runtime/lib/floatundisf.c

/*===-- floatundisf.c - Implement __floatundisf ---------------------------===
 *
 *                     The LLVM Compiler Infrastructure
 *
 * This file is distributed under the University of Illinois Open Source
 * License. See LICENSE.TXT for details.
 *
 * ===----------------------------------------------------------------------===
 *
 * This file implements __floatundisf for the compiler_rt library.
 *
 *===----------------------------------------------------------------------===
 */

#if !defined(__GNUC__) || __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 8) // gcc >= 4.8 implements this in libgcc
#include "int_lib.h"
#include <float.h>

/* Returns: convert a to a float, rounding toward even. */

/* Assumption: float is a IEEE 32 bit floating point type 
 *            du_int is a 64 bit integral type
 */

/* seee eeee emmm mmmm mmmm mmmm mmmm mmmm */

float
__floatundisf(du_int a)
{
    if (a == 0)
        return 0.0F;
    const unsigned N = sizeof(du_int) * CHAR_BIT;
    int sd = N - __builtin_clzll(a);  /* number of significant digits */
    int e = sd - 1;             /* 8 exponent */
    if (sd > FLT_MANT_DIG)
    {
        /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
         *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
         *                                                12345678901234567890123456
         *  1 = msb 1 bit
         *  P = bit FLT_MANT_DIG-1 bits to the right of 1
         *  Q = bit FLT_MANT_DIG bits to the right of 1
         *  R = "or" of all bits to the right of Q
         */
        switch (sd)
        {
        case FLT_MANT_DIG + 1:
            a <<= 1;
            break;
        case FLT_MANT_DIG + 2:
            break;
        default:
            a = (a >> (sd - (FLT_MANT_DIG+2))) |
                ((a & ((du_int)(-1) >> ((N + FLT_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 FLT_MANT_DIG or FLT_MANT_DIG+1 bits */
        if (a & ((du_int)1 << FLT_MANT_DIG))
        {
            a >>= 1;
            ++e;
        }
        /* a is now rounded to FLT_MANT_DIG bits */
    }
    else
    {
        a <<= (FLT_MANT_DIG - sd);
        /* a is now rounded to FLT_MANT_DIG bits */
    }
    float_bits fb;
    fb.u = ((e + 127) << 23)       |  /* exponent */
           ((su_int)a & 0x007FFFFF);  /* mantissa */
    return fb.f;
}
#endif