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      1 /*
      2  * ====================================================
      3  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
      4  *
      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
      8  * is preserved.
      9  * ====================================================
     10  */
     11 
     12 #include <sys/cdefs.h>
     13 __FBSDID("$FreeBSD$");
     14 
     15 /*
     16  * Float version of e_log2.c.  See the latter for most comments.
     17  */
     18 
     19 #include "math.h"
     20 #include "math_private.h"
     21 #include "k_logf.h"
     22 
     23 static const float
     24 two25      =  3.3554432000e+07, /* 0x4c000000 */
     25 ivln2hi    =  1.4428710938e+00, /* 0x3fb8b000 */
     26 ivln2lo    = -1.7605285393e-04; /* 0xb9389ad4 */
     27 
     28 static const float zero   =  0.0;
     29 static volatile float vzero = 0.0;
     30 
     31 float
     32 __ieee754_log2f(float x)
     33 {
     34 	float f,hfsq,hi,lo,r,y;
     35 	int32_t i,k,hx;
     36 
     37 	GET_FLOAT_WORD(hx,x);
     38 
     39 	k=0;
     40 	if (hx < 0x00800000) {			/* x < 2**-126  */
     41 	    if ((hx&0x7fffffff)==0)
     42 		return -two25/vzero;		/* log(+-0)=-inf */
     43 	    if (hx<0) return (x-x)/zero;	/* log(-#) = NaN */
     44 	    k -= 25; x *= two25; /* subnormal number, scale up x */
     45 	    GET_FLOAT_WORD(hx,x);
     46 	}
     47 	if (hx >= 0x7f800000) return x+x;
     48 	if (hx == 0x3f800000)
     49 	    return zero;			/* log(1) = +0 */
     50 	k += (hx>>23)-127;
     51 	hx &= 0x007fffff;
     52 	i = (hx+(0x4afb0d))&0x800000;
     53 	SET_FLOAT_WORD(x,hx|(i^0x3f800000));	/* normalize x or x/2 */
     54 	k += (i>>23);
     55 	y = (float)k;
     56 	f = x - (float)1.0;
     57 	hfsq = (float)0.5*f*f;
     58 	r = k_log1pf(f);
     59 
     60 	/*
     61 	 * We no longer need to avoid falling into the multi-precision
     62 	 * calculations due to compiler bugs breaking Dekker's theorem.
     63 	 * Keep avoiding this as an optimization.  See e_log2.c for more
     64 	 * details (some details are here only because the optimization
     65 	 * is not yet available in double precision).
     66 	 *
     67 	 * Another compiler bug turned up.  With gcc on i386,
     68 	 * (ivln2lo + ivln2hi) would be evaluated in float precision
     69 	 * despite runtime evaluations using double precision.  So we
     70 	 * must cast one of its terms to float_t.  This makes the whole
     71 	 * expression have type float_t, so return is forced to waste
     72 	 * time clobbering its extra precision.
     73 	 */
     74 	if (sizeof(float_t) > sizeof(float))
     75 		return (r - hfsq + f) * ((float_t)ivln2lo + ivln2hi) + y;
     76 
     77 	hi = f - hfsq;
     78 	GET_FLOAT_WORD(hx,hi);
     79 	SET_FLOAT_WORD(hi,hx&0xfffff000);
     80 	lo = (f - hi) - hfsq + r;
     81 	return (lo+hi)*ivln2lo + lo*ivln2hi + hi*ivln2hi + y;
     82 }
     83