2 * ====================================================
3 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
5 * Developed at SunPro, a Sun Microsystems, Inc. business.
6 * Permission to use, copy, modify, and distribute this
7 * software is freely granted, provided that this notice
9 * ====================================================
10 * $FreeBSD: head/lib/msun/src/e_log2f.c 251024 2013-05-27 08:50:10Z das $
14 * Float version of e_log2.c. See the latter for most comments.
18 #include "math_private.h"
22 two25 = 3.3554432000e+07, /* 0x4c000000 */
23 ivln2hi = 1.4428710938e+00, /* 0x3fb8b000 */
24 ivln2lo = -1.7605285393e-04; /* 0xb9389ad4 */
26 static const float zero = 0.0;
27 static volatile float vzero = 0.0;
30 __ieee754_log2f(float x)
32 float f,hfsq,hi,lo,r,y;
38 if (hx < 0x00800000) { /* x < 2**-126 */
39 if ((hx&0x7fffffff)==0)
40 return -two25/vzero; /* log(+-0)=-inf */
41 if (hx<0) return (x-x)/zero; /* log(-#) = NaN */
42 k -= 25; x *= two25; /* subnormal number, scale up x */
45 if (hx >= 0x7f800000) return x+x;
47 return zero; /* log(1) = +0 */
50 i = (hx+(0x4afb0d))&0x800000;
51 SET_FLOAT_WORD(x,hx|(i^0x3f800000)); /* normalize x or x/2 */
55 hfsq = (float)0.5*f*f;
59 * We no longer need to avoid falling into the multi-precision
60 * calculations due to compiler bugs breaking Dekker's theorem.
61 * Keep avoiding this as an optimization. See e_log2.c for more
62 * details (some details are here only because the optimization
63 * is not yet available in double precision).
65 * Another compiler bug turned up. With gcc on i386,
66 * (ivln2lo + ivln2hi) would be evaluated in float precision
67 * despite runtime evaluations using double precision. So we
68 * must cast one of its terms to float_t. This makes the whole
69 * expression have type float_t, so return is forced to waste
70 * time clobbering its extra precision.
72 if (sizeof(float_t) > sizeof(float))
73 return (r - hfsq + f) * ((float_t)ivln2lo + ivln2hi) + y;
76 GET_FLOAT_WORD(hx,hi);
77 SET_FLOAT_WORD(hi,hx&0xfffff000);
78 lo = (f - hi) - hfsq + r;
79 return (lo+hi)*ivln2lo + lo*ivln2hi + hi*ivln2hi + y;