Align up structs to IntPtr.Size if they have gc pointers

src/coreclr/tools/Common/TypeSystem/Common/MetadataFieldLayoutAlgorithm.cs

@@ -965,33 +965,37 @@ private static SizeAndAlignment ComputeInstanceSize(MetadataType type, LayoutInt
         {
             SizeAndAlignment result;
 
-            // Pad the length of structs to be 1 if they are empty so we have no zero-length structures
+            // Pad the length of structs to be 1 if they are empty, so we have no zero-length structures
             if (type.IsValueType && instanceSize == LayoutInt.Zero)
             {
                 instanceSize = LayoutInt.One;
             }
 
             TargetDetails target = type.Context.Target;
 
-            if (classLayoutSize != 0)
+            if (type.IsValueType)
             {
-                LayoutInt parentSize;
-                if (type.IsValueType)
-                    parentSize = new LayoutInt(0);
-                else
-                    parentSize = type.BaseType.InstanceByteCountUnaligned;
-
-                LayoutInt specifiedInstanceSize = parentSize + new LayoutInt(classLayoutSize);
+                if (classLayoutSize != 0)
+                {
+                    instanceSize = LayoutInt.Max(new LayoutInt(classLayoutSize), instanceSize);
 
-                instanceSize = LayoutInt.Max(specifiedInstanceSize, instanceSize);
-            }
-            else
-            {
-                if (type.IsValueType)
+                    // Size of a struct with gc references is always expected to be aligned to pointer size
+                    if (type.ContainsGCPointers)
+                    {
+                        instanceSize = LayoutInt.AlignUp(instanceSize, new LayoutInt(target.PointerSize), target);
+                    }
+                }
+                else
                 {
                     instanceSize = LayoutInt.AlignUp(instanceSize, alignment, target);
                 }
             }
+            else if (classLayoutSize != 0 && type.IsSequentialLayout && !type.ContainsGCPointers)
+            {
+                // For classes, we respect classLayoutSize only for SequentialLayout + no gc fields
+                LayoutInt specifiedInstanceSize = type.BaseType.InstanceByteCountUnaligned + new LayoutInt(classLayoutSize);
+                instanceSize = LayoutInt.Max(specifiedInstanceSize, instanceSize);
+            }
 
             if (type.IsValueType)
             {

src/coreclr/tools/aot/ILCompiler.TypeSystem.Tests/InstanceFieldLayoutTests.cs

@@ -76,7 +76,7 @@ public void TestExplicitLayoutThatIsEmpty()
         public void TestExplicitTypeLayoutWithSize()
         {
             var explicitSizeType = _testModule.GetType("Explicit", "ExplicitSize");
-            Assert.Equal(48, explicitSizeType.InstanceByteCount.AsInt);
+            Assert.Equal(32, explicitSizeType.InstanceByteCount.AsInt);
         }
 
         [Fact]

