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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 #include <sys/cdefs.h>
     14 #if defined(LIBM_SCCS) && !defined(lint)
     15 __RCSID("$NetBSD: s_scalbn.c,v 1.19 2018/03/27 11:59:49 martin Exp $");
     16 #endif
     17 
     18 /*
     19  * scalbn (double x, int n)
     20  * scalbn(x,n) returns x* 2**n  computed by  exponent
     21  * manipulation rather than by actually performing an
     22  * exponentiation or a multiplication.
     23  */
     24 
     25 #include "namespace.h"
     26 #include "math.h"
     27 #include "math_private.h"
     28 
     29 #ifndef _LP64
     30 __strong_alias(_scalbn, _scalbln)
     31 #endif
     32 
     33 #ifndef __HAVE_LONG_DOUBLE
     34 __strong_alias(_scalbnl, _scalbn)
     35 __strong_alias(_scalblnl, _scalbln)
     36 __weak_alias(scalbnl, _scalbnl)
     37 __weak_alias(scalblnl, _scalblnl)
     38 __weak_alias(ldexpl, _scalblnl)
     39 #endif
     40 
     41 #ifdef __weak_alias
     42 __weak_alias(scalbn, _scalbn)
     43 __weak_alias(scalbln, _scalbln)
     44 __weak_alias(ldexp, _scalbn)
     45 #endif
     46 
     47 static const double
     48 two54   =  0x1.0p54,	/* 0x43500000, 0x00000000 */
     49 twom54  =  0x1.0p-54,	/* 0x3C900000, 0x00000000 */
     50 huge   = 1.0e+300,
     51 tiny   = 1.0e-300;
     52 
     53 #ifdef _LP64
     54 double
     55 scalbn(double x, int n)
     56 {
     57 	return scalbln(x, n);
     58 }
     59 #endif
     60 
     61 double
     62 scalbln(double x, long n)
     63 {
     64 	int32_t k,hx,lx;
     65 	EXTRACT_WORDS(hx,lx,x);
     66         k = ((uint32_t)hx&0x7ff00000)>>20;		/* extract exponent */
     67         if (k==0) {				/* 0 or subnormal x */
     68             if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
     69 	    x *= two54;
     70 	    GET_HIGH_WORD(hx,x);
     71 	    k = (((uint32_t)hx&0x7ff00000)>>20) - 54;
     72             if (n< -50000) return tiny*x; 	/*underflow*/
     73 	    }
     74         if (k==0x7ff) return x+x;		/* NaN or Inf */
     75         k = k+n;
     76         if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
     77         if (k > 0) 				/* normal result */
     78 	    {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
     79         if (k <= -54) {
     80             if (n > 50000) 	/* in case integer overflow in n+k */
     81 		return huge*copysign(huge,x);	/*overflow*/
     82 	    else return tiny*copysign(tiny,x); 	/*underflow*/
     83 	}
     84         k += 54;				/* subnormal result */
     85 	SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
     86         return x*twom54;
     87 }
     88