Sum of products of two arrays (dotproduct)

Question

First off, I know my title can be formulated better, but my math classes are so far gone I can\'t remember the correct words anymore..

I need to do something like this (

Answer 1

Even faster is to unroll the loop

Test("Regular Loop", () =>
            {
                int result = 0;
                for (int i = 0; i < digits1.Length; i++)
                {
                    result += digits1[i] * digits2[i];
                }
                return result;
            });
            // This will fail if vectors are not a multiple of 4 in length.
            Test("Unroll 4x", () =>
            {
                int result = 0;
                for (int i = 0; i < digits1.Length; i+=4)
                {
                    result += digits1[i] * digits2[i];
                    result += digits1[i+1] * digits2[i+1];
                    result += digits1[i+2] * digits2[i+2];
                    result += digits1[i+3] * digits2[i+3];
                }
                return result;
            });

            Test("Dynamic unroll", () =>
            {
                int result = 0;
                int limit = (digits1.Length/8)*8;
                int reminderLimit = digits1.Length;

                if (digits1.Length >= 8)
                {
                    for (int i = 0; i < limit; i+=8)
                    {
                        result += digits1[i] * digits2[i];
                        result += digits1[i+1] * digits2[i+1];
                        result += digits1[i+2] * digits2[i+2];
                        result += digits1[i+3] * digits2[i+3];
                        result += digits1[i+4] * digits2[i+4];
                        result += digits1[i+5] * digits2[i+5];
                        result += digits1[i+6] * digits2[i+6];
                        result += digits1[i+7] * digits2[i+7];
                    }

                    reminderLimit = digits1.Length % 8;
                }

                switch(reminderLimit)
                {
                    case 7: {
                                result += digits1[limit] * digits2[limit];
                                result += digits1[limit+1] * digits2[limit+1];
                                result += digits1[limit+2] * digits2[limit+2];
                                result += digits1[limit+3] * digits2[limit+3];
                                result += digits1[limit+4] * digits2[limit+4];
                                result += digits1[limit+5] * digits2[limit+5];
                                result += digits1[limit+6] * digits2[limit+6];
                                break;
                            }

                    case 6: {
                                result += digits1[limit] * digits2[limit];
                                result += digits1[limit+1] * digits2[limit+1];
                                result += digits1[limit+2] * digits2[limit+2];
                                result += digits1[limit+3] * digits2[limit+3];
                                result += digits1[limit+4] * digits2[limit+4];
                                result += digits1[limit+5] * digits2[limit+5];      
                                break;
                            }

                    case 5: {
                                result += digits1[limit] * digits2[limit];
                                result += digits1[limit+1] * digits2[limit+1];
                                result += digits1[limit+2] * digits2[limit+2];
                                result += digits1[limit+3] * digits2[limit+3];
                                result += digits1[limit+4] * digits2[limit+4];
                                break;
                            }

                    case 4: {
                                result += digits1[limit] * digits2[limit];
                                result += digits1[limit+1] * digits2[limit+1];
                                result += digits1[limit+2] * digits2[limit+2];
                                result += digits1[limit+3] * digits2[limit+3];
                                break;
                            }

                    case 3: {
                                result += digits1[limit] * digits2[limit];
                                result += digits1[limit+1] * digits2[limit+1];
                                result += digits1[limit+2] * digits2[limit+2];
                                break;
                            }

                    case 2: {
                                result += digits1[limit] * digits2[limit];
                                result += digits1[limit+1] * digits2[limit+1];
                                break;
                            }

                    case 1: {
                                result += digits1[limit] * digits2[limit];
                                break;
                            }
                    default :
                            {
                                break;
                            }
                }

                return result;
            });

There is a huge difference is running time between debug and release mode of C# code, this is run with release mode:

Regular Loop
Iterations: 1000000
Time(ms):   0/         0/       0 (       596)
Ticks:      1/  2,071213/     455 (   2154248)

Unroll 4x
Iterations: 1000000
Time(ms):   0/     2E-06/       1 (       575)
Ticks:      1/  1,984301/    3876 (   2076105)

Dynamic unroll
Iterations: 1000000
Time(ms):   0/         0/       0 (       430)
Ticks:      1/    1,4635/    3228 (   1554830)

Debug mode:

Regular Loop
Iterations: 1000000
Time(ms):   0/     1E-06/       1 (      1296)
Ticks:      4/  4,529916/    3907 (   4678354)

Unroll 4x
Iterations: 1000000
Time(ms):   0/         0/       0 (       871)
Ticks:      2/  3,048466/     701 (   3145277)

Dynamic unroll
Iterations: 1000000
Time(ms):   0/         0/       0 (       819)
Ticks:      2/  2,858588/    1398 (   2957179)