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Diffstat (limited to 'lib/mlibc/options/ansi/musl-generic-math/log2.c')
| -rw-r--r-- | lib/mlibc/options/ansi/musl-generic-math/log2.c | 122 |
1 files changed, 122 insertions, 0 deletions
diff --git a/lib/mlibc/options/ansi/musl-generic-math/log2.c b/lib/mlibc/options/ansi/musl-generic-math/log2.c new file mode 100644 index 0000000..0aafad4 --- /dev/null +++ b/lib/mlibc/options/ansi/musl-generic-math/log2.c @@ -0,0 +1,122 @@ +/* origin: FreeBSD /usr/src/lib/msun/src/e_log2.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. + * ==================================================== + */ +/* + * Return the base 2 logarithm of x. See log.c for most comments. + * + * Reduce x to 2^k (1+f) and calculate r = log(1+f) - f + f*f/2 + * as in log.c, then combine and scale in extra precision: + * log2(x) = (f - f*f/2 + r)/log(2) + k + */ + +#include <math.h> +#include <stdint.h> + +static const double +ivln2hi = 1.44269504072144627571e+00, /* 0x3ff71547, 0x65200000 */ +ivln2lo = 1.67517131648865118353e-10, /* 0x3de705fc, 0x2eefa200 */ +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 log2(double x) +{ + union {double f; uint64_t i;} u = {x}; + double_t hfsq,f,s,z,R,w,t1,t2,y,hi,lo,val_hi,val_lo; + 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; + + /* + * f-hfsq must (for args near 1) be evaluated in extra precision + * to avoid a large cancellation when x is near sqrt(2) or 1/sqrt(2). + * This is fairly efficient since f-hfsq only depends on f, so can + * be evaluated in parallel with R. Not combining hfsq with R also + * keeps R small (though not as small as a true `lo' term would be), + * so that extra precision is not needed for terms involving R. + * + * Compiler bugs involving extra precision used to break Dekker's + * theorem for spitting f-hfsq as hi+lo, unless double_t was used + * or the multi-precision calculations were avoided when double_t + * has extra precision. These problems are now automatically + * avoided as a side effect of the optimization of combining the + * Dekker splitting step with the clear-low-bits step. + * + * y must (for args near sqrt(2) and 1/sqrt(2)) be added in extra + * precision to avoid a very large cancellation when x is very near + * these values. Unlike the above cancellations, this problem is + * specific to base 2. It is strange that adding +-1 is so much + * harder than adding +-ln2 or +-log10_2. + * + * This uses Dekker's theorem to normalize y+val_hi, so the + * compiler bugs are back in some configurations, sigh. And I + * don't want to used double_t to avoid them, since that gives a + * pessimization and the support for avoiding the pessimization + * is not yet available. + * + * The multi-precision calculations for the multiplications are + * routine. + */ + + /* hi+lo = f - hfsq + s*(hfsq+R) ~ log(1+f) */ + hi = f - hfsq; + u.f = hi; + u.i &= (uint64_t)-1<<32; + hi = u.f; + lo = f - hi - hfsq + s*(hfsq+R); + + val_hi = hi*ivln2hi; + val_lo = (lo+hi)*ivln2lo + lo*ivln2hi; + + /* spadd(val_hi, val_lo, y), except for not using double_t: */ + y = k; + w = y + val_hi; + val_lo += (y - w) + val_hi; + val_hi = w; + + return val_lo + val_hi; +} |