Add a "fork" of musl as //fusl.

fusl/src/math/log2.c

+/* 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;
+}