.net - Simulate tearing a double in C#

Question

I'm running on a 32-bit machine and I'm able to confirm that long values can tear using the following code snippet which hits very quickly.

        static void TestTearingLong()
        {
            System.Threading.Thread A = new System.Threading.Thread(ThreadA);
            A.Start();

            System.Threading.Thread B = new System.Threading.Thread(ThreadB);
            B.Start();
        }

        static ulong s_x;

        static void ThreadA()
        {
            int i = 0;
            while (true)
            {
                s_x = (i & 1) == 0 ? 0x0L : 0xaaaabbbbccccddddL;
                i++;
            }
        }

        static void ThreadB()
        {
            while (true)
            {
                ulong x = s_x;
                Debug.Assert(x == 0x0L || x == 0xaaaabbbbccccddddL);
            }
        }

But when I try something similar with doubles, I'm not able to get any tearing. Does anyone know why? As far as I can tell from the spec, only assignment to a float is atomic. The assignment to a double should have a risk of tearing.

    static double s_x;

    static void TestTearingDouble()
    {
        System.Threading.Thread A = new System.Threading.Thread(ThreadA);
        A.Start();

        System.Threading.Thread B = new System.Threading.Thread(ThreadB);
        B.Start();
    }

    static void ThreadA()
    {
        long i = 0;

        while (true)
        {
            s_x = ((i & 1) == 0) ? 0.0 : double.MaxValue;
            i++;

            if (i % 10000000 == 0)
            {
                Console.Out.WriteLine("i = " + i);
            }
        }
    }

    static void ThreadB()
    {
        while (true)
        {
            double x = s_x;

            System.Diagnostics.Debug.Assert(x == 0.0 || x == double.MaxValue);
        }
    }

Answer 1

As strange as it sounds, that depends on your CPU. While doubles are not guaranteed not to tear, they won't on many current processors. Try an AMD Sempron if you want tearing in this situation.

EDIT: Learned that the hard way a few years ago.