floatdidf.c revision 222656 
1/*===-- floatdidf.c - Implement __floatdidf -------------------------------===
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 __floatdidf for the compiler_rt library.
11 *
12 *===----------------------------------------------------------------------===
13 */
14#include "abi.h"
15
16#include "int_lib.h"
17#include <float.h>
18
19/* Returns: convert a to a double, rounding toward even. */
20
21/* Assumption: double is a IEEE 64 bit floating point type
22 *             di_int is a 64 bit integral type
23 */
24
25/* seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm */
26
27ARM_EABI_FNALIAS(l2d, floatdidf);
28
29#ifndef __SOFT_FP__
30/* Support for systems that have hardware floating-point; we'll set the inexact flag
31 * as a side-effect of this computation.
32 */
33#include <stdint.h>
34
35COMPILER_RT_ABI double
36__floatdidf(di_int a)
37{
38	static const double twop52 = 0x1.0p52;
39	static const double twop32 = 0x1.0p32;
40
41	union { int64_t x; double d; } low = { .d = twop52 };
42
43	const double high = (int32_t)(a >> 32) * twop32;
44	low.x |= a & INT64_C(0x00000000ffffffff);
45
46	const double result = (high - twop52) + low.d;
47	return result;
48}
49
50#else
51/* Support for systems that don't have hardware floating-point; there are no flags to
52 * set, and we don't want to code-gen to an unknown soft-float implementation.
53 */
54
55COMPILER_RT_ABI double
56__floatdidf(di_int a)
57{
58    if (a == 0)
59        return 0.0;
60    const unsigned N = sizeof(di_int) * CHAR_BIT;
61    const di_int s = a >> (N-1);
62    a = (a ^ s) - s;
63    int sd = N - __builtin_clzll(a);  /* number of significant digits */
64    int e = sd - 1;             /* exponent */
65    if (sd > DBL_MANT_DIG)
66    {
67        /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
68         *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
69         *                                                12345678901234567890123456
70         *  1 = msb 1 bit
71         *  P = bit DBL_MANT_DIG-1 bits to the right of 1
72         * Q = bit DBL_MANT_DIG bits to the right of 1
73         *  R = "or" of all bits to the right of Q
74        */
75        switch (sd)
76        {
77        case DBL_MANT_DIG + 1:
78            a <<= 1;
79            break;
80        case DBL_MANT_DIG + 2:
81            break;
82        default:
83            a = ((du_int)a >> (sd - (DBL_MANT_DIG+2))) |
84                ((a & ((du_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0);
85        };
86        /* finish: */
87        a |= (a & 4) != 0;  /* Or P into R */
88        ++a;  /* round - this step may add a significant bit */
89        a >>= 2;  /* dump Q and R */
90        /* a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits */
91        if (a & ((du_int)1 << DBL_MANT_DIG))
92        {
93            a >>= 1;
94            ++e;
95        }
96        /* a is now rounded to DBL_MANT_DIG bits */
97    }
98    else
99    {
100        a <<= (DBL_MANT_DIG - sd);
101        /* a is now rounded to DBL_MANT_DIG bits */
102    }
103    double_bits fb;
104    fb.u.high = ((su_int)s & 0x80000000) |        /* sign */
105                ((e + 1023) << 20)      |        /* exponent */
106                ((su_int)(a >> 32) & 0x000FFFFF); /* mantissa-high */
107    fb.u.low = (su_int)a;                         /* mantissa-low */
108    return fb.f;
109}
110#endif
111