrath 28f540f45b initial import
1995-02-18 01:27:10 +00:00

487 lines
14 KiB
C
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/*
* Copyright (c) 1992, 1993
* The Regents of the University of California. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. All advertising materials mentioning features or use of this software
* must display the following acknowledgement:
* This product includes software developed by the University of
* California, Berkeley and its contributors.
* 4. Neither the name of the University nor the names of its contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
#ifndef lint
static char sccsid[] = "@(#)log.c 8.2 (Berkeley) 11/30/93";
#endif /* not lint */
#include <math.h>
#include <errno.h>
#include "mathimpl.h"
/* Table-driven natural logarithm.
*
* This code was derived, with minor modifications, from:
* Peter Tang, "Table-Driven Implementation of the
* Logarithm in IEEE Floating-Point arithmetic." ACM Trans.
* Math Software, vol 16. no 4, pp 378-400, Dec 1990).
*
* Calculates log(2^m*F*(1+f/F)), |f/j| <= 1/256,
* where F = j/128 for j an integer in [0, 128].
*
* log(2^m) = log2_hi*m + log2_tail*m
* since m is an integer, the dominant term is exact.
* m has at most 10 digits (for subnormal numbers),
* and log2_hi has 11 trailing zero bits.
*
* log(F) = logF_hi[j] + logF_lo[j] is in tabular form in log_table.h
* logF_hi[] + 512 is exact.
*
* log(1+f/F) = 2*f/(2*F + f) + 1/12 * (2*f/(2*F + f))**3 + ...
* the leading term is calculated to extra precision in two
* parts, the larger of which adds exactly to the dominant
* m and F terms.
* There are two cases:
* 1. when m, j are non-zero (m | j), use absolute
* precision for the leading term.
* 2. when m = j = 0, |1-x| < 1/256, and log(x) ~= (x-1).
* In this case, use a relative precision of 24 bits.
* (This is done differently in the original paper)
*
* Special cases:
* 0 return signalling -Inf
* neg return signalling NaN
* +Inf return +Inf
*/
#if defined(vax) || defined(tahoe)
#define _IEEE 0
#define TRUNC(x) x = (double) (float) (x)
#else
#define _IEEE 1
#define endian (((*(int *) &one)) ? 1 : 0)
#define TRUNC(x) *(((int *) &x) + endian) &= 0xf8000000
#define infnan(x) 0.0
#endif
#define N 128
/* Table of log(Fj) = logF_head[j] + logF_tail[j], for Fj = 1+j/128.
* Used for generation of extend precision logarithms.
* The constant 35184372088832 is 2^45, so the divide is exact.
* It ensures correct reading of logF_head, even for inaccurate
* decimal-to-binary conversion routines. (Everybody gets the
* right answer for integers less than 2^53.)
* Values for log(F) were generated using error < 10^-57 absolute
* with the bc -l package.
*/
static double A1 = .08333333333333178827;
static double A2 = .01250000000377174923;
static double A3 = .002232139987919447809;
static double A4 = .0004348877777076145742;
static double logF_head[N+1] = {
0.,
.007782140442060381246,
.015504186535963526694,
.023167059281547608406,
.030771658666765233647,
.038318864302141264488,
.045809536031242714670,
.053244514518837604555,
.060624621816486978786,
.067950661908525944454,
.075223421237524235039,
.082443669210988446138,
.089612158689760690322,
.096729626458454731618,
.103796793681567578460,
.110814366340264314203,
.117783035656430001836,
.124703478501032805070,
.131576357788617315236,
.138402322859292326029,
.145182009844575077295,
.151916042025732167530,
.158605030176659056451,
.165249572895390883786,
.171850256926518341060,
.178407657472689606947,
.184922338493834104156,
.191394852999565046047,
.197825743329758552135,
.204215541428766300668,
.210564769107350002741,
.216873938300523150246,
.223143551314024080056,
.229374101064877322642,
.235566071312860003672,
.241719936886966024758,
.247836163904594286577,
.253915209980732470285,
.259957524436686071567,
.265963548496984003577,
.271933715484010463114,
.277868451003087102435,
.283768173130738432519,
.289633292582948342896,
.295464212893421063199,
.301261330578199704177,
.307025035294827830512,
.312755710004239517729,
.318453731118097493890,
.324119468654316733591,
.329753286372579168528,
.335355541920762334484,
.340926586970454081892,
.346466767346100823488,
.351976423156884266063,
.357455888922231679316,
.362905493689140712376,
