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      1 /* @(#)s_scalbn.c 5.1 93/09/24 */
      2 /*
      3  * ====================================================
      4  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
      5  *
      6  * Developed at SunPro, a Sun Microsystems, Inc. business.
      7  * Permission to use, copy, modify, and distribute this
      8  * software is freely granted, provided that this notice
      9  * is preserved.
     10  * ====================================================
     11  */
     12 
     13 #ifndef lint
     14 static char rcsid[] = "$FreeBSD$";
     15 #endif
     16 
     17 /*
     18  * scalbn (double x, int n)
     19  * scalbn(x,n) returns x* 2**n  computed by  exponent
     20  * manipulation rather than by actually performing an
     21  * exponentiation or a multiplication.
     22  */
     23 
     24 #include <sys/cdefs.h>
     25 #include <float.h>
     26 
     27 #include "math.h"
     28 #include "math_private.h"
     29 
     30 static const double
     31 two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
     32 twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
     33 huge   = 1.0e+300,
     34 tiny   = 1.0e-300;
     35 
     36 double
     37 scalbn (double x, int n)
     38 {
     39 	int32_t k,hx,lx;
     40 	EXTRACT_WORDS(hx,lx,x);
     41         k = (hx&0x7ff00000)>>20;		/* extract exponent */
     42         if (k==0) {				/* 0 or subnormal x */
     43             if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
     44 	    x *= two54;
     45 	    GET_HIGH_WORD(hx,x);
     46 	    k = ((hx&0x7ff00000)>>20) - 54;
     47             if (n< -50000) return tiny*x; 	/*underflow*/
     48 	    }
     49         if (k==0x7ff) return x+x;		/* NaN or Inf */
     50         k = k+n;
     51         if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
     52         if (k > 0) 				/* normal result */
     53 	    {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
     54         if (k <= -54)
     55             if (n > 50000) 	/* in case integer overflow in n+k */
     56 		return huge*copysign(huge,x);	/*overflow*/
     57 	    else return tiny*copysign(tiny,x); 	/*underflow*/
     58         k += 54;				/* subnormal result */
     59 	SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
     60         return x*twom54;
     61 }
     62 
     63 #if (LDBL_MANT_DIG == 53)
     64 __weak_reference(scalbn, ldexpl);
     65 __weak_reference(scalbn, scalbnl);
     66 #endif
     67