1 /* ===-- floatuntidf.c - Implement __floatuntidf ---------------------------===
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 __floatuntidf for the compiler_rt library.
11 *
12 * ===----------------------------------------------------------------------===
13 */
14
15 #if __x86_64
16
17 #include "int_lib.h"
18 #include <float.h>
19
20 /* Returns: convert a to a double, rounding toward even. */
21
22 /* Assumption: double is a IEEE 64 bit floating point type
23 * tu_int is a 128 bit integral type
24 */
25
26 /* seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm */
27
28 si_int __clzti2(ti_int a);
29
30 double
__floatuntidf(tu_int a)31 __floatuntidf(tu_int a)
32 {
33 if (a == 0)
34 return 0.0;
35 const unsigned N = sizeof(tu_int) * CHAR_BIT;
36 int sd = N - __clzti2(a); /* number of significant digits */
37 int e = sd - 1; /* exponent */
38 if (sd > DBL_MANT_DIG)
39 {
40 /* start: 0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
41 * finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
42 * 12345678901234567890123456
43 * 1 = msb 1 bit
44 * P = bit DBL_MANT_DIG-1 bits to the right of 1
45 * Q = bit DBL_MANT_DIG bits to the right of 1
46 * R = "or" of all bits to the right of Q
47 */
48 switch (sd)
49 {
50 case DBL_MANT_DIG + 1:
51 a <<= 1;
52 break;
53 case DBL_MANT_DIG + 2:
54 break;
55 default:
56 a = (a >> (sd - (DBL_MANT_DIG+2))) |
57 ((a & ((tu_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0);
58 };
59 /* finish: */
60 a |= (a & 4) != 0; /* Or P into R */
61 ++a; /* round - this step may add a significant bit */
62 a >>= 2; /* dump Q and R */
63 /* a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits */
64 if (a & ((tu_int)1 << DBL_MANT_DIG))
65 {
66 a >>= 1;
67 ++e;
68 }
69 /* a is now rounded to DBL_MANT_DIG bits */
70 }
71 else
72 {
73 a <<= (DBL_MANT_DIG - sd);
74 /* a is now rounded to DBL_MANT_DIG bits */
75 }
76 double_bits fb;
77 fb.u.s.high = ((e + 1023) << 20) | /* exponent */
78 ((su_int)(a >> 32) & 0x000FFFFF); /* mantissa-high */
79 fb.u.s.low = (su_int)a; /* mantissa-low */
80 return fb.f;
81 }
82
83 #endif
84