ConditionSurprise2 https://twitter.com/badamczewski01/status/1391787991132164096?s=20

ConditionSurprise2.cs

// https://sharplab.io/#v2:CYLg1APgAgTAjAWAFBQMwAJboMLoN7LpGYZQAs6AsgBQCU+hxAvsi0smpjOgEICmAOwDGACwC2AQwBOAa2QEkxdAAcpASwBuEgC590UvhOAB7AQBsAnujUDtAbQC66APoSA3MkZFO/YeOkydF4MikrErugAvOgCfADu1raO+OhwADToMBmoGWQZAKwZAGwZAOzoTB6hRGzBnDba6AAyggDm2iJB1SFhxFpS6BJRLu7BSmoAZujUEgB0LQLtIlHRcOgAZOtjvYPzbR0rmbTbRAo7YVDlcFXnbOeX6AAMN8ye3fW2zfsiTcbGyl0lGcdv1BsNXC9ehNjANqA1rMNngiADypNzWMBgWjA85ESbTOYLJaHNabE64wnfQ4wY7dc443FKB7XclKO47dm9B7PYJsNhAA===
using System;
public class C {
    public void M() {
    }
}

public class Benchmark
{
    private readonly int[] _a;

    public Benchmark()
    {
        _a = new int[] { 1, 2, 3, 4, 5, 6, 7 };
    }

    public int Length()
    {
        var a = _a;
        if (a.Length == 1 &&
            a.Length == 2)
        {
            return 1;
        }
        return 0;
    }

    public int LengthLoop()
    {
        var a = _a;
        for (int i = 0; i < 1; i++){
            if (a.Length == 1 &&
                a.Length == 2)
            {
                return 1;
            }
        }
        return 0;
    }
}

ConditionSurprise2_Benchmark.cs

void Main()
{
    var summary = BenchmarkRunner.Run<Benchmark>();
}


[MemoryDiagnoser]
public class Benchmark
{
    private int[] _a;

    [GlobalSetup]
    public void Init()
    {
        _a = new int[] { 1, 2, 3, 4, 5, 6, 7 };
    }

    [Benchmark]
    public int Length()
    {
        var a = _a;
        if (a.Length == 1 &&
            a.Length == 2)
        {
            return 1;
        }
        return 0;
    }

    [Benchmark]
    public int LengthLoop()
    {
        var a = _a;
        for (int i = 0; i < 1; i++){
            if (a.Length == 1 &&
                a.Length == 2)
            {
                return 1;
            }
        }
        return 0;
    }
}

ConditionSurprise2_CompilerGenerated_Debug.cs

using System;
using System.Diagnostics;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Security;
using System.Security.Permissions;

[assembly: CompilationRelaxations(8)]
[assembly: RuntimeCompatibility(WrapNonExceptionThrows = true)]
[assembly: Debuggable(DebuggableAttribute.DebuggingModes.Default | DebuggableAttribute.DebuggingModes.DisableOptimizations | DebuggableAttribute.DebuggingModes.IgnoreSymbolStoreSequencePoints | DebuggableAttribute.DebuggingModes.EnableEditAndContinue)]
[assembly: SecurityPermission(SecurityAction.RequestMinimum, SkipVerification = true)]
[assembly: AssemblyVersion("0.0.0.0")]
[module: UnverifiableCode]
public class C
{
    public void M()
    {
    }
}
public class Benchmark
{
    private readonly int[] _a;

    public Benchmark()
    {
        int[] array = new int[7];
        RuntimeHelpers.InitializeArray(array, (RuntimeFieldHandle)/*OpCode not supported: LdMemberToken*/);
        _a = array;
    }

    public int Length()
    {
        int[] a = _a;
        if (a.Length == 1 && a.Length == 2)
        {
            return 1;
        }
        return 0;
    }

    public int LengthLoop()
    {
        int[] a = _a;
        int num = 0;
        while (num < 1)
        {
            if (a.Length == 1 && a.Length == 2)
            {
                return 1;
            }
            num++;
        }
        return 0;
    }
}
[CompilerGenerated]
internal sealed class <PrivateImplementationDetails>
{
    [StructLayout(LayoutKind.Explicit, Pack = 1, Size = 28)]
    private struct __StaticArrayInitTypeSize=28
    {
    }

    internal static readonly __StaticArrayInitTypeSize=28 4C816952BA53CC361D8E45BD833338DC6427E4A5D5F06EBAD5351FD46439A15A/* Not supported: data(01 00 00 00 02 00 00 00 03 00 00 00 04 00 00 00 05 00 00 00 06 00 00 00 07 00 00 00) */;
}

