1 files changed, 84 insertions, 0 deletions
diff --git a/src/math/log10.c b/src/math/log10.c
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+++ b/src/math/log10.c
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+/* origin: FreeBSD /usr/src/lib/msun/src/e_log10.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 10 logarithm of x.  See e_log.c and k_log.h for most
+ * comments.
+ *
+ *    log10(x) = (f - 0.5*f*f + k_log1p(f)) / ln10 + k * log10(2)
+ * in not-quite-routine extra precision.
+ */
+
+#include "libm.h"
+#include "__log1p.h"
+
+static const double
+two54     = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
+ivln10hi  = 4.34294481878168880939e-01, /* 0x3fdbcb7b, 0x15200000 */
+ivln10lo  = 2.50829467116452752298e-11, /* 0x3dbb9438, 0xca9aadd5 */
+log10_2hi = 3.01029995663611771306e-01, /* 0x3FD34413, 0x509F6000 */
+log10_2lo = 3.69423907715893078616e-13; /* 0x3D59FEF3, 0x11F12B36 */
+
+static const double zero = 0.0;
+
+double log10(double x)
+{
+	double f,hfsq,hi,lo,r,val_hi,val_lo,w,y,y2;
+	int32_t i,k,hx;
+	uint32_t lx;
+
+	EXTRACT_WORDS(hx, lx, x);
+
+	k = 0;
+	if (hx < 0x00100000) {  /* x < 2**-1022  */
+		if (((hx&0x7fffffff)|lx) == 0)
+			return -two54/zero;  /* log(+-0)=-inf */
+		if (hx<0)
+			return (x-x)/zero;   /* log(-#) = NaN */
+		/* subnormal number, scale up x */
+		k -= 54;
+		x *= two54;
+		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 = __log1p(f);
+
+	/* See log2.c for details. */
+	hi = f - hfsq;
+	SET_LOW_WORD(hi, 0);
+	lo = (f - hi) - hfsq + r;
+	val_hi = hi*ivln10hi;
+	y2 = y*log10_2hi;
+	val_lo = y*log10_2lo + (lo+hi)*ivln10lo + lo*ivln10hi;
+
+	/*
+	 * Extra precision in for adding y*log10_2hi is not strictly needed
+	 * since there is no very large cancellation near x = sqrt(2) or
+	 * x = 1/sqrt(2), but we do it anyway since it costs little on CPUs
+	 * with some parallelism and it reduces the error for many args.
+	 */
+	w = y2 + val_hi;
+	val_lo += (y2 - w) + val_hi;
+	val_hi = w;
+
+	return val_lo + val_hi;
+}