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Slightly improve performance and maintainability in SequentialGuidValueGenerator #35981

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Slightly improve performance and maintainability in SequentialGuidValueGenerator #35981

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35 changes: 24 additions & 11 deletions src/EFCore/ValueGeneration/SequentialGuidValueGenerator.cs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Buffers.Binary;
using System.Runtime.InteropServices;

namespace Microsoft.EntityFrameworkCore.ValueGeneration;
Expand Down Expand Up @@ -35,17 +36,29 @@ public class SequentialGuidValueGenerator : ValueGenerator<Guid>
/// <returns>The value to be assigned to a property.</returns>
public override Guid Next(EntityEntry entry)
{
Span<byte> guidBytes = stackalloc byte[16];
var succeeded = Guid.NewGuid().TryWriteBytes(guidBytes);
Check.DebugAssert(succeeded, "Could not write Guid to Span");
var incrementedCounter = Interlocked.Increment(ref _counter);
Span<byte> counterBytes = stackalloc byte[sizeof(long)];
MemoryMarshal.Write(counterBytes, in incrementedCounter);
var guid = Guid.NewGuid();

if (!BitConverter.IsLittleEndian)
{
counterBytes.Reverse();
}
var counter = BitConverter.IsLittleEndian
? Interlocked.Increment(ref _counter)
: BinaryPrimitives.ReverseEndianness(Interlocked.Increment(ref _counter));

var counterBytes = MemoryMarshal.AsBytes(
new ReadOnlySpan<long>(in counter));

// Guid uses a sequential layout where the first 8 bytes (_a, _b, _c)
// are subject to byte-swapping on big-endian systems when reading from
// or writing to a byte array (e.g., via MemoryMarshal or Guid constructors).
// The remaining 8 bytes (_d through _k) are interpreted as-is,
// regardless of endianness.
//
// Since we only modify the last 8 bytes of the Guid (bytes 8–15),
// byte order does not affect the result.
//
// This allows us to safely use MemoryMarshal.AsBytes to directly access
// and modify the Guid's underlying bytes without any extra conversions,
// which also slightly improves performance on big-endian architectures.
var guidBytes = MemoryMarshal.AsBytes(
new Span<Guid>(ref guid));

guidBytes[08] = counterBytes[1];
guidBytes[09] = counterBytes[0];
Expand All @@ -56,7 +69,7 @@ public override Guid Next(EntityEntry entry)
guidBytes[14] = counterBytes[3];
guidBytes[15] = counterBytes[2];

return new Guid(guidBytes);
return guid;
}

/// <summary>
Expand Down