ConditionSurprise2_CompilerGenerated_Release.cs

using System;
using System.Diagnostics;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Security;
using System.Security.Permissions;

[assembly: CompilationRelaxations(8)]
[assembly: RuntimeCompatibility(WrapNonExceptionThrows = true)]
[assembly: Debuggable(DebuggableAttribute.DebuggingModes.IgnoreSymbolStoreSequencePoints)]
[assembly: SecurityPermission(SecurityAction.RequestMinimum, SkipVerification = true)]
[assembly: AssemblyVersion("0.0.0.0")]
[module: UnverifiableCode]
public class C
{
    public void M()
    {
    }
}
public class Benchmark
{
    private readonly int[] _a;

    public Benchmark()
    {
        int[] array = new int[7];
        RuntimeHelpers.InitializeArray(array, (RuntimeFieldHandle)/*OpCode not supported: LdMemberToken*/);
        _a = array;
    }

    public int Length()
    {
        int[] a = _a;
        if (a.Length == 1 && a.Length == 2)
        {
            return 1;
        }
        return 0;
    }

    public int LengthLoop()
    {
        int[] a = _a;
        int num = 0;
        while (num < 1)
        {
            if (a.Length == 1 && a.Length == 2)
            {
                return 1;
            }
            num++;
        }
        return 0;
    }
}
[CompilerGenerated]
internal sealed class <PrivateImplementationDetails>
{
    [StructLayout(LayoutKind.Explicit, Pack = 1, Size = 28)]
    private struct __StaticArrayInitTypeSize=28
    {
    }

    internal static readonly __StaticArrayInitTypeSize=28 4C816952BA53CC361D8E45BD833338DC6427E4A5D5F06EBAD5351FD46439A15A/* Not supported: data(01 00 00 00 02 00 00 00 03 00 00 00 04 00 00 00 05 00 00 00 06 00 00 00 07 00 00 00) */;
}

JIT.asm

; Core CLR v5.0.421.11614 on x86

C..ctor()
    L0000: ret

C.M()
    L0000: ret

Benchmark..ctor()
    L0000: push esi
    L0001: vzeroupper
    L0004: mov esi, ecx
    L0006: mov ecx, 0x63eb9d0
    L000b: mov edx, 7
    L0010: call 0x061f3224
    L0015: mov edx, 0x189808e8
    L001a: vmovdqu xmm0, [edx]
    L001e: vmovdqu [eax+8], xmm0
    L0023: mov ecx, [edx+0x10]
    L0026: mov [eax+0x18], ecx
    L0029: mov ecx, [edx+0x14]
    L002c: mov [eax+0x1c], ecx
    L002f: mov ecx, [edx+0x18]
    L0032: mov [eax+0x20], ecx
    L0035: lea edx, [esi+4]
    L0038: call 0x6fb2d3b0
    L003d: pop esi
    L003e: ret

Benchmark.Length()
    L0000: push ebp
    L0001: mov ebp, esp
    L0003: mov eax, [ecx+4]
    L0006: mov eax, [eax+4]
    L0009: cmp eax, 1
    L000c: jne short L001a
    L000e: cmp eax, 2
    L0011: jne short L001a
    L0013: mov eax, 1
    L0018: pop ebp
    L0019: ret
    L001a: xor eax, eax
    L001c: pop ebp
    L001d: ret

Benchmark.LengthLoop()
    L0000: push ebp
    L0001: mov ebp, esp
    L0003: mov eax, [ecx+4]
    L0006: xor edx, edx
    L0008: mov eax, [eax+4]
    L000b: inc edx
    L000c: test edx, edx
    L000e: jle short L000b
    L0010: xor eax, eax
    L0012: pop ebp
    L0013: ret