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Merged
mattwar merged 1 commit into from
Sep 5, 2026
Merged

Reorder fielsd to possibly save memory footprint #19

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14 changes: 8 additions & 6 deletions scratch/Program.cs
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Original file line number Diff line number Diff line change
@@ -1,9 +1,11 @@
var u = new MyUnion(new A(1, 2.0f));
using System.Runtime.CompilerServices;

Console.WriteLine(u);
var u = new MyUnion(new A(1, 2));

if (u is A)
Console.WriteLine("I'm an A");
Console.WriteLine($"size: {Unsafe.SizeOf<MyUnion>()}");

if (u is byte)
Console.WriteLine("I'm a byte");

if (u is IFoo)
Console.WriteLine("I'm an IFoo");
Expand All @@ -20,12 +22,12 @@ partial void Cases(
int x,
IBar bar,
IFoo foo,
A a
byte y
);
}

public interface IFoo {}
public interface IBar {}
public record struct A(int X, float Y) : IFoo;
public record struct A(int X, byte Y) : IFoo;


2 changes: 1 addition & 1 deletion scratch/Scratch.csproj
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@

<ItemGroup>
<!-- version must match SourceGenerators.Package build output; clear ~/.nuget/packages/uniontypes.toolkit.generators after rebuilding -->
<PackageReference Include="UnionTypes.Toolkit.Generators" Version="0.0.0" PrivateAssets="all" />
<PackageReference Include="UnionTypes.Toolkit.Generator" Version="0.0.0" PrivateAssets="all" />
</ItemGroup>

</Project>
3 changes: 2 additions & 1 deletion src/GeneratedTypes/SimplePrimitivesUnion_generated.cs
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Expand Up @@ -12,10 +12,11 @@
#pragma warning disable CS8618

[System.Runtime.CompilerServices.Union]
[StructLayout(LayoutKind.Sequential)]
public partial struct SimplePrimitivesUnion : System.Runtime.CompilerServices.IUnion
{
private readonly int _kind;
private readonly Overlapped _overlapped;
private readonly byte _kind;

[StructLayout(LayoutKind.Explicit)]
private struct Overlapped
Expand Down
3 changes: 2 additions & 1 deletion src/GeneratedTypes/TagLikeUnion_generated.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,10 +12,11 @@
#pragma warning disable CS8618

[System.Runtime.CompilerServices.Union]
[StructLayout(LayoutKind.Sequential)]
public partial struct TagLikeUnion : System.Runtime.CompilerServices.IUnion
{
private readonly int _kind;
private readonly Overlapped _overlapped;
private readonly byte _kind;

[StructLayout(LayoutKind.Explicit)]
private struct Overlapped
Expand Down
10 changes: 5 additions & 5 deletions src/Generators.Tests/CustomUnionGeneratorTests.cs
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Expand Up @@ -48,7 +48,7 @@ public void TestOverlappablePrimitiveCases()
generatedText =>
{
// prove that the two primitives got overlapped into the overlapped field
Assert.IsTrue(generatedText.Contains("int _kind"));
Assert.IsTrue(generatedText.Contains("_kind"));
Assert.IsTrue(generatedText.Contains("_overlapped"));
Assert.IsFalse(generatedText.Contains("_value1"));

Expand Down Expand Up @@ -79,7 +79,7 @@ public record struct B(float Value);
),
generatedText =>
{
Assert.IsTrue(generatedText.Contains("int _kind"));
Assert.IsTrue(generatedText.Contains("_kind"));
Assert.IsTrue(generatedText.Contains("_overlapped"));
Assert.IsFalse(generatedText.Contains("_value1"));
Assert.IsTrue(generatedText.Contains("A Case1"));
Expand Down Expand Up @@ -167,7 +167,7 @@ public record struct B(float Value, int Value2, string Value3);
),
generatedText =>
{
Assert.IsTrue(generatedText.Contains("int _kind"));
Assert.IsTrue(generatedText.Contains("_kind"));
Assert.IsTrue(generatedText.Contains("_overlapped"));
Assert.IsTrue(generatedText.Contains("_value1"));
Assert.IsTrue(generatedText.Contains("int Case1"));
Expand Down Expand Up @@ -225,7 +225,7 @@ public record struct F(int Value, int Value2);
),
generatedText =>
{
Assert.IsTrue(generatedText.Contains("int _kind"));
Assert.IsTrue(generatedText.Contains("_kind"));
Assert.IsTrue(generatedText.Contains("_overlapped"));
Assert.IsTrue(generatedText.Contains("object? _value1"));
Assert.IsTrue(generatedText.Contains("object? _value2"));
Expand Down Expand Up @@ -256,7 +256,7 @@ public record struct B();
),
generatedText =>
{
Assert.IsTrue(generatedText.Contains("int _kind"));
Assert.IsTrue(generatedText.Contains("_kind"));
// there is no overlapped data so the field should not exist
Assert.IsFalse(generatedText.Contains("_overlapped"));
// there is no data other than the tag _kind, so no value fields should exist
Expand Down
16 changes: 12 additions & 4 deletions src/Generators/CustomUnionGenerator.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -68,6 +68,7 @@ private void WriteUnionType(UnionInfo union)
void WriteUnion()
{
_writer.WriteLine("[System.Runtime.CompilerServices.Union]");
_writer.WriteLine($"[StructLayout(LayoutKind.Sequential)]");
_writer.WriteLine($"{union.Accessibility} partial struct {union.DeclarationName} : System.Runtime.CompilerServices.IUnion");
_writer.WriteBraceNested(() =>
{
Expand All @@ -91,15 +92,22 @@ void WriteUnion()
/// </summary>
private void WriteStorageFields(UnionLayout layout)
{
if (layout.TagField != null)
_writer.WriteLine($"private readonly {layout.TagField.Type.TypeName} {layout.TagField.Name};");
// place data fields up front since they are either reference type fiels or
// likely contain reference types (otherwise they would have been overlapped)
// and will always be pointer-size aligned.
foreach (var field in layout.DataFields)
{
_writer.WriteLine($"private readonly {field.Type.TypeName} {field.Name};");
}

if (layout.OverlappedField != null)
_writer.WriteLine($"private readonly {layout.OverlappedField.Type.TypeName} {layout.OverlappedField.Name};");

foreach (var field in layout.DataFields)
// put tag field at end so it may fit inside any otherwise padding space left over from the overlapped field being less than pointer size aligned.
if (layout.TagField != null)
{
_writer.WriteLine($"private readonly {field.Type.TypeName} {field.Name};");
var type = layout.CaseLayouts.Count <= 255 ? "byte" : "ushort";
_writer.WriteLine($"private readonly {type} {layout.TagField.Name};");
}
}

Expand Down
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