[mono] Fix min alignment of structures with explicit layout.

src/mono/mono/metadata/marshal.c

@@ -5764,21 +5764,18 @@ mono_marshal_load_type_info (MonoClass* klass)
 	if (m_class_get_byval_arg (klass)->type == MONO_TYPE_PTR)
 		info->native_size = TARGET_SIZEOF_VOID_P;
 
+	gboolean align_size = TRUE;
 	if (layout != TYPE_ATTRIBUTE_AUTO_LAYOUT) {
 		info->native_size = MAX (native_size, info->native_size);
-		/*
-		 * If the provided Size is equal or larger than the calculated size, and there
-		 * was no Pack attribute, we set min_align to 1 to avoid native_size being increased
-		 */
 		if (layout == TYPE_ATTRIBUTE_EXPLICIT_LAYOUT) {
 			if (native_size && native_size == info->native_size && m_class_get_packing_size (klass) == 0)
-				min_align = 1;
+				align_size = FALSE;
 			else
 				min_align = MIN (min_align, packing);
 		}
 	}
 
-	if (info->native_size & (min_align - 1)) {
+	if (info->native_size & (min_align - 1) && align_size) {
 		info->native_size += min_align - 1;
 		info->native_size &= ~(min_align - 1);
 	}

src/tests/JIT/Regression/JitBlue/Runtime_90531/Runtime_90531.cs

@@ -0,0 +1,120 @@
+// 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.Runtime.InteropServices;
+using Xunit;
+
+public class Runtime_90531
+{
+    [StructLayout(LayoutKind.Explicit, Size = 8)]
+    public struct StructA
+    {
+        [FieldOffset(0)]
+        public long Field0;
+    }
+
+    public struct StructB
+    {
+        public int Field0;
+        // padded 4
+        public StructA Field1;
+        // no padding
+        public int Field2;
+    }
+
+	// Size should be 16 in both 32 and 64 bits win/linux
+	// Size should be 12 on 32bits OSX size alignment of long is 4
+	[StructLayout (LayoutKind.Explicit)]
+	struct TestStruct8 {
+		[FieldOffset (0)]
+		public int a;
+		[FieldOffset (4)]
+		public ulong b;
+	}
+
+	// Size should be 12 in both 32 and 64 bits
+	[StructLayout (LayoutKind.Explicit, Size=12)]
+	struct TestStruct9 {
+		[FieldOffset (0)]
+		public int a;
+		[FieldOffset (4)]
+		public ulong b;
+	}
+
+	// Size should be 16 in both 32 and 64 bits
+	// Size should be 12 on 32bits OSX size alignment of long is 4
+	[StructLayout (LayoutKind.Explicit)]
+	struct TestStruct10 {
+		[FieldOffset (0)]
+		public int a;
+		[FieldOffset (3)]
+		public ulong b;
+	}
+
+	// Size should be 11 in both 32 and 64 bits
+	[StructLayout (LayoutKind.Explicit, Size=11)]
+	struct TestStruct11 {
+		[FieldOffset (0)]
+		public int a;
+		[FieldOffset (3)]
+		public ulong b;
+	}
+
+	[StructLayout (LayoutKind.Explicit, Pack=1)]
+	struct TestStruct12 {
+		[FieldOffset (0)]
+		public short a;
+		[FieldOffset (2)]
+		public int b;
+	}
+
+	// Size should always be 12, since pack = 0, size = 0 and min alignment = 4
+	//When pack is not set, we default to 8, so min (8, min alignment) -> 4
+	[StructLayout (LayoutKind.Explicit)]
+	struct TestStruct13 {
+		[FieldOffset(0)]
+		int one;
+		[FieldOffset(4)]
+		int two;
+		[FieldOffset(8)]
+		int three;
+	}
+
+	// Size should always be 12, since pack = 8, size = 0 and min alignment = 4
+	//It's aligned to min (pack, min alignment) -> 4
+	[StructLayout (LayoutKind.Explicit)]
+	struct TestStruct14 {
+		[FieldOffset(0)]
+		int one;
+		[FieldOffset(4)]
+		int two;
+		[FieldOffset(8)]
+		int three;
+	}
+
+    [Fact]    
+    public unsafe static int EntryPoint()
+    {
+        void* mem = stackalloc byte[24];
+        Marshal.WriteInt32((IntPtr)mem, 0, 1);
+        Marshal.WriteInt64((IntPtr)mem, 8, 2);
+        Marshal.WriteInt32((IntPtr)mem, 16, 3);
+
+        var s = Marshal.PtrToStructure<StructB>((IntPtr)mem);
+
+        if (s.Field1.Field0 != 2)
+            return 101;
+        if(Marshal.SizeOf(typeof(TestStruct8)) != 16)
+            return 102;
+        if(Marshal.SizeOf(typeof(TestStruct9)) != 12)
+            return 103;
+        if(Marshal.SizeOf(typeof(TestStruct10)) != 16)
+            return 104;
+        if(Marshal.SizeOf(typeof(TestStruct11)) != 11)
+            return 105;
+        if(Marshal.SizeOf(typeof(TestStruct12)) != 6)
+            return 106;
+        return 100;
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_90531/Runtime_90531.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>