floatuntixf.c revision 263763 
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
29si_int __clzti2(ti_int a);
30
31long double
32__floatuntixf(tu_int a)
33{
34    if (a == 0)
35        return 0.0;
36    const unsigned N = sizeof(tu_int) * CHAR_BIT;
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 = (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 = (e + 16383);                  /* exponent */
79    fb.u.low.all = (du_int)a;                     /* mantissa */
80    return fb.f;
81}
82
83#endif
84