floattisf.c revision 263763 
1/* ===-- floattisf.c - Implement __floattisf -------------------------------===
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 __floattisf 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 float, rounding toward even. */
20
21/* Assumption: float is a IEEE 32 bit floating point type
22 *             ti_int is a 128 bit integral type
23 */
24
25/* seee eeee emmm mmmm mmmm mmmm mmmm mmmm */
26
27si_int __clzti2(ti_int a);
28
29float
30__floattisf(ti_int a)
31{
32    if (a == 0)
33        return 0.0F;
34    const unsigned N = sizeof(ti_int) * CHAR_BIT;
35    const ti_int s = a >> (N-1);
36    a = (a ^ s) - s;
37    int sd = N - __clzti2(a);  /* number of significant digits */
38    int e = sd - 1;             /* exponent */
39    if (sd > FLT_MANT_DIG)
40    {
41        /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
42        * finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
43        *                                                12345678901234567890123456
44        *  1 = msb 1 bit
45        *  P = bit FLT_MANT_DIG-1 bits to the right of 1
46        *  Q = bit FLT_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 FLT_MANT_DIG + 1:
52            a <<= 1;
53            break;
54        case FLT_MANT_DIG + 2:
55            break;
56        default:
57            a = ((tu_int)a >> (sd - (FLT_MANT_DIG+2))) |
58                ((a & ((tu_int)(-1) >> ((N + FLT_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 FLT_MANT_DIG or FLT_MANT_DIG+1 bits */
65        if (a & ((tu_int)1 << FLT_MANT_DIG))
66        {
67            a >>= 1;
68            ++e;
69        }
70        /* a is now rounded to FLT_MANT_DIG bits */
71    }
72    else
73    {
74        a <<= (FLT_MANT_DIG - sd);
75        /* a is now rounded to FLT_MANT_DIG bits */
76    }
77    float_bits fb;
78    fb.u = ((su_int)s & 0x80000000) |  /* sign */
79           ((e + 127) << 23)       |  /* exponent */
80           ((su_int)a & 0x007FFFFF);   /* mantissa */
81    return fb.f;
82}
83
84#endif /* CRT_HAS_128BIT */
85