sys/external/bsd/compiler_rt/dist/lib/builtins/floattidf.c

   1 /* ===-- floattidf.c - Implement __floattidf -------------------------------===
   2  *
   3  *                    The LLVM Compiler Infrastructure
   4  *
   5  * This file is dual licensed under the MIT and the University of Illinois Open
   6  * Source Licenses. See LICENSE.TXT for details.
   7  *
   8  * ===----------------------------------------------------------------------===
   9  *
  10  * This file implements __floattidf for the compiler_rt library.
  11  *
  12  * ===----------------------------------------------------------------------===
  13  */
  14
  15 #include "int_lib.h"
  16
  17 #ifdef CRT_HAS_128BIT
  18
  19 /* Returns: convert a to a double, rounding toward even.*/
  20
  21 /* Assumption: double is a IEEE 64 bit floating point type
  22  *            ti_int is a 128 bit integral type
  23  */
  24
  25 /* seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm */
  26
  27 COMPILER_RT_ABI double
  28 __floattidf(ti_int a)
  29 {
  30     if (a == 0)
  31         return 0.0;
  32     const unsigned N = sizeof(ti_int) * CHAR_BIT;
  33     const ti_int s = a >> (N-1);
  34     a = (a ^ s) - s;
  35     int sd = N - __clzti2(a);  /* number of significant digits */
  36     int e = sd - 1;             /* exponent */
  37     if (sd > DBL_MANT_DIG)
  38     {
  39         /* start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
  40          *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
  41          *                                               12345678901234567890123456
  42          * 1 = msb 1 bit
  43          * P = bit DBL_MANT_DIG-1 bits to the right of 1
  44          * Q = bit DBL_MANT_DIG bits to the right of 1
  45          * R = "or" of all bits to the right of Q
  46          */
  47         switch (sd)
  48         {
  49         case DBL_MANT_DIG + 1:
  50             a <<= 1;
  51             break;
  52         case DBL_MANT_DIG + 2:
  53             break;
  54         default:
  55             a = ((tu_int)a >> (sd - (DBL_MANT_DIG+2))) |
  56                 ((a & ((tu_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0);
  57         };
  58         /* finish: */
  59         a |= (a & 4) != 0;  /* Or P into R */
  60         ++a;  /* round - this step may add a significant bit */
  61         a >>= 2;  /* dump Q and R */
  62         /* a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits */
  63         if (a & ((tu_int)1 << DBL_MANT_DIG))
  64         {
  65             a >>= 1;
  66             ++e;
  67         }
  68         /* a is now rounded to DBL_MANT_DIG bits */
  69     }
  70     else
  71     {
  72         a <<= (DBL_MANT_DIG - sd);
  73         /* a is now rounded to DBL_MANT_DIG bits */
  74     }
  75     double_bits fb;
  76     fb.u.s.high = ((su_int)s & 0x80000000) |        /* sign */
  77                 ((e + 1023) << 20)      |        /* exponent */
  78                 ((su_int)(a >> 32) & 0x000FFFFF); /* mantissa-high */
  79     fb.u.s.low = (su_int)a;                         /* mantissa-low */
  80     return fb.f;
  81 }
  82
  83 #endif /* CRT_HAS_128BIT */