floatundidf.c revision b3a6901e66f55b35aa9e01bcb24134e6a65ea004 
1//===-- floatundidf.c - Implement __floatundidf ---------------------------===//
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 __floatundidf for the compiler_rt library.
11//
12//===----------------------------------------------------------------------===//
13
14#include "int_lib.h"
15#include <float.h>
16
17// Returns: convert a to a double, rounding toward even.
18
19// Assumption: double is a IEEE 64 bit floating point type
20//             du_int is a 64 bit integral type
21
22// seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm
23
24#ifndef __SOFT_FP__
25// Support for systems that have hardware floating-point; we'll set the inexact flag
26// as a side-effect of this computation.
27#include <stdint.h>
28
29double
30__floatundidf(du_int a)
31{
32	static const double twop52 = 0x1.0p52;
33	static const double twop84 = 0x1.0p84;
34	static const double twop84_plus_twop52 = 0x1.00000001p84;
35
36	union { uint64_t x; double d; } high = { .d = twop84 };
37	union { uint64_t x; double d; } low = { .d = twop52 };
38
39	high.x |= a >> 32;
40	low.x |= a & UINT64_C(0x00000000ffffffff);
41
42	const double result = (high.d - twop84_plus_twop52) + low.d;
43	return result;
44}
45
46#else
47// Support for systems that don't have hardware floating-point; there are no flags to
48// set, and we don't want to code-gen to an unknown soft-float implementation.
49
50double
51__floatundidf(du_int a)
52{
53    if (a == 0)
54        return 0.0;
55    const unsigned N = sizeof(du_int) * CHAR_BIT;
56    int sd = N - __builtin_clzll(a);  // number of significant digits
57    int e = sd - 1;             // exponent
58    if (sd > DBL_MANT_DIG)
59    {
60        //  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
61        //  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
62        //                                                12345678901234567890123456
63        //  1 = msb 1 bit
64        //  P = bit DBL_MANT_DIG-1 bits to the right of 1
65        //  Q = bit DBL_MANT_DIG bits to the right of 1
66        //  R = "or" of all bits to the right of Q
67        switch (sd)
68        {
69        case DBL_MANT_DIG + 1:
70            a <<= 1;
71            break;
72        case DBL_MANT_DIG + 2:
73            break;
74        default:
75            a = (a >> (sd - (DBL_MANT_DIG+2))) |
76                ((a & ((du_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0);
77        };
78        // finish:
79        a |= (a & 4) != 0;  // Or P into R
80        ++a;  // round - this step may add a significant bit
81        a >>= 2;  // dump Q and R
82        // a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits
83        if (a & ((du_int)1 << DBL_MANT_DIG))
84        {
85            a >>= 1;
86            ++e;
87        }
88        // a is now rounded to DBL_MANT_DIG bits
89    }
90    else
91    {
92        a <<= (DBL_MANT_DIG - sd);
93        // a is now rounded to DBL_MANT_DIG bits
94    }
95    double_bits fb;
96    fb.u.high = ((e + 1023) << 20)      |        // exponent
97                ((su_int)(a >> 32) & 0x000FFFFF); // mantissa-high
98    fb.u.low = (su_int)a;                         // mantissa-low
99    return fb.f;
100}
101#endif