1 files changed, 81 insertions, 0 deletions
diff --git a/src/math/log2f.c b/src/math/log2f.c
new file mode 100644
index 00000000..a968984d
--- /dev/null
+++ b/src/math/log2f.c
@@ -0,0 +1,81 @@
+/* origin: FreeBSD /usr/src/lib/msun/src/e_log2f.c */
+/*
+ * ====================================================
+ * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
+ *
+ * Developed at SunPro, a Sun Microsystems, Inc. business.
+ * Permission to use, copy, modify, and distribute this
+ * software is freely granted, provided that this notice
+ * is preserved.
+ * ====================================================
+ */
+/*
+ * See comments in log2.c.
+ */
+
+#include "libm.h"
+#include "__log1pf.h"
+
+static const float
+two25   =  3.3554432000e+07, /* 0x4c000000 */
+ivln2hi =  1.4428710938e+00, /* 0x3fb8b000 */
+ivln2lo = -1.7605285393e-04; /* 0xb9389ad4 */
+
+static const float zero = 0.0;
+
+float log2f(float x)
+{
+	float f,hfsq,hi,lo,r,y;
+	int32_t i,k,hx;
+
+	GET_FLOAT_WORD(hx, x);
+
+	k = 0;
+	if (hx < 0x00800000) {  /* x < 2**-126  */
+		if ((hx&0x7fffffff) == 0)
+			return -two25/zero;  /* log(+-0)=-inf */
+		if (hx < 0)
+			return (x-x)/zero;   /* log(-#) = NaN */
+		/* subnormal number, scale up x */
+		k -= 25;
+		x *= two25;
+		GET_FLOAT_WORD(hx, x);
+	}
+	if (hx >= 0x7f800000)
+		return x+x;
+	if (hx == 0x3f800000)
+		return zero;  /* log(1) = +0 */
+	k += (hx>>23) - 127;
+	hx &= 0x007fffff;
+	i = (hx+(0x4afb0d))&0x800000;
+	SET_FLOAT_WORD(x, hx|(i^0x3f800000));  /* normalize x or x/2 */
+	k += i>>23;
+	y = (float)k;
+	f = x - (float)1.0;
+	hfsq = (float)0.5*f*f;
+	r = __log1pf(f);
+
+	/*
+	 * We no longer need to avoid falling into the multi-precision
+	 * calculations due to compiler bugs breaking Dekker's theorem.
+	 * Keep avoiding this as an optimization.  See log2.c for more
+	 * details (some details are here only because the optimization
+	 * is not yet available in double precision).
+	 *
+	 * Another compiler bug turned up.  With gcc on i386,
+	 * (ivln2lo + ivln2hi) would be evaluated in float precision
+	 * despite runtime evaluations using double precision.  So we
+	 * must cast one of its terms to float_t.  This makes the whole
+	 * expression have type float_t, so return is forced to waste
+	 * time clobbering its extra precision.
+	 */
+// FIXME
+//      if (sizeof(float_t) > sizeof(float))
+//              return (r - hfsq + f) * ((float_t)ivln2lo + ivln2hi) + y;
+
+	hi = f - hfsq;
+	GET_FLOAT_WORD(hx,hi);
+	SET_FLOAT_WORD(hi,hx&0xfffff000);
+	lo = (f - hi) - hfsq + r;
+	return (lo+hi)*ivln2lo + lo*ivln2hi + hi*ivln2hi + y;
+}