Nullable optimization. use is to determine Nullable<T> to T. https://twitter.com/badamczewski01/status/1452188929747308544

NullableOptimization.cs

BenchmarkRunner.Run<BenchmarkNullable>();

[BenchmarkDotNet.Attributes.MemoryDiagnoser]
public class BenchmarkNullable
{
    [Benchmark]
    public void Slow()
    {
        NullaleSlow(0);
    }

    [Benchmark]
    public void Fast()
    {
        NullaleFast(0);
    }

    int? NullaleSlow(int? x)
    {
        for (int i = 0; i < 1000; i++)
        {
            x++;
        }
        return x;
    }

    int? NullaleFast(int? x)
    {
        if (x is int i32)
        {
            for (int i = 0; i < 1000; i++)
            {
                i32++;
            }
            x = i32;
        }
        return x;
    }
}

Result_BenchmarkRunner_NET5.txt

// Validating benchmarks:
Assembly LINQPadQuery, Version=1.0.0.139, Culture=neutral, PublicKeyToken=null is located in temp. If you are running benchmarks from xUnit you need to disable shadow copy. It's not supported by design.
// ***** BenchmarkRunner: Start   *****

// * Summary *

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.22000
AMD Ryzen 9 5900X, 1 CPU, 24 logical and 12 physical cores
.NET SDK=6.0.100-rc.2.21505.57
  [Host] : .NET 5.0.11 (5.0.1121.47308), X64 RyuJIT


| Method |     Mean |   Error |  StdDev | Allocated |
|------- |---------:|--------:|--------:|----------:|
|   Slow | 648.4 ns | 7.86 ns | 7.35 ns |         - |
|   Fast | 223.5 ns | 0.62 ns | 0.55 ns |         - |

// * Legends *
  Mean      : Arithmetic mean of all measurements
  Error     : Half of 99.9% confidence interval
  StdDev    : Standard deviation of all measurements
  Allocated : Allocated memory per single operation (managed only, inclusive, 1KB = 1024B)
  1 ns      : 1 Nanosecond (0.000000001 sec)

// * Diagnostic Output - MemoryDiagnoser *


// ***** BenchmarkRunner: End *****
// ** Remained 0 benchmark(s) to run **
Run time: 00:00:44 (44.52 sec), executed benchmarks: 2

Global total time: 00:00:44 (44.56 sec), executed benchmarks: 2
// * Artifacts cleanup *

Result_BenchmarkRunner_NET6.txt

// Validating benchmarks:
Assembly LINQPadQuery, Version=1.0.0.6, Culture=neutral, PublicKeyToken=null is located in temp. If you are running benchmarks from xUnit you need to disable shadow copy. It's not supported by design.
// ***** BenchmarkRunner: Start   *****

// * Summary *

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.22000
AMD Ryzen 9 5900X, 1 CPU, 24 logical and 12 physical cores
.NET SDK=6.0.100-rc.2.21505.57
  [Host] : .NET 6.0.0 (6.0.21.48005), X64 RyuJIT


| Method |     Mean |   Error |  StdDev | Allocated |
|------- |---------:|--------:|--------:|----------:|
|   Slow | 637.5 ns | 0.88 ns | 0.73 ns |         - |
|   Fast | 221.0 ns | 0.53 ns | 0.42 ns |         - |

// * Legends *
  Mean      : Arithmetic mean of all measurements
  Error     : Half of 99.9% confidence interval
  StdDev    : Standard deviation of all measurements
  Allocated : Allocated memory per single operation (managed only, inclusive, 1KB = 1024B)
  1 ns      : 1 Nanosecond (0.000000001 sec)

// * Diagnostic Output - MemoryDiagnoser *


// ***** BenchmarkRunner: End *****
// ** Remained 0 benchmark(s) to run **
Run time: 00:00:41 (41.7 sec), executed benchmarks: 2

Global total time: 00:00:41 (41.73 sec), executed benchmarks: 2
// * Artifacts cleanup *