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
__floattidf(ti_int a)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 */
84