将uint64_t转换为double后出现意外结果

Question

In the following code:

#include <iostream>

...

uint64_t t1 = 1510763846;
uint64_t t2 = 1510763847;
double d1 = (double)t1;
double d2 = (double)t2;
// d1 == t2 => evaluates to true somehow?
// t1 == d2 => evaluates to true somehow?
// d1 == d2 => evaluates to true somehow?
// t1 == t2 => evaluates to false, of course.
std::cout << std::fixed << 
        "uint64_t: " << t1 << ", " << t2 << ", " <<
        "double: " << d1 << ", " << d2 << ", " << (d2+1) << std::endl;

I get this output:

uint64_t: 1510763846, 1510763847, double: 1510763904.000000, 1510763904.000000, 1510763905.000000

And I don't understand why. This answer: biggest integer that can be stored in a double says that an integral number up to 2^53 (9007199254740992) can be stored in a double without losing precision.

I actually get errors when I start doing calculations with the doubles, so it's not only a printing issue. (eg 1510763846 and 1510763847 both give 1510763904)

It's also very weird that the double can just be added to and then come out correct (d2+1 == 1510763905.000000)

Rationale: I'm converting these numbers to doubles because I need to work with them in Lua, which only supports floating point numbers. I'm sure I'm compiling the Lua lib with double as the lua_Number type, not float .

std::cout << sizeof(t1) << ", " << sizeof(d2) << std::endl;

Outputs

8, 8

I'm using VS 2012 with target MachineX86, toolkit v110_xp. Floating point model "Precise (/fp:precise)"

Addendum

With the help of people who replied and this article Why are doubles added incorrectly in a specific Visual Studio 2008 project? , I've been able to pinpoint the problem. A library is using a function like _set_controlfp, _control87, _controlfp or __control87_2 to change the precision of my executable to "single". That is why a uint64_t conversion to a double behaves as if it's a float.

When doing a file search for the above function names and "MCW_PC", which is used for Precision Control, I found the following libraries that might have set it:

• Android NDK
• boost::math
• boost::numeric
• DirectX (We're using June 2010)
• FMod (non-EX)
• Pyro particle engine

Now I'd like to rephrase my question:

How do I make sure converting from a uint64_t to a double goes correctly every time, without:

1. having to call _fpreset() each and every time a possible conversion occurs (think about the function parameters)
2. having to worry about a library's thread changing the floating point precision in between my _fpreset() and the conversion?

Naive code would be something like this:

double toDouble(uint64_t i)
{
    double d;
    do {
        _fpreset();
        d = i;
        _fpreset();
    } while (d != i);
    return d;
}

double toDouble(int64_t i)
{
    double d;
    do {
        _fpreset();
        d = i;
        _fpreset();
    } while (d != i);
    return d;
}

This solution assumes the odds of a thread messing with the Floating Point Precision twice are astronomically small. Problem is, the values I'm working with, are timers that represent real-world value. So I shouldn't be taking any chances. Is there a silver bullet for this problem?

Answer 1

From ieee754 floating point conversion it looks like your implementation of double is actually float, which is of course allowed by the standard, that mandates that sizeof double >= sizeof float .

The most accurate representation of 1510763846 is 1.510763904E9.