floatundisf.c revision 214152 
1/*===-- floatundisf.c - Implement __floatundisf ---------------------------===
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 __floatundisf for the compiler_rt library.
11 *
12 *===----------------------------------------------------------------------===
13 */
14
15#include "int_lib.h"
16#include <float.h>
17
18/* Returns: convert a to a float, rounding toward even. */
19
20/* Assumption: float is a IEEE 32 bit floating point type
21 *            du_int is a 64 bit integral type
22 */
23
24/* seee eeee emmm mmmm mmmm mmmm mmmm mmmm */
25
26float
27__floatundisf(du_int a)
28{
29    if (a == 0)
30        return 0.0F;
31    const unsigned N = sizeof(du_int) * CHAR_BIT;
32    int sd = N - __builtin_clzll(a);  /* number of significant digits */
33    int e = sd - 1;             /* 8 exponent */
34    if (sd > FLT_MANT_DIG)
35    {
36        /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
37         *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
38         *                                                12345678901234567890123456
39         *  1 = msb 1 bit
40         *  P = bit FLT_MANT_DIG-1 bits to the right of 1
41         *  Q = bit FLT_MANT_DIG bits to the right of 1
42         *  R = "or" of all bits to the right of Q
43         */
44        switch (sd)
45        {
46        case FLT_MANT_DIG + 1:
47            a <<= 1;
48            break;
49        case FLT_MANT_DIG + 2:
50            break;
51        default:
52            a = (a >> (sd - (FLT_MANT_DIG+2))) |
53                ((a & ((du_int)(-1) >> ((N + FLT_MANT_DIG+2) - sd))) != 0);
54        };
55        /* finish: */
56        a |= (a & 4) != 0;  /* Or P into R */
57        ++a;  /* round - this step may add a significant bit */
58        a >>= 2;  /* dump Q and R */
59        /* a is now rounded to FLT_MANT_DIG or FLT_MANT_DIG+1 bits */
60        if (a & ((du_int)1 << FLT_MANT_DIG))
61        {
62            a >>= 1;
63            ++e;
64        }
65        /* a is now rounded to FLT_MANT_DIG bits */
66    }
67    else
68    {
69        a <<= (FLT_MANT_DIG - sd);
70        /* a is now rounded to FLT_MANT_DIG bits */
71    }
72    float_bits fb;
73    fb.u = ((e + 127) << 23)       |  /* exponent */
74           ((su_int)a & 0x007FFFFF);  /* mantissa */
75    return fb.f;
76}
77