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
__floatuntidf(tu_int a)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 */
81