Is Array.Copy() faster than for loop, for 2D arrays?

Question

I recently changed

        this.FieldValues = new object[2, fieldValues.GetUpperBound(1) + 1];
        for (int i = 0; i < FieldCount; i++)            
           


        

Answer 1

In my own experience, I've found that I can't trust my intuition about anything when it comes to performance. Consequently, I keep a quick-and-dirty benchmarking app around (that I call "StupidPerformanceTricks"), which I use to test these scenarios. This is invaluable, as I've made all sorts of surprising and counter-intuitive discoveries about performance tricks. It's also important to remember to run your benchmark app in release mode, without a debugger attached, as you otherwise don't get JIT optimizations, and those optimizations can make a significant difference: technique A might be slower than technique B in debug mode, but significantly faster in release mode, with optimized code.

That said, in general, my own testing experience indicates that if your array is < ~32 elements, you'll get better performance by rolling your own copy loop - presumably because you don't have the method call overhead, which can be significant. However, if the loop is larger than ~32 elements, you'll get better performance by using Array.Copy(). (If you're copying ints or floats or similar sorts of things, you might also want to investigate Buffer.BlockCopy(), which is ~10% faster than Array.Copy() for small arrays.)

But all that said, the real answer is, "Write your own tests that match these precise alternatives as closely as possible, wrap them each with a loop, give the loop enough iterations for it to chew up at least 2-3 seconds worth of CPU, and then compare the alternatives yourself."