lib/floatundidf.c

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

#include "int_lib.h"
#include <float.h>

// Returns: convert a to a double, rounding toward even.

// Assumption: double is a IEEE 64 bit floating point type 
//             du_int is a 64 bit integral type

// seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm

#ifndef __SOFT_FP__
// Support for systems that have hardware floating-point; we'll set the inexact flag
// as a side-effect of this computation.
#include <stdint.h>

double
__floatundidf(du_int a)
{
	static const double twop52 = 0x1.0p52;
	static const double twop84 = 0x1.0p84;
	static const double twop84_plus_twop52 = 0x1.00000001p84;
	
	union { uint64_t x; double d; } high = { .d = twop84 };
	union { uint64_t x; double d; } low = { .d = twop52 };
	
	high.x |= a >> 32;
	low.x |= a & UINT64_C(0x00000000ffffffff);
	
	const double result = (high.d - twop84_plus_twop52) + low.d;
	return result;
}

#else
// Support for systems that don't have hardware floating-point; there are no flags to
// set, and we don't want to code-gen to an unknown soft-float implementation.

double
__floatundidf(du_int a)
{
    if (a == 0)
        return 0.0;
    const unsigned N = sizeof(du_int) * CHAR_BIT;
    int sd = N - __builtin_clzll(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 = (a >> (sd - (DBL_MANT_DIG+2))) |
                ((a & ((du_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 & ((du_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.high = ((e + 1023) << 20)      |        // exponent
                ((su_int)(a >> 32) & 0x000FFFFF); // mantissa-high
    fb.u.low = (su_int)a;                         // mantissa-low
    return fb.f;
}
#endif