floatundidf.c revision 2d1fdb26e458c4ddc04155c1d421bced3ba90cd0 
1/* ===-- floatundidf.c - Implement __floatundidf ---------------------------===
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 __floatundidf for the compiler_rt library.
11 *
12 * ===----------------------------------------------------------------------===
13 */
14
15/* Returns: convert a to a double, rounding toward even. */
16
17/* Assumption: double is a IEEE 64 bit floating point type
18 *             du_int is a 64 bit integral type
19 */
20
21/* seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm */
22
23#include "int_lib.h"
24
25ARM_EABI_FNALIAS(ul2d, floatundidf)
26
27#ifndef __SOFT_FP__
28/* Support for systems that have hardware floating-point; we'll set the inexact flag
29 * as a side-effect of this computation.
30 */
31
32COMPILER_RT_ABI double
33__floatundidf(du_int a)
34{
35	static const double twop52 = 0x1.0p52;
36	static const double twop84 = 0x1.0p84;
37	static const double twop84_plus_twop52 = 0x1.00000001p84;
38
39	union { uint64_t x; double d; } high = { .d = twop84 };
40	union { uint64_t x; double d; } low = { .d = twop52 };
41
42	high.x |= a >> 32;
43	low.x |= a & UINT64_C(0x00000000ffffffff);
44
45	const double result = (high.d - twop84_plus_twop52) + low.d;
46	return result;
47}
48
49#else
50/* Support for systems that don't have hardware floating-point; there are no flags to
51 * set, and we don't want to code-gen to an unknown soft-float implementation.
52 */
53
54COMPILER_RT_ABI double
55__floatundidf(du_int a)
56{
57    if (a == 0)
58        return 0.0;
59    const unsigned N = sizeof(du_int) * CHAR_BIT;
60    int sd = N - __builtin_clzll(a);  /* number of significant digits */
61    int e = sd - 1;             /* exponent */
62    if (sd > DBL_MANT_DIG)
63    {
64        /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
65         *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
66         *                                                12345678901234567890123456
67         *  1 = msb 1 bit
68         *  P = bit DBL_MANT_DIG-1 bits to the right of 1
69         *  Q = bit DBL_MANT_DIG bits to the right of 1
70         *  R = "or" of all bits to the right of Q
71         */
72        switch (sd)
73        {
74        case DBL_MANT_DIG + 1:
75            a <<= 1;
76            break;
77        case DBL_MANT_DIG + 2:
78            break;
79        default:
80            a = (a >> (sd - (DBL_MANT_DIG+2))) |
81                ((a & ((du_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0);
82        };
83        /* finish: */
84        a |= (a & 4) != 0;  /* Or P into R */
85        ++a;  /* round - this step may add a significant bit */
86        a >>= 2;  /* dump Q and R */
87        /* a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits */
88        if (a & ((du_int)1 << DBL_MANT_DIG))
89        {
90            a >>= 1;
91            ++e;
92        }
93        /* a is now rounded to DBL_MANT_DIG bits */
94    }
95    else
96    {
97        a <<= (DBL_MANT_DIG - sd);
98        /* a is now rounded to DBL_MANT_DIG bits */
99    }
100    double_bits fb;
101    fb.u.high = ((e + 1023) << 20)      |        /* exponent */
102                ((su_int)(a >> 32) & 0x000FFFFF); /* mantissa-high */
103    fb.u.low = (su_int)a;                         /* mantissa-low  */
104    return fb.f;
105}
106#endif
107