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 
15 #include "int_lib.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  *             di_int is a 64 bit integral type
21  */
22 
23 /* seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm */
24 
ARM_EABI_FNALIAS(l2d,floatdidf)25 ARM_EABI_FNALIAS(l2d, floatdidf)
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 
32 COMPILER_RT_ABI double
33 __floatdidf(di_int a)
34 {
35 	static const double twop52 = 0x1.0p52;
36 	static const double twop32 = 0x1.0p32;
37 
38 	union { int64_t x; double d; } low = { .d = twop52 };
39 
40 	const double high = (int32_t)(a >> 32) * twop32;
41 	low.x |= a & INT64_C(0x00000000ffffffff);
42 
43 	const double result = (high - twop52) + low.d;
44 	return result;
45 }
46 
47 #else
48 /* Support for systems that don't have hardware floating-point; there are no flags to
49  * set, and we don't want to code-gen to an unknown soft-float implementation.
50  */
51 
52 COMPILER_RT_ABI double
53 __floatdidf(di_int a)
54 {
55     if (a == 0)
56         return 0.0;
57     const unsigned N = sizeof(di_int) * CHAR_BIT;
58     const di_int s = a >> (N-1);
59     a = (a ^ s) - s;
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 = ((du_int)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 = ((su_int)s & 0x80000000) |        /* sign */
102                 ((e + 1023) << 20)      |        /* exponent */
103                 ((su_int)(a >> 32) & 0x000FFFFF); /* mantissa-high */
104     fb.u.low = (su_int)a;                         /* mantissa-low */
105     return fb.f;
106 }
107 #endif
108