sysdeps/ieee754/dbl-64/s_scalbn.c

   1 /*
   2  * ====================================================
   3  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
   4  *
   5  * Developed at SunPro, a Sun Microsystems, Inc. business.
   6  * Permission to use, copy, modify, and distribute this
   7  * software is freely granted, provided that this notice
   8  * is preserved.
   9  * ====================================================
  10  */
  11
  12 /*
  13  * scalbn (double x, int n)
  14  * scalbn(x,n) returns x* 2**n  computed by  exponent
  15  * manipulation rather than by actually performing an
  16  * exponentiation or a multiplication.
  17  */
  18
  19 #include <math.h>
  20 #include <math_private.h>
  21
  22 static const double
  23 two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
  24 twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
  25 huge   = 1.0e+300,
  26 tiny   = 1.0e-300;
  27
  28 double
  29 __scalbn (double x, int n)
  30 {
  31         int32_t k,hx,lx;
  32         EXTRACT_WORDS(hx,lx,x);
  33         k = (hx&0x7ff00000)>>20;                /* extract exponent */
  34         if (__builtin_expect(k==0, 0)) {        /* 0 or subnormal x */
  35             if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
  36             x *= two54;
  37             GET_HIGH_WORD(hx,x);
  38             k = ((hx&0x7ff00000)>>20) - 54;
  39             }
  40         if (__builtin_expect(k==0x7ff, 0)) return x+x;  /* NaN or Inf */
  41         if (__builtin_expect(n< -50000, 0))
  42           return tiny*__copysign(tiny,x); /*underflow*/
  43         if (__builtin_expect(n> 50000 || k+n > 0x7fe, 0))
  44           return huge*__copysign(huge,x); /* overflow  */
  45         /* Now k and n are bounded we know that k = k+n does not
  46            overflow.  */
  47         k = k+n;
  48         if (__builtin_expect(k > 0, 1))         /* normal result */
  49             {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
  50         if (k <= -54)
  51           return tiny*__copysign(tiny,x);       /*underflow*/
  52         k += 54;                                /* subnormal result */
  53         SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
  54         return x*twom54;
  55 }
  56 weak_alias (__scalbn, scalbn)
  57 #ifdef NO_LONG_DOUBLE
  58 strong_alias (__scalbn, __scalbnl)
  59 weak_alias (__scalbn, scalbnl)
  60 #endif