1 /*===-- floatdisf.c - Implement __floatdisf -------------------------------===
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 __floatdisf 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 * di_int is a 64 bit integral type
22 */
23
24 /* seee eeee emmm mmmm mmmm mmmm mmmm mmmm */
25
26 float
__floatdisf(di_int a)27 __floatdisf(di_int a)
28 {
29 if (a == 0)
30 return 0.0F;
31 const unsigned N = sizeof(di_int) * CHAR_BIT;
32 const di_int s = a >> (N-1);
33 a = (a ^ s) - s;
34 int sd = N - __builtin_clzll(a); /* number of significant digits */
35 int e = sd - 1; /* exponent */
36 if (sd > FLT_MANT_DIG)
37 {
38 /* start: 0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
39 * finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
40 * 12345678901234567890123456
41 * 1 = msb 1 bit
42 * P = bit FLT_MANT_DIG-1 bits to the right of 1
43 * Q = bit FLT_MANT_DIG bits to the right of 1
44 * R = "or" of all bits to the right of Q
45 */
46 switch (sd)
47 {
48 case FLT_MANT_DIG + 1:
49 a <<= 1;
50 break;
51 case FLT_MANT_DIG + 2:
52 break;
53 default:
54 a = ((du_int)a >> (sd - (FLT_MANT_DIG+2))) |
55 ((a & ((du_int)(-1) >> ((N + FLT_MANT_DIG+2) - sd))) != 0);
56 };
57 /* finish: */
58 a |= (a & 4) != 0; /* Or P into R */
59 ++a; /* round - this step may add a significant bit */
60 a >>= 2; /* dump Q and R */
61 /* a is now rounded to FLT_MANT_DIG or FLT_MANT_DIG+1 bits */
62 if (a & ((du_int)1 << FLT_MANT_DIG))
63 {
64 a >>= 1;
65 ++e;
66 }
67 /* a is now rounded to FLT_MANT_DIG bits */
68 }
69 else
70 {
71 a <<= (FLT_MANT_DIG - sd);
72 /* a is now rounded to FLT_MANT_DIG bits */
73 }
74 float_bits fb;
75 fb.u = ((su_int)s & 0x80000000) | /* sign */
76 ((e + 127) << 23) | /* exponent */
77 ((su_int)a & 0x007FFFFF); /* mantissa */
78 return fb.f;
79 }
80