Array boundaries check optimization in a for-loop

Question

        var ar = new int[500000000];

        var sw = new Stopwatch();
        sw.Start();

        var length = ar.Length;
        for (var i = 0; i < length; i++)
        {
            if (ar[i] == 0);
        }

        sw.Stop();

sw.ElapsedMilliseconds: ~2930ms

        var ar = new int[500000000];

        var sw = new Stopwatch();
        sw.Start();

        for (var i = 0; i < ar.Length; i++)
        {
            if (ar[i] == 0);
        }

        sw.Stop();

sw.ElapsedMilliseconds: ~3520ms

Win8x64, VS12, .NET4.5, Release build, "Optimize code" on.

As far as I know the second approach should be faster because of an array boundaries check optimization. Am I missing something?

Answer 1

I'm also using Win8 x64, .NET 4.5, Release build, outside of the debugger (this is an important one); I get:

0: 813ms vs 421ms
1: 439ms vs 420ms
2: 440ms vs 420ms
3: 431ms vs 429ms
4: 433ms vs 427ms
5: 424ms vs 437ms
6: 427ms vs 434ms
7: 430ms vs 432ms
8: 432ms vs 435ms
9: 430ms vs 430ms
10: 427ms vs 418ms
11: 422ms vs 421ms
12: 434ms vs 420ms
13: 439ms vs 425ms
14: 426ms vs 429ms
15: 426ms vs 426ms
16: 417ms vs 432ms
17: 442ms vs 425ms
18: 420ms vs 429ms
19: 420ms vs 422ms

The first pays a JIT / "fusion" cost, but overall it is about the same (some in each column look faster, but overall not much to speak about).

using System;
using System.Diagnostics;
static class Program
{
    static void Main()
    {
        var ar = new int[500000000];

        for (int j = 0; j < 20; j++)
        {
            var sw = Stopwatch.StartNew();
            var length = ar.Length;
            for (var i = 0; i < length; i++)
            {
                if (ar[i] == 0) ;
            }

            sw.Stop();
            long hoisted = sw.ElapsedMilliseconds;

            sw = Stopwatch.StartNew();
            for (var i = 0; i < ar.Length; i++)
            {
                if (ar[i] == 0) ;
            }
            sw.Stop();
            long direct = sw.ElapsedMilliseconds;

            Console.WriteLine("{0}: {1}ms vs {2}ms", j, hoisted, direct);
        }
    }
}