I've been doing some performance testing, mainly so I can understand the difference between iterators and simple for loops. As part of this I created a simple set of tests and was then totally surprised by the results. For some methods, 64 bit was nearly 10 times faster than 32 bit.

What I'm looking for is some explanation for why this is happening.

[The answer below states this is due to 64 bit arithmetic in a 32 bit app. Changing the longs to ints results in good performance on 32 and 64 bit systems.]

Here are the 3 methods in question.

private static long ForSumArray(long[] array)
{
    var result = 0L;
    for (var i = 0L; i < array.LongLength; i++)
    {
        result += array[i];
    }
    return result;
}

private static long ForSumArray2(long[] array)
{
    var length = array.LongLength;
    var result = 0L;
    for (var i = 0L; i < length; i++)
    {
        result += array[i];
    }
    return result;
}

private static long IterSumArray(long[] array)
{
    var result = 0L;
    foreach (var entry in array)
    {
        result += entry;
    }
    return result;
}

I have a simple test harness that tests this

var repeat = 10000;

var arrayLength = 100000;
var array = new long[arrayLength];
for (var i = 0; i < arrayLength; i++)
{
    array[i] = i;
}

Console.WriteLine("For: {0}", AverageRunTime(repeat, () => ForSumArray(array)));

repeat = 100000;
Console.WriteLine("For2: {0}", AverageRunTime(repeat, () => ForSumArray2(array)));
Console.WriteLine("Iter: {0}", AverageRunTime(repeat, () => IterSumArray(array)));

private static TimeSpan AverageRunTime(int count, Action method)
{
    var stopwatch = new Stopwatch();
    stopwatch.Start();
    for (var i = 0; i < count; i++)
    {
        method();
    }
    stopwatch.Stop();
    var average = stopwatch.Elapsed.Ticks / count;
    return new TimeSpan(average);
}

When I run these, I get the following

c# performance
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