lib/libc/gen/ldexp.c

   1 /* @(#)s_scalbn.c 5.1 93/09/24 */
   2 /* @(#)fdlibm.h 5.1 93/09/24 */
   3 /* $FreeBSD: head/lib/libc/gen/ldexp.c 140607 2005-01-22 06:03:40Z das $ */
   4 /*
   5  * ====================================================
   6  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
   7  *
   8  * Developed at SunPro, a Sun Microsystems, Inc. business.
   9  * Permission to use, copy, modify, and distribute this
  10  * software is freely granted, provided that this notice
  11  * is preserved.
  12  * ====================================================
  13  */
  14
  15 #include <sys/types.h>
  16 #include <machine/endian.h>
  17 #include <math.h>
  18
  19 /* Bit fiddling routines copied from msun/src/math_private.h,v 1.15 */
  20
  21 #if BYTE_ORDER == BIG_ENDIAN
  22
  23 typedef union
  24 {
  25   double value;
  26   struct
  27   {
  28     u_int32_t msw;
  29     u_int32_t lsw;
  30   } parts;
  31 } ieee_double_shape_type;
  32
  33 #endif
  34
  35 #if BYTE_ORDER == LITTLE_ENDIAN
  36
  37 typedef union
  38 {
  39   double value;
  40   struct
  41   {
  42     u_int32_t lsw;
  43     u_int32_t msw;
  44   } parts;
  45 } ieee_double_shape_type;
  46
  47 #endif
  48
  49 /* Get two 32 bit ints from a double.  */
  50
  51 #define EXTRACT_WORDS(ix0,ix1,d)                                \
  52 do {                                                            \
  53   ieee_double_shape_type ew_u;                                  \
  54   ew_u.value = (d);                                             \
  55   (ix0) = ew_u.parts.msw;                                       \
  56   (ix1) = ew_u.parts.lsw;                                       \
  57 } while (0)
  58
  59 /* Get the more significant 32 bit int from a double.  */
  60
  61 #define GET_HIGH_WORD(i,d)                                      \
  62 do {                                                            \
  63   ieee_double_shape_type gh_u;                                  \
  64   gh_u.value = (d);                                             \
  65   (i) = gh_u.parts.msw;                                         \
  66 } while (0)
  67
  68 /* Set the more significant 32 bits of a double from an int.  */
  69
  70 #define SET_HIGH_WORD(d,v)                                      \
  71 do {                                                            \
  72   ieee_double_shape_type sh_u;                                  \
  73   sh_u.value = (d);                                             \
  74   sh_u.parts.msw = (v);                                         \
  75   (d) = sh_u.value;                                             \
  76 } while (0)
  77
  78
  79 static const double
  80 two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
  81 twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
  82 huge   = 1.0e+300,
  83 tiny   = 1.0e-300;
  84
  85 static double
  86 _copysign(double x, double y)
  87 {
  88         u_int32_t hx,hy;
  89         GET_HIGH_WORD(hx,x);
  90         GET_HIGH_WORD(hy,y);
  91         SET_HIGH_WORD(x,(hx&0x7fffffff)|(hy&0x80000000));
  92         return x;
  93 }
  94
  95 double
  96 ldexp(double x, int n)
  97 {
  98         int32_t k,hx,lx;
  99         EXTRACT_WORDS(hx,lx,x);
 100         k = (hx&0x7ff00000)>>20;                /* extract exponent */
 101         if (k==0) {                             /* 0 or subnormal x */
 102             if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
 103             x *= two54;
 104             GET_HIGH_WORD(hx,x);
 105             k = ((hx&0x7ff00000)>>20) - 54;
 106             if (n< -50000) return tiny*x;       /*underflow*/
 107             }
 108         if (k==0x7ff) return x+x;               /* NaN or Inf */
 109         k = k+n;
 110         if (k >  0x7fe) return huge*_copysign(huge,x); /* overflow  */
 111         if (k > 0)                              /* normal result */
 112             {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
 113         if (k <= -54) {
 114             if (n > 50000)      /* in case integer overflow in n+k */
 115                 return huge*_copysign(huge,x);  /*overflow*/
 116             else return tiny*_copysign(tiny,x);         /*underflow*/
 117         }
 118         k += 54;                                /* subnormal result */
 119         SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
 120         return x*twom54;
 121 }