runtime: for loop little slower on .NET Core 3.0

The (very) basic benchmark below shows that a for loop on .NET Core 3.0 is a little slower than on .NET Core 2.1.5. What surprised me most, is that a foreach is “a lot” faster than a for loop on .NET Core 3.0.

using BenchmarkDotNet.Attributes;
using System.Linq;

namespace Benchmarks
{
    public class LoopBenchmark
    {
        private int[] _ints;
        private string[] _strings;

        [Benchmark]
        public void Int32_ForEach()
        {
            foreach (var s in _ints)
            {
                if (s == -1)
                    break;
            }
        }

        [Benchmark]
        public void Int32_For()
        {
            for (var i = 0; i < _ints.Length; i++)
            {
                if (_ints[i] == -1)
                    break;
            }
        }

        [Benchmark]
        public void String_ForEach()
        {
            foreach (var s in _strings)
            {
                if (s == null)
                    break;
            }
        }

        [Benchmark]
        public void String_For()
        {
            for (var i = 0; i < _strings.Length; i++)
            {
                if (_strings[i] == null)
                    break;
            }
        }

        [GlobalSetup]
        public void GlobalSetup()
        {
            _ints = Enumerable.Range(1, 100).ToArray();
            _strings = _ints.Select(p => p.ToString()).ToArray();
        }
    }
}

.NET Core 3.0

BenchmarkDotNet=v0.11.2, OS=Windows 10.0.17134.345 (1803/April2018Update/Redstone4)
Intel Core i7-6700K CPU 4.00GHz (Skylake), 1 CPU, 8 logical and 4 physical cores
Frequency=3914070 Hz, Resolution=255.4885 ns, Timer=TSC
.NET Core SDK=3.0.100-alpha1-009640
  [Host]     : .NET Core 3.0.0-preview1-27004-04 (CoreCLR 4.6.27003.04, CoreFX 4.6.27003.02), 64bit RyuJIT
  DefaultJob : .NET Core 3.0.0-preview1-27004-04 (CoreCLR 4.6.27003.04, CoreFX 4.6.27003.02), 64bit RyuJIT

Method	Mean	Error	StdDev
Int32_ForEach	40.81 ns	0.2533 ns	0.2369 ns
Int32_For	56.18 ns	0.8982 ns	0.8402 ns
String_ForEach	41.19 ns	0.2446 ns	0.2169 ns
String_For	56.52 ns	0.3497 ns	0.3271 ns

.NET Core 2.1.5

BenchmarkDotNet=v0.11.2, OS=Windows 10.0.17134.345 (1803/April2018Update/Redstone4)
Intel Core i7-6700K CPU 4.00GHz (Skylake), 1 CPU, 8 logical and 4 physical cores
Frequency=3914070 Hz, Resolution=255.4885 ns, Timer=TSC
.NET Core SDK=3.0.100-alpha1-009640
  [Host]     : .NET Core 2.1.5 (CoreCLR 4.6.26919.02, CoreFX 4.6.26919.02), 64bit RyuJIT
  DefaultJob : .NET Core 2.1.5 (CoreCLR 4.6.26919.02, CoreFX 4.6.26919.02), 64bit RyuJIT

Method	Mean	Error	StdDev
Int32_ForEach	52.81 ns	0.3626 ns	0.3392 ns
Int32_For	54.38 ns	0.2734 ns	0.2557 ns
String_ForEach	52.76 ns	0.3920 ns	0.3274 ns
String_For	54.35 ns	0.3005 ns	0.2811 ns

I always (wrongly?) assumed that a for loop would produce far (?) more optimized code.

category:cq theme:loop-opt skill-level:intermediate cost:medium