newlib/libm/common/s_scalbn.c

   1
   2 /* @(#)s_scalbn.c 5.1 93/09/24 */
   3 /*
   4  * ====================================================
   5  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
   6  *
   7  * Developed at SunPro, a Sun Microsystems, Inc. business.
   8  * Permission to use, copy, modify, and distribute this
   9  * software is freely granted, provided that this notice
  10  * is preserved.
  11  * ====================================================
  12  */
  13
  14 /*
  15 FUNCTION
  16 <<scalbn>>, <<scalbnf>>, <<scalbln>>, <<scalblnf>>---scale by power of FLT_RADIX (=2)
  17 INDEX
  18         scalbn
  19 INDEX
  20         scalbnf
  21 INDEX
  22         scalbln
  23 INDEX
  24         scalblnf
  25
  26 SYNOPSIS
  27         #include <math.h>
  28         double scalbn(double <[x]>, int <[n]>);
  29         float scalbnf(float <[x]>, int <[n]>);
  30         double scalbln(double <[x]>, long int <[n]>);
  31         float scalblnf(float <[x]>, long int <[n]>);
  32
  33 DESCRIPTION
  34 The <<scalbn>> and <<scalbln>> functions compute
  35         @ifnottex
  36         <[x]> times FLT_RADIX to the power <[n]>.
  37         @end ifnottex
  38         @tex
  39         $x \cdot FLT\_RADIX^n$.
  40         @end tex
  41 efficiently.  The result is computed by manipulating the exponent, rather than
  42 by actually performing an exponentiation or multiplication.  In this
  43 floating-point implementation FLT_RADIX=2, which makes the <<scalbn>>
  44 functions equivalent to the <<ldexp>> functions.
  45
  46 RETURNS
  47 <[x]> times 2 to the power <[n]>.  A range error may occur.
  48
  49 PORTABILITY
  50 ANSI C, POSIX
  51
  52 SEEALSO
  53 <<ldexp>>
  54
  55 */
  56
  57 /*
  58  * scalbn (double x, int n)
  59  * scalbn(x,n) returns x* 2**n  computed by  exponent
  60  * manipulation rather than by actually performing an
  61  * exponentiation or a multiplication.
  62  */
  63
  64 #include "fdlibm.h"
  65
  66 #ifndef _DOUBLE_IS_32BITS
  67
  68 #ifdef __STDC__
  69 static const double
  70 #else
  71 static double
  72 #endif
  73 two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
  74 twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
  75 huge   = 1.0e+300,
  76 tiny   = 1.0e-300;
  77
  78 #ifdef __STDC__
  79         double scalbn (double x, int n)
  80 #else
  81         double scalbn (x,n)
  82         double x; int n;
  83 #endif
  84 {
  85         __int32_t  k,hx,lx;
  86         EXTRACT_WORDS(hx,lx,x);
  87         k = (hx&0x7ff00000)>>20;                /* extract exponent */
  88         if (k==0) {                             /* 0 or subnormal x */
  89             if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
  90             x *= two54;
  91             GET_HIGH_WORD(hx,x);
  92             k = ((hx&0x7ff00000)>>20) - 54;
  93             if (n< -50000) return tiny*x;       /*underflow*/
  94             }
  95         if (k==0x7ff) return x+x;               /* NaN or Inf */
  96         if (n > 50000)  /* in case integer overflow in n+k */
  97             return huge*copysign(huge,x);       /*overflow*/
  98         k = k+n;
  99         if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
 100         if (k > 0)                              /* normal result */
 101             {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
 102         if (k <= -54)
 103             return tiny*copysign(tiny,x);       /*underflow*/
 104         k += 54;                                /* subnormal result */
 105         SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
 106         return x*twom54;
 107 }
 108
 109 #endif /* _DOUBLE_IS_32BITS */