lib/libm/src/s_scalbn.c

   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  * $NetBSD: s_scalbn.c,v 1.12 2002/05/26 22:01:58 wiz Exp $
  13  * $DragonFly: src/lib/libm/src/s_scalbn.c,v 1.1 2005/07/26 21:15:20 joerg Exp $
  14  */
  15
  16 /*
  17  * scalbn (double x, int n)
  18  * scalbn(x,n) returns x* 2**n  computed by  exponent
  19  * manipulation rather than by actually performing an
  20  * exponentiation or a multiplication.
  21  */
  22
  23 #include <math.h>
  24 #include "math_private.h"
  25
  26 static const double
  27 two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
  28 twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
  29 huge   = 1.0e+300,
  30 tiny   = 1.0e-300;
  31
  32 double
  33 scalbn(double x, int n)
  34 {
  35         int32_t k,hx,lx;
  36         EXTRACT_WORDS(hx,lx,x);
  37         k = (hx&0x7ff00000)>>20;                /* extract exponent */
  38         if (k==0) {                             /* 0 or subnormal x */
  39             if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
  40             x *= two54;
  41             GET_HIGH_WORD(hx,x);
  42             k = ((hx&0x7ff00000)>>20) - 54;
  43             if (n< -50000) return tiny*x;       /*underflow*/
  44             }
  45         if (k==0x7ff) return x+x;               /* NaN or Inf */
  46         k = k+n;
  47         if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
  48         if (k > 0)                              /* normal result */
  49             {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
  50         if (k <= -54) {
  51             if (n > 50000)      /* in case integer overflow in n+k */
  52                 return huge*copysign(huge,x);   /*overflow*/
  53             else return tiny*copysign(tiny,x);  /*underflow*/
  54         }
  55         k += 54;                                /* subnormal result */
  56         SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
  57         return x*twom54;
  58 }