release mode uses double precision even for float variables

问题

My algorithm is calculating the epsilon for single precision floating point arithmetic. It is supposed to be something around 1.1921e-007. Here is the code:

static void Main(string[] args) {
    // start with some small magic number
    float a = 0.000000000000000013877787807814457f;
    for (; ; ) {
        // add the small a to 1
        float temp = 1f + a;
        // break, if a + 1 really is > '1'
        if (temp - 1f != 0f) break;
        // otherwise a is too small -> increase it
        a *= 2f;
        Console.Out.WriteLine("current increment: " + a); 
    }
    Console.Out.WriteLine("Found epsilon: " + a); 
    Console.ReadKey(); 
}

In debug mode, it gives the following reasonable output (abbreviated):

current increment: 2,775558E-17
current increment: 5,551115E-17
...
current increment: 2,980232E-08
current increment: 5,960464E-08
current increment: 1,192093E-07
Found epsilon: 1,192093E-07

However, when switching to release mode (no matter with/ Without optimization!), the code gives the following result:

current increment: 2,775558E-17
current increment: 5,551115E-17
current increment: 1,110223E-16
current increment: 2,220446E-16
Found epsilon: 2,220446E-16

which corresponds to the value for double precision. So I assume, some optimizations cause the computations to be done on double values. Of course the result is wrong in this case!

Also: this happens only, if targeting X86 Release in the project options. Again: optimization on/off does not matter. I am on 64 bit WIN7, VS 2010 Ultimate, targeting .NET 4.0.

What might cause that behaviour? Some WOW issue? How to get around it in a reliable way? How to prevent the CLR to generate code which makes use of double precision instead of single precision calculations?

Note: switching to "Any CPU" or even "X64" as platform target is no option - even if the problem does not occur here. But we have some native libraries, in different versions for 32/64 bit. So the target must be specific.

回答1:

As discussed in the comments, this is expected. It can be side-stepped by removing the JIT's ability to keep the value in a register (which will be wider than the actual value) - by forcing it down to a field (which has clearly-defined size):

class WorkingContext
{ 
    public float Value; // you'll excuse me a public field here, I trust
    public override string ToString()
    {
        return Value.ToString();
    }
}
static void Main()
{
    // start with some small magic number
    WorkingContext a = new WorkingContext(), temp = new WorkingContext();
    a.Value = 0.000000000000000013877787807814457f;
    for (; ; )
    {
        // add the small a to 1
        temp.Value = 1f + a.Value;
        // break, if a + 1 really is > '1'
        if (temp.Value - 1f != 0f) break;
        // otherwise a is too small -> increase it
        a.Value *= 2f;
        Console.Out.WriteLine("current increment: " + a);
    }
    Console.Out.WriteLine("Found epsilon: " + a);
    Console.ReadKey();
}

Interestingly, I tried this with a struct first, but the JIT was able to see past my cheating (presumably because it is all on the stack).

来源：https://stackoverflow.com/questions/6277623/release-mode-uses-double-precision-even-for-float-variables

标签

.net

cross-platform

floating-point-precision