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Diffstat (limited to 'lib/mlibc/options/ansi/musl-generic-math/log.c')
| -rw-r--r-- | lib/mlibc/options/ansi/musl-generic-math/log.c | 118 |
1 files changed, 118 insertions, 0 deletions
diff --git a/lib/mlibc/options/ansi/musl-generic-math/log.c b/lib/mlibc/options/ansi/musl-generic-math/log.c new file mode 100644 index 0000000..e61e113 --- /dev/null +++ b/lib/mlibc/options/ansi/musl-generic-math/log.c @@ -0,0 +1,118 @@ +/* origin: FreeBSD /usr/src/lib/msun/src/e_log.c */ +/* + * ==================================================== + * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. + * + * Developed at SunSoft, a Sun Microsystems, Inc. business. + * Permission to use, copy, modify, and distribute this + * software is freely granted, provided that this notice + * is preserved. + * ==================================================== + */ +/* log(x) + * Return the logarithm of x + * + * Method : + * 1. Argument Reduction: find k and f such that + * x = 2^k * (1+f), + * where sqrt(2)/2 < 1+f < sqrt(2) . + * + * 2. Approximation of log(1+f). + * Let s = f/(2+f) ; based on log(1+f) = log(1+s) - log(1-s) + * = 2s + 2/3 s**3 + 2/5 s**5 + ....., + * = 2s + s*R + * We use a special Remez algorithm on [0,0.1716] to generate + * a polynomial of degree 14 to approximate R The maximum error + * of this polynomial approximation is bounded by 2**-58.45. In + * other words, + * 2 4 6 8 10 12 14 + * R(z) ~ Lg1*s +Lg2*s +Lg3*s +Lg4*s +Lg5*s +Lg6*s +Lg7*s + * (the values of Lg1 to Lg7 are listed in the program) + * and + * | 2 14 | -58.45 + * | Lg1*s +...+Lg7*s - R(z) | <= 2 + * | | + * Note that 2s = f - s*f = f - hfsq + s*hfsq, where hfsq = f*f/2. + * In order to guarantee error in log below 1ulp, we compute log + * by + * log(1+f) = f - s*(f - R) (if f is not too large) + * log(1+f) = f - (hfsq - s*(hfsq+R)). (better accuracy) + * + * 3. Finally, log(x) = k*ln2 + log(1+f). + * = k*ln2_hi+(f-(hfsq-(s*(hfsq+R)+k*ln2_lo))) + * Here ln2 is split into two floating point number: + * ln2_hi + ln2_lo, + * where n*ln2_hi is always exact for |n| < 2000. + * + * Special cases: + * log(x) is NaN with signal if x < 0 (including -INF) ; + * log(+INF) is +INF; log(0) is -INF with signal; + * log(NaN) is that NaN with no signal. + * + * Accuracy: + * according to an error analysis, the error is always less than + * 1 ulp (unit in the last place). + * + * Constants: + * The hexadecimal values are the intended ones for the following + * constants. The decimal values may be used, provided that the + * compiler will convert from decimal to binary accurately enough + * to produce the hexadecimal values shown. + */ + +#include <math.h> +#include <stdint.h> + +static const double +ln2_hi = 6.93147180369123816490e-01, /* 3fe62e42 fee00000 */ +ln2_lo = 1.90821492927058770002e-10, /* 3dea39ef 35793c76 */ +Lg1 = 6.666666666666735130e-01, /* 3FE55555 55555593 */ +Lg2 = 3.999999999940941908e-01, /* 3FD99999 9997FA04 */ +Lg3 = 2.857142874366239149e-01, /* 3FD24924 94229359 */ +Lg4 = 2.222219843214978396e-01, /* 3FCC71C5 1D8E78AF */ +Lg5 = 1.818357216161805012e-01, /* 3FC74664 96CB03DE */ +Lg6 = 1.531383769920937332e-01, /* 3FC39A09 D078C69F */ +Lg7 = 1.479819860511658591e-01; /* 3FC2F112 DF3E5244 */ + +double log(double x) +{ + union {double f; uint64_t i;} u = {x}; + double_t hfsq,f,s,z,R,w,t1,t2,dk; + uint32_t hx; + int k; + + hx = u.i>>32; + k = 0; + if (hx < 0x00100000 || hx>>31) { + if (u.i<<1 == 0) + return -1/(x*x); /* log(+-0)=-inf */ + if (hx>>31) + return (x-x)/0.0; /* log(-#) = NaN */ + /* subnormal number, scale x up */ + k -= 54; + x *= 0x1p54; + u.f = x; + hx = u.i>>32; + } else if (hx >= 0x7ff00000) { + return x; + } else if (hx == 0x3ff00000 && u.i<<32 == 0) + return 0; + + /* reduce x into [sqrt(2)/2, sqrt(2)] */ + hx += 0x3ff00000 - 0x3fe6a09e; + k += (int)(hx>>20) - 0x3ff; + hx = (hx&0x000fffff) + 0x3fe6a09e; + u.i = (uint64_t)hx<<32 | (u.i&0xffffffff); + x = u.f; + + f = x - 1.0; + hfsq = 0.5*f*f; + s = f/(2.0+f); + z = s*s; + w = z*z; + t1 = w*(Lg2+w*(Lg4+w*Lg6)); + t2 = z*(Lg1+w*(Lg3+w*(Lg5+w*Lg7))); + R = t2 + t1; + dk = k; + return s*(hfsq+R) + dk*ln2_lo - hfsq + f + dk*ln2_hi; +} |