.368325561158599157352,
.373716409793814818840,
.379078352934811846353,
.384411698910298582632,
.389716751140440464951,
.394993808240542421117,
.400243164127459749579,
.405465108107819105498,
.410659924985338875558,
.415827895143593195825,
.420969294644237379543,
.426084395310681429691,
.431173464818130014464,
.436236766774527495726,
.441274560805140936281,
.446287102628048160113,
.451274644139630254358,
.456237433481874177232,
.461175715122408291790,
.466089729924533457960,
.470979715219073113985,
.475845904869856894947,
.480688529345570714212,
.485507815781602403149,
.490303988045525329653,
.495077266798034543171,
.499827869556611403822,
.504556010751912253908,
.509261901790523552335,
.513945751101346104405,
.518607764208354637958,
.523248143765158602036,
.527867089620485785417,
.532464798869114019908,
.537041465897345915436,
.541597282432121573947,
.546132437597407260909,
.550647117952394182793,
.555141507540611200965,
.559615787935399566777,
.564070138285387656651,
.568504735352689749561,
.572919753562018740922,
.577315365035246941260,
.581691739635061821900,
.586049045003164792433,
.590387446602107957005,
.594707107746216934174,
.599008189645246602594,
.603290851438941899687,
.607555250224322662688,
.611801541106615331955,
.616029877215623855590,
.620240409751204424537,
.624433288012369303032,
.628608659422752680256,
.632766669570628437213,
.636907462236194987781,
.641031179420679109171,
.645137961373620782978,
.649227946625615004450,
.653301272011958644725,
.657358072709030238911,
.661398482245203922502,
.665422632544505177065,
.669430653942981734871,
.673422675212350441142,
.677398823590920073911,
.681359224807238206267,
.685304003098281100392,
.689233281238557538017,
.693147180560117703862
};
static double logF_tail[N+1] = {
0.,
-.00000000000000543229938420049,
.00000000000000172745674997061,
-.00000000000001323017818229233,
-.00000000000001154527628289872,
-.00000000000000466529469958300,
.00000000000005148849572685810,
-.00000000000002532168943117445,
-.00000000000005213620639136504,
-.00000000000001819506003016881,
.00000000000006329065958724544,
.00000000000008614512936087814,
-.00000000000007355770219435028,
.00000000000009638067658552277,
.00000000000007598636597194141,
.00000000000002579999128306990,
-.00000000000004654729747598444,
-.00000000000007556920687451336,
.00000000000010195735223708472,
-.00000000000017319034406422306,
-.00000000000007718001336828098,
.00000000000010980754099855238,
-.00000000000002047235780046195,
-.00000000000008372091099235912,
.00000000000014088127937111135,
.00000000000012869017157588257,
.00000000000017788850778198106,
.00000000000006440856150696891,
.00000000000016132822667240822,
-.00000000000007540916511956188,
-.00000000000000036507188831790,
.00000000000009120937249914984,
.00000000000018567570959796010,
-.00000000000003149265065191483,
-.00000000000009309459495196889,
.00000000000017914338601329117,
-.00000000000001302979717330866,
.00000000000023097385217586939,
.00000000000023999540484211737,
.00000000000015393776174455408,
-.00000000000036870428315837678,
.00000000000036920375082080089,
-.00000000000009383417223663699,
.00000000000009433398189512690,
.00000000000041481318704258568,
-.00000000000003792316480209314,
.00000000000008403156304792424,
-.00000000000034262934348285429,
.00000000000043712191957429145,
-.00000000000010475750058776541,
-.00000000000011118671389559323,
.00000000000037549577257259853,
.00000000000013912841212197565,
.00000000000010775743037572640,
.00000000000029391859187648000,
-.00000000000042790509060060774,
.00000000000022774076114039555,
.00000000000010849569622967912,
-.00000000000023073801945705758,
.00000000000015761203773969435,
.00000000000003345710269544082,
-.00000000000041525158063436123,
.00000000000032655698896907146,
-.00000000000044704265010452446,
.00000000000034527647952039772,
-.00000000000007048962392109746,
.00000000000011776978751369214,
-.00000000000010774341461609578,
.00000000000021863343293215910,
.00000000000024132639491333131,
.00000000000039057462209830700,
-.00000000000026570679203560751,
.00000000000037135141919592021,
-.00000000000017166921336082431,
-.00000000000028658285157914353,
-.00000000000023812542263446809,
.00000000000006576659768580062,
