Double.IsNaN test 100 times faster?

Question

I found this in the .NET Source Code: It claims to be 100 times faster than System.Double.IsNaN. Is there a reason to not use this function instead of Sys

Answer 1

Here's a naive benchmark:

public static void Main()
{
    int iterations = 500 * 1000 * 1000;

    double nan = double.NaN;
    double notNan = 42;

    Stopwatch sw = Stopwatch.StartNew();

    bool isNan;
    for (int i = 0; i < iterations; i++)
    {
        isNan = IsNaN(nan);     // true
        isNan = IsNaN(notNan);  // false
    }

    sw.Stop();
    Console.WriteLine("IsNaN: {0}", sw.ElapsedMilliseconds);

    sw = Stopwatch.StartNew();

    for (int i = 0; i < iterations; i++)
    {
        isNan = double.IsNaN(nan);     // true
        isNan = double.IsNaN(notNan);  // false
    }

    sw.Stop();
    Console.WriteLine("double.IsNaN: {0}", sw.ElapsedMilliseconds);

    Console.Read();
}

Obviously they're wrong:

IsNaN: 15012

double.IsNaN: 6243

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EDIT + NOTE: I'm sure the timing will change depending on input values, many other factors etc., but claiming that generally speaking this wrapper is 100x faster than the default implementation seems just wrong.