Eliminate memory allocations in GuidCombGenerator under .NET 8+

[cd21h]

Replace memory allocations with stack allocations in GuidComboGenerator under .NET 8+

ToByteArray() and GetBytes() are replaced with non-allocating equivalents.

[hazzik]

What is performance gain here?

[hazzik]

I would just go with this implementation:

protected static Guid GenerateComb(Guid guid, DateTime utcNow)
{
#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER
	Span<byte> guidArray = stackalloc byte[16];
	guid.TryWriteBytes(guidArray);
#else
	var guidArray = guid.ToByteArray();
#endif
	// Get the days and milliseconds which will be used to build the byte string 
	var ts = new TimeSpan(utcNow.Ticks - BaseDateTicks);
	var days = ts.Days;
	guidArray[10] = (byte) (days >> 8);
	guidArray[11] = (byte) days;
			
	// Note that SQL Server is accurate to 1/300th of a millisecond so we divide by 3.333333 
	var msecs = (long) (utcNow.TimeOfDay.TotalMilliseconds / 3.333333);
	guidArray[12] = (byte) (msecs >> 24);
	guidArray[13] = (byte) (msecs >> 16);
	guidArray[14] = (byte) (msecs >> 8);
	guidArray[15] = (byte) msecs;

	return new Guid(guidArray);
}

[cd21h]

What is performance gain here?
@hazzik

Optimized1 - proposed version in PR
Optimized2 - your version

// * Summary *

BenchmarkDotNet v0.14.0, Windows 11 (10.0.22621.4169/22H2/2022Update/SunValley2)
11th Gen Intel Core i7-11800H 2.30GHz, 1 CPU, 16 logical and 8 physical cores
.NET SDK 9.0.100-preview.4.24267.66
  [Host]   : .NET 8.0.8 (8.0.824.36612), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI
  .NET 8.0 : .NET 8.0.8 (8.0.824.36612), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI
  .NET 9.0 : .NET 9.0.0 (9.0.24.26619), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI


| Method         | Job      | Runtime  | Mean      | Error     | StdDev    | Ratio | RatioSD | Gen0   | Allocated | Alloc Ratio |
|--------------- |--------- |--------- |----------:|----------:|----------:|------:|--------:|-------:|----------:|------------:|
| OriginalMethod | .NET 8.0 | .NET 8.0 | 27.930 ns | 0.5818 ns | 0.7565 ns |  1.00 |    0.04 | 0.0083 |     104 B |        1.00 |
| Optimized1     | .NET 8.0 | .NET 8.0 |  6.973 ns | 0.1096 ns | 0.1025 ns |  0.25 |    0.01 |      - |         - |        0.00 |
| Optimized2     | .NET 8.0 | .NET 8.0 |  5.481 ns | 0.0431 ns | 0.0382 ns |  0.20 |    0.01 |      - |         - |        0.00 |
|                |          |          |           |           |           |       |         |        |           |             |
| OriginalMethod | .NET 9.0 | .NET 9.0 | 25.708 ns | 0.2555 ns | 0.2134 ns |  1.00 |    0.01 | 0.0083 |     104 B |        1.00 |
| Optimized1     | .NET 9.0 | .NET 9.0 |  8.416 ns | 0.0594 ns | 0.0496 ns |  0.33 |    0.00 |      - |         - |        0.00 |
| Optimized2     | .NET 9.0 | .NET 9.0 |  5.687 ns | 0.0257 ns | 0.0201 ns |  0.22 |    0.00 |      - |         - |        0.00 |

// * Hints *
Outliers
  GuidCombGeneratorBenchmark.OriginalMethod: .NET 8.0 -> 1 outlier  was  removed (32.05 ns)
  GuidCombGeneratorBenchmark.Optimized2: .NET 8.0     -> 1 outlier  was  removed (7.44 ns)
  GuidCombGeneratorBenchmark.OriginalMethod: .NET 9.0 -> 2 outliers were removed (28.37 ns, 29.39 ns)
  GuidCombGeneratorBenchmark.Optimized1: .NET 9.0     -> 2 outliers were removed, 3 outliers were detected (10.00 ns, 10.23 ns, 10.27 ns)
  GuidCombGeneratorBenchmark.Optimized2: .NET 9.0     -> 3 outliers were removed (7.47 ns..7.66 ns)

// * Legends *
  Mean        : Arithmetic mean of all measurements
  Error       : Half of 99.9% confidence interval
  StdDev      : Standard deviation of all measurements
  Ratio       : Mean of the ratio distribution ([Current]/[Baseline])
  RatioSD     : Standard deviation of the ratio distribution ([Current]/[Baseline])
  Gen0        : GC Generation 0 collects per 1000 operations
  Allocated   : Allocated memory per single operation (managed only, inclusive, 1KB = 1024B)
  Alloc Ratio : Allocated memory ratio distribution ([Current]/[Baseline])
  1 ns        : 1 Nanosecond (0.000000001 sec)