floatuntixf.c revision 214152 
1/* ===-- floatuntixf.c - Implement __floatuntixf ---------------------------===
2 *
3 *                     The LLVM Compiler Infrastructure
4 *
5 * This file is distributed under the University of Illinois Open Source
6 * License. See LICENSE.TXT for details.
7 *
8 * ===----------------------------------------------------------------------===
9 *
10 * This file implements __floatuntixf for the compiler_rt library.
11 *
12 * ===----------------------------------------------------------------------===
13 */
14
15#if __x86_64
16
17#include "int_lib.h"
18#include <float.h>
19
20/* Returns: convert a to a long double, rounding toward even. */
21
22/* Assumption: long double is a IEEE 80 bit floating point type padded to 128 bits
23 *             tu_int is a 128 bit integral type
24 */
25
26/* gggg gggg gggg gggg gggg gggg gggg gggg | gggg gggg gggg gggg seee eeee eeee eeee |
27 * 1mmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm
28 */
29
30si_int __clzti2(ti_int a);
31
32long double
33__floatuntixf(tu_int a)
34{
35    if (a == 0)
36        return 0.0;
37    const unsigned N = sizeof(tu_int) * CHAR_BIT;
38    int sd = N - __clzti2(a);  /* number of significant digits */
39    int e = sd - 1;             /* exponent */
40    if (sd > LDBL_MANT_DIG)
41    {
42        /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
43         *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
44         *                                                12345678901234567890123456
45         *  1 = msb 1 bit
46         *  P = bit LDBL_MANT_DIG-1 bits to the right of 1
47         *  Q = bit LDBL_MANT_DIG bits to the right of 1
48         *  R = "or" of all bits to the right of Q
49	 */
50        switch (sd)
51        {
52        case LDBL_MANT_DIG + 1:
53            a <<= 1;
54            break;
55        case LDBL_MANT_DIG + 2:
56            break;
57        default:
58            a = (a >> (sd - (LDBL_MANT_DIG+2))) |
59                ((a & ((tu_int)(-1) >> ((N + LDBL_MANT_DIG+2) - sd))) != 0);
60        };
61        /* finish: */
62        a |= (a & 4) != 0;  /* Or P into R */
63        ++a;  /* round - this step may add a significant bit */
64        a >>= 2;  /* dump Q and R */
65        /* a is now rounded to LDBL_MANT_DIG or LDBL_MANT_DIG+1 bits */
66        if (a & ((tu_int)1 << LDBL_MANT_DIG))
67        {
68            a >>= 1;
69            ++e;
70        }
71        /* a is now rounded to LDBL_MANT_DIG bits */
72    }
73    else
74    {
75        a <<= (LDBL_MANT_DIG - sd);
76        /* a is now rounded to LDBL_MANT_DIG bits */
77    }
78    long_double_bits fb;
79    fb.u.high.s.low = (e + 16383);                  /* exponent */
80    fb.u.low.all = (du_int)a;                     /* mantissa */
81    return fb.f;
82}
83
84#endif
85