lib/builtins/floattixf.c

   1 /* ===-- floattixf.c - Implement __floattixf -------------------------------===
   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 __floattixf 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  *             ti_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 __floattixf(ti_int a)
  31 {
  32     if (a == 0)
  33         return 0.0;
  34     const unsigned N = sizeof(ti_int) * CHAR_BIT;
  35     const ti_int s = a >> (N-1);
  36     a = (a ^ s) - s;
  37     int sd = N - __clzti2(a);  /* number of significant digits */
  38     int e = sd - 1;             /* exponent */
  39     if (sd > LDBL_MANT_DIG)
  40     {
  41         /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
  42          *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
  43          *                                                12345678901234567890123456
  44          *  1 = msb 1 bit
  45          *  P = bit LDBL_MANT_DIG-1 bits to the right of 1
  46          *  Q = bit LDBL_MANT_DIG bits to the right of 1
  47          *  R = "or" of all bits to the right of Q
  48          */
  49         switch (sd)
  50         {
  51         case LDBL_MANT_DIG + 1:
  52             a <<= 1;
  53             break;
  54         case LDBL_MANT_DIG + 2:
  55             break;
  56         default:
  57             a = ((tu_int)a >> (sd - (LDBL_MANT_DIG+2))) |
  58                 ((a & ((tu_int)(-1) >> ((N + LDBL_MANT_DIG+2) - sd))) != 0);
  59         };
  60         /* finish: */
  61         a |= (a & 4) != 0;  /* Or P into R */
  62         ++a;  /* round - this step may add a significant bit */
  63         a >>= 2;  /* dump Q and R */
  64         /* a is now rounded to LDBL_MANT_DIG or LDBL_MANT_DIG+1 bits */
  65         if (a & ((tu_int)1 << LDBL_MANT_DIG))
  66         {
  67             a >>= 1;
  68             ++e;
  69         }
  70         /* a is now rounded to LDBL_MANT_DIG bits */
  71     }
  72     else
  73     {
  74         a <<= (LDBL_MANT_DIG - sd);
  75         /* a is now rounded to LDBL_MANT_DIG bits */
  76     }
  77     long_double_bits fb;
  78     fb.u.high.s.low = ((su_int)s & 0x8000) |        /* sign */
  79                     (e + 16383);                  /* exponent */
  80     fb.u.low.all = (du_int)a;                     /* mantissa */
  81     return fb.f;
  82 }
  83
  84 #endif /* CRT_HAS_128BIT */