src/tests/Loader/classloader/explicitlayout/Regressions/100220/Runtime_100220.cs

@@ -0,0 +1,190 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Reflection;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using Xunit;
+
+public static class Runtime_100220
+{
+    [Fact]
+    [ActiveIssue("https://github.com/dotnet/runtime/issues/100220", TestRuntimes.Mono)]
+    // Also, Mono needs RuntimeHelpers.GetRawObjectDataSize or equivalent to get an object size
+    public static int TestEntryPoint()
+    {
+        if (IntPtr.Size == 8)
+        {
+            // Classes
+            Assert.Equal(16, ObjectSize<MyClassAuto>());
+            Assert.Equal(16, ObjectSize<MyClass1000Exp>());
+            Assert.Equal(16, ObjectSize<MyClass1000Seq>());
+            Assert.Equal(16, ObjectSize<MyClass1000NoGcExp>());
+            Assert.Equal(1000, ObjectSize<MyClass1000NoGcSeq>());
+            Assert.Equal(24, ObjectSize<BaseClassSeq>());
+            Assert.Equal(40, ObjectSize<SubclassSeq>());
+            Assert.Equal(48, ObjectSize<SubclassSubclassSeq>());
+            Assert.Equal(32, ObjectSize<SubclassWithGcSeq>());
+            Assert.Equal(16, ObjectSize<SubclassOfBaseWithGcSeq>());
+
+            // Structs
+            Assert.Equal(16, Unsafe.SizeOf<MyStructAuto>());
+            Assert.Equal(16, Unsafe.SizeOf<MyStruct9Seq>());
+            Assert.Equal(16, Unsafe.SizeOf<MyStruct9Exp>());
+            Assert.Equal(16, Unsafe.SizeOf<MyStruct1000Seq>());
+            Assert.Equal(1000, Unsafe.SizeOf<MyStruct1000Exp>());
+        }
+        else
+        {
+            // Classes
+            Assert.Equal(8, ObjectSize<MyClassAuto>());
+            Assert.Equal(12, ObjectSize<MyClass1000Exp>());
+            Assert.Equal(8, ObjectSize<MyClass1000Seq>());
+            Assert.Equal(12, ObjectSize<MyClass1000NoGcExp>());
+            Assert.Equal(1000, ObjectSize<MyClass1000NoGcSeq>());
+            Assert.Equal(20, ObjectSize<BaseClassSeq>());
+            Assert.Equal(36, ObjectSize<SubclassSeq>());
+            Assert.Equal(44, ObjectSize<SubclassSubclassSeq>());
+            Assert.Equal(28, ObjectSize<SubclassWithGcSeq>());
+            Assert.Equal(8, ObjectSize<SubclassOfBaseWithGcSeq>());
+
+            // Structs
+            Assert.Equal(8, Unsafe.SizeOf<MyStructAuto>());
+            Assert.Equal(8, Unsafe.SizeOf<MyStruct9Seq>());
+            Assert.Equal(12, Unsafe.SizeOf<MyStruct9Exp>());
+            Assert.Equal(8, Unsafe.SizeOf<MyStruct1000Seq>());
+            Assert.Equal(1000, Unsafe.SizeOf<MyStruct1000Exp>());
+        }
+
+        // Field offsets:
+        Assert.Equal("(5, 3, 4, 1, 2, 6)", FieldOffsets(new SubclassSubclassSeq { A = 1, B = 2, C = 3, D = 4, E = 5, F = 6 }).ToString());
+        return 100;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static object FieldOffsets(SubclassSubclassSeq f) => (f.E, f.C, f.D, f.A, f.B, f.F);
+
+    [StructLayout(LayoutKind.Sequential, Size = 20)]
+    public class BaseClassSeq
+    {
+        public byte A;
+        public nint B;
+    }
+
+    [StructLayout(LayoutKind.Sequential, Size = 20)]
+    public class BaseClassWithGcSeq
+    {
+        public byte A;
+        public string B;
+    }
+
+    public class SubclassOfBaseWithGcSeq : BaseClassWithGcSeq
+    {
+        public byte C;
+    }
+
+    [StructLayout(LayoutKind.Sequential, Size = 16)]
+    public class SubclassSeq : BaseClassSeq
+    {
+        public byte C;
+        public nint D;
+    }
+
+    [StructLayout(LayoutKind.Sequential, Size = 32)]
+    public class SubclassWithGcSeq : BaseClassSeq
+    {
+        public byte C;
+        public object D;
+    }
+
+    public class SubclassSubclassSeq : SubclassSeq
+    {
+        public byte E;
+        public nint F;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+    private static int ObjectSize<T>() where T : new()
+    {
+        return (int)(nuint)typeof(RuntimeHelpers)
+            .GetMethod("GetRawObjectDataSize", BindingFlags.Static | BindingFlags.NonPublic)
+            .Invoke(null, [new T()]);
+    }
+
+    private class MyClassAuto
+    {
+        private byte[] a;
+        private byte b;
+    }
+
+    [StructLayout(LayoutKind.Explicit, Size = 1000)]
+    private class MyClass1000Exp
+    {
+        [FieldOffset(0)]
+        private byte[] a;
+        [FieldOffset(8)]
+        private byte b;
+    }
+
+    [StructLayout(LayoutKind.Sequential, Size = 1000)]
+    private class MyClass1000Seq
+    {
+        private byte[] a;
+        private byte b;
+    }
+
+    [StructLayout(LayoutKind.Explicit, Size = 1000)]
+    private class MyClass1000NoGcExp
+    {
+        [FieldOffset(0)]
+        private nint a;
+        [FieldOffset(8)]
+        private byte b;
+    }
+
+    [StructLayout(LayoutKind.Sequential, Size = 1000)]
+    private class MyClass1000NoGcSeq
+    {
+        private nint a;
+        private byte b;
+    }
+
+    private struct MyStructAuto
+    {
+        private byte[] a;
+        private byte b;
+    }
+
+    [StructLayout(LayoutKind.Explicit, Size = 1000)]
+    private struct MyStruct1000Exp
+    {
+        [FieldOffset(0)]
+        private byte[] a;
+        [FieldOffset(8)]
+        private byte b;
+    }
+
+    [StructLayout(LayoutKind.Sequential, Size = 1000)]
+    private struct MyStruct1000Seq
+    {
+        private byte[] a;
+        private byte b;
+    }
+
+    [StructLayout(LayoutKind.Explicit, Size = 9)]
+    private struct MyStruct9Exp
+    {
+        [FieldOffset(0)]
+        private byte[] a;
+        [FieldOffset(8)]
+        private byte b;
+    }
+
+    [StructLayout(LayoutKind.Sequential, Size = 9)]
+    private struct MyStruct9Seq
+    {
+        private byte[] a;
+        private byte b;
+    }
+}

src/tests/Loader/classloader/explicitlayout/Regressions/100220/Runtime_100220.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>