text: /* @(#)s_logb.c 5.1 93/09/24 */
  /*
   * ====================================================
   * 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.
   * ====================================================
   */
  
  #include <sys/cdefs.h>
  __FBSDID("$FreeBSD: src/lib/msun/src/s_logb.c,v 1.12 2008/02/08 01:22:13 bde Exp $");
  
  /*
   * double logb(x)
   * IEEE 754 logb. Included to pass IEEE test suite. Not recommend.
   * Use ilogb instead.
   */
  
  #include <float.h>
  
  #include "math.h"
  #include "math_private.h"
  
  static const double
  two54 = 1.80143985094819840000e+16;	/* 43500000 00000000 */
  
  double
  logb(double x)
  {
  	int32_t lx,ix;
  	EXTRACT_WORDS(ix,lx,x);
  	ix &= 0x7fffffff;			/* high |x| */
  	if((ix|lx)==0) return -1.0/fabs(x);
  	if(ix>=0x7ff00000) return x*x;
  	if(ix<0x00100000) {
  		x *= two54;		 /* convert subnormal x to normal */
  		GET_HIGH_WORD(ix,x);
  		ix &= 0x7fffffff;
  		return (double) ((ix>>20)-1023-54);
  	} else
  		return (double) ((ix>>20)-1023);
  }
  
  #if (LDBL_MANT_DIG == 53)
  __weak_reference(logb, logbl);
  #endif
  