Incorrect and unexpected optimizer behavior for Interlocked.Exchange with short value

[stefan-schweiger]

NOTE: Most likely this does not just affect Interlocked.Exchange, but it's easy to reproduce this way.

When turning on the optimization for the following code you get different (and also wrong) output values than from building the application without turning on the optimizer.

using System;
using System.Threading;

class Program
{
    private static int _intToExchange = -1;
    private static short _innerShort = 2;

    static void Main( string[] args )
    {
        var oldValue = Interlocked.Exchange(ref _intToExchange, _innerShort);
        Console.WriteLine( "It was:   {0}", oldValue );
        Console.WriteLine( "It is:    {0}", _intToExchange );
        Console.WriteLine( "Expected: {0}", _innerShort );
    }
}

Without using optimization the output is as expected:

It was:   -1
It is:    2
Expected: 2

But when you turn on optimization (e.g. with <Optimize>true</Optimize> in .csproj) this is the output:

It was:   65535
It is:    -65534
Expected: 2

More about this bug can also be found here on stackoverflow.

[mikedn]

Most likely this does not just affect Interlocked.Exchange, but it's easy to reproduce this way.

As far as I can tell this affects Interlocked.Add and Interlocked.Exchange. Do you have other examples?

[stefan-schweiger]

I don't have any other examples, it was just my best guess that this would probably also affect other methods with similar signatures or internals.