Make SequentialGuidValueGenerator non-allocating

[NKnusperer]

The current implementation of SequentialGuidValueGenerator.Next() allocates some heap memory due to byte[] usages:

efcore/src/EFCore/ValueGeneration/SequentialGuidValueGenerator.cs

Lines 34 to 54 in bdd9846

	public override Guid Next(EntityEntry entry)
	{
	var guidBytes = Guid.NewGuid().ToByteArray();
	var counterBytes = BitConverter.GetBytes(Interlocked.Increment(ref _counter));
	if (!BitConverter.IsLittleEndian)
	{
	Array.Reverse(counterBytes);
	}
	guidBytes[08] = counterBytes[1];
	guidBytes[09] = counterBytes[0];
	guidBytes[10] = counterBytes[7];
	guidBytes[11] = counterBytes[6];
	guidBytes[12] = counterBytes[5];
	guidBytes[13] = counterBytes[4];
	guidBytes[14] = counterBytes[3];
	guidBytes[15] = counterBytes[2];
	return new Guid(guidBytes);
	}

We can rewrite this to a Span based version to avoid any allocations like this:

public override Guid Next(EntityEntry entry)
{
    Span<byte> guidBytes = stackalloc byte[16];
    Guid.NewGuid().TryWriteBytes(guidBytes);
    long incrementedCounter = Interlocked.Increment(ref _counter);
    Span<byte> counterBytes = MemoryMarshal.AsBytes(MemoryMarshal.CreateSpan(ref incrementedCounter, 1));
    
    if (!BitConverter.IsLittleEndian)
    {
        counterBytes.Reverse();
    }
    
    guidBytes[08] = counterBytes[1];
    guidBytes[09] = counterBytes[0];
    guidBytes[10] = counterBytes[7];
    guidBytes[11] = counterBytes[6];
    guidBytes[12] = counterBytes[5];
    guidBytes[13] = counterBytes[4];
    guidBytes[14] = counterBytes[3];
    guidBytes[15] = counterBytes[2];
    
    return new Guid(guidBytes);
}

Comparison:

BenchmarkDotNet=v0.13.5, OS=Windows 10 (10.0.19045.2728/22H2/2022Update)
11th Gen Intel Core i7-11850H 2.50GHz, 1 CPU, 16 logical and 8 physical cores
.NET SDK=7.0.202
  [Host]     : .NET 6.0.15 (6.0.1523.11507), X64 RyuJIT AVX2
  DefaultJob : .NET 6.0.15 (6.0.1523.11507), X64 RyuJIT AVX2


| Method |     Mean |    Error |   StdDev | Ratio |   Gen0 | Allocated | Alloc Ratio |
|------- |---------:|---------:|---------:|------:|-------:|----------:|------------:|
|    Old | 65.44 ns | 0.611 ns | 0.572 ns |  1.00 | 0.0057 |      72 B |        1.00 |
|    New | 58.45 ns | 0.639 ns | 0.598 ns |  0.89 |      - |         - |        0.00 |

I can open a PR once this has been reviewed and approved.

[roji]

@NKnusperer LGTM - go ahead and submit a PR.

[NKnusperer]

@roji I just noticed that

Span<byte> counterBytes = MemoryMarshal.AsBytes(MemoryMarshal.CreateSpan(ref incrementedCounter, 1));

can also be expressed as

Span<byte> counterBytes = stackalloc byte[sizeof(long)];
BitConverter.TryWriteBytes(counterBytes, incrementedCounter);

which is probably the preferred version right?

[roji]

Sure, that's slightly better. You should also be able to just MemoryMarshal.Write, which is very similar but slightly terser (no real need for Try here).