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
27COMPILER_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 */
81