lib/builtins/floatuntidf.c

   1 /* ===-- floatuntidf.c - Implement __floatuntidf ---------------------------===
   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 __floatuntidf 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  *             tu_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 __floatuntidf(tu_int a)
  29 {
  30     if (a == 0)
  31         return 0.0;
  32     const unsigned N = sizeof(tu_int) * CHAR_BIT;
  33     int sd = N - __clzti2(a);  /* number of significant digits */
  34     int e = sd - 1;             /* exponent */
  35     if (sd > DBL_MANT_DIG)
  36     {
  37         /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
  38          *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
  39          *                                                12345678901234567890123456
  40          *  1 = msb 1 bit
  41          *  P = bit DBL_MANT_DIG-1 bits to the right of 1
  42          *  Q = bit DBL_MANT_DIG bits to the right of 1
  43          *  R = "or" of all bits to the right of Q
  44          */
  45         switch (sd)
  46         {
  47         case DBL_MANT_DIG + 1:
  48             a <<= 1;
  49             break;
  50         case DBL_MANT_DIG + 2:
  51             break;
  52         default:
  53             a = (a >> (sd - (DBL_MANT_DIG+2))) |
  54                 ((a & ((tu_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0);
  55         };
  56         /* finish: */
  57         a |= (a & 4) != 0;  /* Or P into R */
  58         ++a;  /* round - this step may add a significant bit */
  59         a >>= 2;  /* dump Q and R */
  60         /* a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits */
  61         if (a & ((tu_int)1 << DBL_MANT_DIG))
  62         {
  63             a >>= 1;
  64             ++e;
  65         }
  66         /* a is now rounded to DBL_MANT_DIG bits */
  67     }
  68     else
  69     {
  70         a <<= (DBL_MANT_DIG - sd);
  71         /* a is now rounded to DBL_MANT_DIG bits */
  72     }
  73     double_bits fb;
  74     fb.u.s.high = ((e + 1023) << 20)      |        /* exponent */
  75                 ((su_int)(a >> 32) & 0x000FFFFF); /* mantissa-high */
  76     fb.u.s.low = (su_int)a;                         /* mantissa-low */
  77     return fb.f;
  78 }
  79
  80 #endif /* CRT_HAS_128BIT */