floattidf.c

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/* ===-- floattidf.c - Implement __floattidf -------------------------------===
 *
 *                    The LLVM Compiler Infrastructure
 *
 * This file is dual licensed under the MIT and the University of Illinois Open
 * Source Licenses. See LICENSE.TXT for details.
 *
 * ===----------------------------------------------------------------------===
 *
 * This file implements __floattidf for the compiler_rt library.
 *
 * ===----------------------------------------------------------------------===
 */
 
/* Returns: convert a to a double, rounding toward even.*/
 
/* Assumption: double is a IEEE 64 bit floating point type
 *            ti_int is a 128 bit integral type
 */
 
/* seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm */
 
#include <float.h> // needed for DBL_MANT_DIG
#include "int_t.h"
 
double ___floattidf(__int128 a)
{
    if (a == 0)
        return 0.0;
    const unsigned N = sizeof(ti_int) * CHAR_BIT;
    const ti_int s = a >> (N-1);
    a = (a ^ s) - s;
    int sd = N - __clzti2(a);  /* number of significant digits */
    int e = sd - 1;             /* exponent */
    if (sd > DBL_MANT_DIG)
    {
        /* start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
         *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
         *                                               12345678901234567890123456
         * 1 = msb 1 bit
         * P = bit DBL_MANT_DIG-1 bits to the right of 1
         * Q = bit DBL_MANT_DIG bits to the right of 1
         * R = "or" of all bits to the right of Q
         */
        switch (sd)
        {
        case DBL_MANT_DIG + 1:
            a <<= 1;
            break;
        case DBL_MANT_DIG + 2:
            break;
        default:
            a = ((tu_int)a >> (sd - (DBL_MANT_DIG+2))) |
                ((a & ((tu_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0);
        };
        /* finish: */
        a |= (a & 4) != 0;  /* Or P into R */
        ++a;  /* round - this step may add a significant bit */
        a >>= 2;  /* dump Q and R */
        /* a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits */
        if (a & ((tu_int)1 << DBL_MANT_DIG))
        {
            a >>= 1;
            ++e;
        }
        /* a is now rounded to DBL_MANT_DIG bits */
    }
    else
    {
        a <<= (DBL_MANT_DIG - sd);
        /* a is now rounded to DBL_MANT_DIG bits */
    }
    double_bits fb;
    fb.u.s.high = ((uint32_t)s & 0x80000000) |        /* sign */
                ((e + 1023) << 20)      |        /* exponent */
                ((uint32_t)(a >> 32) & 0x000FFFFF); /* mantissa-high */
    fb.u.s.low = (uint32_t)a;                         /* mantissa-low */
    return fb.f;
}