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
  13 #include <sys/cdefs.h>
  14 #if defined(LIBM_SCCS) && !defined(lint)
  15 __RCSID("$NetBSD: s_scalbn.c,v 1.14 2010/04/23 19:17:07 drochner 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 #ifdef __weak_alias
  30 __weak_alias(scalbn, _scalbn)
  31 #endif
  32
  33 static const double
  34 two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
  35 twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
  36 huge   = 1.0e+300,
  37 tiny   = 1.0e-300;
  38
  39 double
  40 scalbn(double x, int n)
  41 {
  42         int32_t k,hx,lx;
  43         EXTRACT_WORDS(hx,lx,x);
  44         k = ((uint32_t)hx&0x7ff00000)>>20;              /* extract exponent */
  45         if (k==0) {                             /* 0 or subnormal x */
  46             if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
  47             x *= two54;
  48             GET_HIGH_WORD(hx,x);
  49             k = (((uint32_t)hx&0x7ff00000)>>20) - 54;
  50             if (n< -50000) return tiny*x;       /*underflow*/
  51             }
  52         if (k==0x7ff) return x+x;               /* NaN or Inf */
  53         k = k+n;
  54         if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
  55         if (k > 0)                              /* normal result */
  56             {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
  57         if (k <= -54) {
  58             if (n > 50000)      /* in case integer overflow in n+k */
  59                 return huge*copysign(huge,x);   /*overflow*/
  60             else return tiny*copysign(tiny,x);  /*underflow*/
  61         }
  62         k += 54;                                /* subnormal result */
  63         SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
  64         return x*twom54;
  65 }