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+
+/* @(#)e_log10.c 1.3 95/01/18 */
+/*
+ * ====================================================
+ * 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.
+ * ====================================================
+ */
+
+//#include <sys/cdefs.h>
+//__FBSDID("$FreeBSD$");
+
+/*
+ * Return the base 2 logarithm of x. See e_log.c and k_log.h for most
+ * comments.
+ *
+ * This reduces x to {k, 1+f} exactly as in e_log.c, then calls the kernel,
+ * then does the combining and scaling steps
+ * log2(x) = (f - 0.5*f*f + k_log1p(f)) / ln2 + k
+ * in not-quite-routine extra precision.
+ */
+
+#include <float.h>
+
+#include "math_private.h"
+#include "k_log.h"
+
+static const double
+two54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
+ivln2hi = 1.44269504072144627571e+00, /* 0x3ff71547, 0x65200000 */
+ivln2lo = 1.67517131648865118353e-10; /* 0x3de705fc, 0x2eefa200 */
+
+static const double zero = 0.0;
+static volatile double vzero = 0.0;
+
+double
+__ieee754_log2(double x)
+{
+ double f,hfsq,hi,lo,r,val_hi,val_lo,w,y;
+ int32_t i,k,hx;
+ u_int32_t lx;
+
+ EXTRACT_WORDS(hx,lx,x);
+
+ k=0;
+ if (hx < 0x00100000) { /* x < 2**-1022 */
+ if (((hx&0x7fffffff)|lx)==0)
+ return -two54/vzero; /* log(+-0)=-inf */
+ if (hx<0) return (x-x)/zero; /* log(-#) = NaN */
+ k -= 54; x *= two54; /* subnormal number, scale up x */
+ GET_HIGH_WORD(hx,x);
+ }
+ if (hx >= 0x7ff00000) return x+x;
+ if (hx == 0x3ff00000 && lx == 0)
+ return zero; /* log(1) = +0 */
+ k += (hx>>20)-1023;
+ hx &= 0x000fffff;
+ i = (hx+0x95f64)&0x100000;
+ SET_HIGH_WORD(x,hx|(i^0x3ff00000)); /* normalize x or x/2 */
+ k += (i>>20);
+ y = (double)k;
+ f = x - 1.0;
+ hfsq = 0.5*f*f;
+ r = k_log1p(f);
+
+ /*
+ * 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 = f - hfsq;
+ SET_LOW_WORD(hi,0);
+ lo = (f - hi) - 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: */
+ w = y + val_hi;
+ val_lo += (y - w) + val_hi;
+ val_hi = w;
+
+ return val_lo + val_hi;
+}