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-rw-r--r--src/math/log2f.c81
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diff --git a/src/math/log2f.c b/src/math/log2f.c
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+/* 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;
+}