-.00000000000028210143846181267,
.00000000000010701931762114254,
.00000000000018119346366441110,
.00000000000009840465278232627,
-.00000000000033149150282752542,
-.00000000000018302857356041668,
-.00000000000016207400156744949,
.00000000000048303314949553201,
-.00000000000071560553172382115,
.00000000000088821239518571855,
-.00000000000030900580513238244,
-.00000000000061076551972851496,
.00000000000035659969663347830,
.00000000000035782396591276383,
-.00000000000046226087001544578,
.00000000000062279762917225156,
.00000000000072838947272065741,
.00000000000026809646615211673,
-.00000000000010960825046059278,
.00000000000002311949383800537,
-.00000000000058469058005299247,
-.00000000000002103748251144494,
-.00000000000023323182945587408,
-.00000000000042333694288141916,
-.00000000000043933937969737844,
.00000000000041341647073835565,
.00000000000006841763641591466,
.00000000000047585534004430641,
.00000000000083679678674757695,
-.00000000000085763734646658640,
.00000000000021913281229340092,
-.00000000000062242842536431148,
-.00000000000010983594325438430,
.00000000000065310431377633651,
-.00000000000047580199021710769,
-.00000000000037854251265457040,
.00000000000040939233218678664,
.00000000000087424383914858291,
.00000000000025218188456842882,
-.00000000000003608131360422557,
-.00000000000050518555924280902,
.00000000000078699403323355317,
-.00000000000067020876961949060,
.00000000000016108575753932458,
.00000000000058527188436251509,
-.00000000000035246757297904791,
-.00000000000018372084495629058,
.00000000000088606689813494916,
.00000000000066486268071468700,
.00000000000063831615170646519,
.00000000000025144230728376072,
-.00000000000017239444525614834
};
double
#ifdef _ANSI_SOURCE
log(double x)
#else
log(x) double x;
#endif
{
int m, j;
double F, f, g, q, u, u2, v, zero = 0.0, one = 1.0;
volatile double u1;
/* Catch special cases */
if (x <= 0)
if (_IEEE && x == zero) /* log(0) = -Inf */
return (-one/zero);
else if (_IEEE) /* log(neg) = NaN */
return (zero/zero);
else if (x == zero) /* NOT REACHED IF _IEEE */
return (infnan(-ERANGE));
else
return (infnan(EDOM));
else if (!finite(x))
if (_IEEE) /* x = NaN, Inf */
return (x+x);
else
return (infnan(ERANGE));
/* Argument reduction: 1 <= g < 2; x/2^m = g; */
/* y = F*(1 + f/F) for |f| <= 2^-8 */
m = logb(x);
g = ldexp(x, -m);
if (_IEEE && m == -1022) {
j = logb(g), m += j;
g = ldexp(g, -j);
}
j = N*(g-1) + .5;
F = (1.0/N) * j + 1; /* F*128 is an integer in [128, 512] */
f = g - F;
/* Approximate expansion for log(1+f/F) ~= u + q */
g = 1/(2*F+f);
u = 2*f*g;
v = u*u;
q = u*v*(A1 + v*(A2 + v*(A3 + v*A4)));
/* case 1: u1 = u rounded to 2^-43 absolute. Since u < 2^-8,
* u1 has at most 35 bits, and F*u1 is exact, as F has < 8 bits.
* It also adds exactly to |m*log2_hi + log_F_head[j] | < 750
*/
if (m | j)
u1 = u + 513, u1 -= 513;
/* case 2: |1-x| < 1/256. The m- and j- dependent terms are zero;
* u1 = u to 24 bits.
*/
else
u1 = u, TRUNC(u1);
u2 = (2.0*(f - F*u1) - u1*f) * g;
/* u1 + u2 = 2f/(2F+f) to extra precision. */
/* log(x) = log(2^m*F*(1+f/F)) = */
/* (m*log2_hi+logF_head[j]+u1) + (m*log2_lo+logF_tail[j]+q); */
/* (exact) + (tiny) */
u1 += m*logF_head[N] + logF_head[j]; /* exact */
u2 = (u2 + logF_tail[j]) + q; /* tiny */
u2 += logF_tail[N]*m;
return (u1 + u2);
}
/*
* Extra precision variant, returning struct {double a, b;};
* log(x) = a+b to 63 bits, with a is rounded to 26 bits.
*/
struct Double
#ifdef _ANSI_SOURCE
__log__D(double x)
#else
__log__D(x) double x;
#endif
{
int m, j;
double F, f, g, q, u, v, u2, one = 1.0;
volatile double u1;
struct Double r;
/* Argument reduction: 1 <= g < 2; x/2^m = g; */
/* y = F*(1 + f/F) for |f| <= 2^-8 */
m = logb(x);
g = ldexp(x, -m);
if (_IEEE && m == -1022) {
j = logb(g), m += j;
g = ldexp(g, -j);
}
j = N*(g-1) + .5;
F = (1.0/N) * j + 1;
f = g - F;
g = 1/(2*F+f);
u = 2*f*g;
v = u*u;
q = u*v*(A1 + v*(A2 + v*(A3 + v*A4)));
if (m | j)
u1 = u + 513, u1 -= 513;
else
u1 = u, TRUNC(u1);
u2 = (2.0*(f - F*u1) - u1*f) * g;
u1 += m*logF_head[N] + logF_head[j];
u2 += logF_tail[j]; u2 += q;
u2 += logF_tail[N]*m;
r.a = u1 + u2; /* Only difference is here */
TRUNC(r.a);
r.b = (u1 - r.a) + u2;
return (r);
}
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