src/compiler-rt/lib/builtins/floatuntixf.c

   1 /* ===-- floatuntixf.c - Implement __floatuntixf ---------------------------===
   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 __floatuntixf 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 long double, rounding toward even. */
  20
  21 /* Assumption: long double is a IEEE 80 bit floating point type padded to 128 bits
  22  *             tu_int is a 128 bit integral type
  23  */
  24
  25 /* gggg gggg gggg gggg gggg gggg gggg gggg | gggg gggg gggg gggg seee eeee eeee eeee |
  26  * 1mmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm
  27  */
  28
  29 COMPILER_RT_ABI long double
  30 __floatuntixf(tu_int a)
  31 {
  32     if (a == 0)
  33         return 0.0;
  34     const unsigned N = sizeof(tu_int) * CHAR_BIT;
  35     int sd = N - __clzti2(a);  /* number of significant digits */
  36     int e = sd - 1;             /* exponent */
  37     if (sd > LDBL_MANT_DIG)
  38     {
  39         /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
  40          *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
  41          *                                                12345678901234567890123456
  42          *  1 = msb 1 bit
  43          *  P = bit LDBL_MANT_DIG-1 bits to the right of 1
  44          *  Q = bit LDBL_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 LDBL_MANT_DIG + 1:
  50             a <<= 1;
  51             break;
  52         case LDBL_MANT_DIG + 2:
  53             break;
  54         default:
  55             a = (a >> (sd - (LDBL_MANT_DIG+2))) |
  56                 ((a & ((tu_int)(-1) >> ((N + LDBL_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 LDBL_MANT_DIG or LDBL_MANT_DIG+1 bits */
  63         if (a & ((tu_int)1 << LDBL_MANT_DIG))
  64         {
  65             a >>= 1;
  66             ++e;
  67         }
  68         /* a is now rounded to LDBL_MANT_DIG bits */
  69     }
  70     else
  71     {
  72         a <<= (LDBL_MANT_DIG - sd);
  73         /* a is now rounded to LDBL_MANT_DIG bits */
  74     }
  75     long_double_bits fb;
  76     fb.u.high.s.low = (e + 16383);                  /* exponent */
  77     fb.u.low.all = (du_int)a;                     /* mantissa */
  78     return fb.f;
  79 }
  80
  81 #endif