js/src/fdlibm/s_scalbn.c

   1 /* -*- Mode: C; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
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  12  * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
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  14  * License.
  15  *
  16  * The Original Code is Mozilla Communicator client code, released
  17  * March 31, 1998.
  18  *
  19  * The Initial Developer of the Original Code is
  20  * Sun Microsystems, Inc.
  21  * Portions created by the Initial Developer are Copyright (C) 1998
  22  * the Initial Developer. All Rights Reserved.
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  39
  40 /* @(#)s_scalbn.c 1.3 95/01/18 */
  41 /*
  42  * ====================================================
  43  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
  44  *
  45  * Developed at SunSoft, a Sun Microsystems, Inc. business.
  46  * Permission to use, copy, modify, and distribute this
  47  * software is freely granted, provided that this notice
  48  * is preserved.
  49  * ====================================================
  50  */
  51
  52 /*
  53  * scalbn (double x, int n)
  54  * scalbn(x,n) returns x* 2**n  computed by  exponent
  55  * manipulation rather than by actually performing an
  56  * exponentiation or a multiplication.
  57  */
  58
  59 #include "fdlibm.h"
  60
  61 #ifdef __STDC__
  62 static const double
  63 #else
  64 static double
  65 #endif
  66 two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
  67 twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
  68 really_big   = 1.0e+300,
  69 tiny   = 1.0e-300;
  70
  71 #ifdef __STDC__
  72         double fd_scalbn (double x, int n)
  73 #else
  74         double fd_scalbn (x,n)
  75         double x; int n;
  76 #endif
  77 {
  78         fd_twoints u;
  79         int  k,hx,lx;
  80         u.d = x;
  81         hx = __HI(u);
  82         lx = __LO(u);
  83         k = (hx&0x7ff00000)>>20;                /* extract exponent */
  84         if (k==0) {                             /* 0 or subnormal x */
  85             if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
  86             x *= two54;
  87             u.d = x;
  88             hx = __HI(u);
  89             k = ((hx&0x7ff00000)>>20) - 54;
  90             if (n< -50000) return tiny*x;       /*underflow*/
  91             }
  92         if (k==0x7ff) return x+x;               /* NaN or Inf */
  93         k = k+n;
  94         if (k >  0x7fe) return really_big*fd_copysign(really_big,x); /* overflow  */
  95         if (k > 0)                              /* normal result */
  96             {u.d = x; __HI(u) = (hx&0x800fffff)|(k<<20); x = u.d; return x;}
  97         if (k <= -54) {
  98             if (n > 50000)      /* in case integer overflow in n+k */
  99                 return really_big*fd_copysign(really_big,x);    /*overflow*/
 100             else return tiny*fd_copysign(tiny,x);       /*underflow*/
 101         }
 102         k += 54;                                /* subnormal result */
 103         u.d = x;
 104         __HI(u) = (hx&0x800fffff)|(k<<20);
 105         x = u.d;
 106         return x*twom54;
 107 }