Should I prefer variables or multiple indirections on arrays in C# in perf critical code?

Question

in some perf critical program (single threaded), if I have arrays of primitive types, and need to access the same index of those more than once in loops.

Should I us

Answer 1

Lets test this:

  int[] arr = new int[]{1, 2, 3, 4, 5, 6, 7};
  int t = arr[3];
  int a = 0;
  var start = DateTime.UtcNow;
  for (int i = 0; i < 1000000000; i++)
  {
    a += t;
  }
  Console.WriteLine(a);
  Console.WriteLine(DateTime.UtcNow-start);
  a = 0;
  start = DateTime.UtcNow;
  for (int i = 0; i < 1000000000; i++)
  {
    a += arr[3];
  }
  Console.WriteLine(a);
  Console.WriteLine(DateTime.UtcNow - start);

Output:

-294967296
00:00:02.1925000
-294967296
00:00:03.4250000

Yes its slower to access the array repeatedly.

Answer 2

Array access always involves indirection, so if there are frequent accesses the variable will likely be faster.

That said, I find it incredibly unlikely that you will be able to measure the difference. This is an example of micro-optimization.

Answer 3

In general access to an array is slower than a temporary because it has 2 additional pieces of overhead

• A layer of indirection
• A bounds check

However I would not be changing any code I had today based on that knowledge unless a profiler clearly showed a tight loop to be a significant perf problem in my application. And furthermore showed that changing to a cached local produced a significant perf benefit.