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+/*
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+ * ====================================================
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+ * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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+ *
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+ * Developed at SunPro, a Sun Microsystems, Inc. business.
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+ * Permission to use, copy, modify, and distribute this
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+ * software is freely granted, provided that this notice
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+ * is preserved.
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+ * ====================================================
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+ */
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+
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+#include "cdefs-compat.h"
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+//__FBSDID("$FreeBSD: src/lib/msun/src/e_log2f.c,v 1.5 2011/10/15 05:23:28 das Exp $");
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+
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+/*
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+ * Float version of e_log2.c. See the latter for most comments.
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+ */
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+
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+#include <openlibm_math.h>
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+
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+#include "math_private.h"
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+#include "k_logf.h"
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+
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+// VBS
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+#define float_t float
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+
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+static const float
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+two25 = 3.3554432000e+07, /* 0x4c000000 */
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+ivln2hi = 1.4428710938e+00, /* 0x3fb8b000 */
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+ivln2lo = -1.7605285393e-04; /* 0xb9389ad4 */
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+
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+static const float zero = 0.0;
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+
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+OLM_DLLEXPORT float
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+__ieee754_log2f(float x)
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+{
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+ float f,hfsq,hi,lo,r,y;
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+ int32_t i,k,hx;
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+
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+ GET_FLOAT_WORD(hx,x);
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+
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+ k=0;
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+ if (hx < 0x00800000) { /* x < 2**-126 */
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+ if ((hx&0x7fffffff)==0)
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+ return -two25/zero; /* log(+-0)=-inf */
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+ if (hx<0) return (x-x)/zero; /* log(-#) = NaN */
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+ k -= 25; x *= two25; /* subnormal number, scale up x */
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+ GET_FLOAT_WORD(hx,x);
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+ }
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+ if (hx >= 0x7f800000) return x+x;
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+ if (hx == 0x3f800000)
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+ return zero; /* log(1) = +0 */
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+ k += (hx>>23)-127;
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+ hx &= 0x007fffff;
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+ i = (hx+(0x4afb0d))&0x800000;
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+ SET_FLOAT_WORD(x,hx|(i^0x3f800000)); /* normalize x or x/2 */
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+ k += (i>>23);
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+ y = (float)k;
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+ f = x - (float)1.0;
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+ hfsq = (float)0.5*f*f;
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+ r = k_log1pf(f);
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+
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+ /*
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+ * We no longer need to avoid falling into the multi-precision
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+ * calculations due to compiler bugs breaking Dekker's theorem.
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+ * Keep avoiding this as an optimization. See e_log2.c for more
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+ * details (some details are here only because the optimization
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+ * is not yet available in double precision).
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+ *
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+ * Another compiler bug turned up. With gcc on i386,
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+ * (ivln2lo + ivln2hi) would be evaluated in float precision
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+ * despite runtime evaluations using double precision. So we
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+ * must cast one of its terms to float_t. This makes the whole
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+ * expression have type float_t, so return is forced to waste
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+ * time clobbering its extra precision.
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+ */
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+ if (sizeof(float_t) > sizeof(float))
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+ return (r - hfsq + f) * ((float_t)ivln2lo + ivln2hi) + y;
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+
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+ hi = f - hfsq;
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+ GET_FLOAT_WORD(hx,hi);
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+ SET_FLOAT_WORD(hi,hx&0xfffff000);
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+ lo = (f - hi) - hfsq + r;
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+ return (lo+hi)*ivln2lo + lo*ivln2hi + hi*ivln2hi + y;
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+} |