floattidf.c revision 239138 
18876Srgrimes/* ===-- floattidf.c - Implement __floattidf -------------------------------===
24Srgrimes *
34Srgrimes *                    The LLVM Compiler Infrastructure
44Srgrimes *
58876Srgrimes * This file is dual licensed under the MIT and the University of Illinois Open
64Srgrimes * Source Licenses. See LICENSE.TXT for details.
74Srgrimes *
84Srgrimes * ===----------------------------------------------------------------------===
94Srgrimes *
104Srgrimes * This file implements __floattidf for the compiler_rt library.
118876Srgrimes *
128876Srgrimes * ===----------------------------------------------------------------------===
134Srgrimes */
144Srgrimes
158876Srgrimes#include "int_lib.h"
164Srgrimes
178876Srgrimes#if __x86_64
184Srgrimes
194Srgrimes/* Returns: convert a to a double, rounding toward even.*/
204Srgrimes
214Srgrimes/* Assumption: double is a IEEE 64 bit floating point type
228876Srgrimes *            ti_int is a 128 bit integral type
234Srgrimes */
244Srgrimes
254Srgrimes/* seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm */
2650477Speter
274Srgrimessi_int __clzti2(ti_int a);
28623Srgrimes
29716Swollmandouble
3012472Sbde__floattidf(ti_int a)
31716Swollman{
324Srgrimes    if (a == 0)
334Srgrimes        return 0.0;
344Srgrimes    const unsigned N = sizeof(ti_int) * CHAR_BIT;
354Srgrimes    const ti_int s = a >> (N-1);
364Srgrimes    a = (a ^ s) - s;
374Srgrimes    int sd = N - __clzti2(a);  /* number of significant digits */
384Srgrimes    int e = sd - 1;             /* exponent */
394Srgrimes    if (sd > DBL_MANT_DIG)
404Srgrimes    {
4112472Sbde        /* start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
4212472Sbde         *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
4312472Sbde         *                                               12345678901234567890123456
4412472Sbde         * 1 = msb 1 bit
4512472Sbde         * P = bit DBL_MANT_DIG-1 bits to the right of 1
46         * Q = bit DBL_MANT_DIG bits to the right of 1
47         * R = "or" of all bits to the right of Q
48         */
49        switch (sd)
50        {
51        case DBL_MANT_DIG + 1:
52            a <<= 1;
53            break;
54        case DBL_MANT_DIG + 2:
55            break;
56        default:
57            a = ((tu_int)a >> (sd - (DBL_MANT_DIG+2))) |
58                ((a & ((tu_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0);
59        };
60        /* finish: */
61        a |= (a & 4) != 0;  /* Or P into R */
62        ++a;  /* round - this step may add a significant bit */
63        a >>= 2;  /* dump Q and R */
64        /* a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits */
65        if (a & ((tu_int)1 << DBL_MANT_DIG))
66        {
67            a >>= 1;
68            ++e;
69        }
70        /* a is now rounded to DBL_MANT_DIG bits */
71    }
72    else
73    {
74        a <<= (DBL_MANT_DIG - sd);
75        /* a is now rounded to DBL_MANT_DIG bits */
76    }
77    double_bits fb;
78    fb.u.s.high = ((su_int)s & 0x80000000) |        /* sign */
79                ((e + 1023) << 20)      |        /* exponent */
80                ((su_int)(a >> 32) & 0x000FFFFF); /* mantissa-high */
81    fb.u.s.low = (su_int)a;                         /* mantissa-low */
82    return fb.f;
83}
84
85#endif
86