[Mono AOT] Fix error when returning zero sized struct (#106013)

* Fix error when returning zero sized struct

* Add test

* Fix x64 errors

Author: jkurdek

src/mono/mono/mini/mini-llvm.c

@@ -1593,7 +1593,7 @@ sig_to_llvm_sig_full (EmitContext *ctx, MonoMethodSignature *sig, LLVMCallInfo *
 			ret_type = LLVMStructType (members, 1, FALSE);
 		} else if (cinfo->ret.pair_storage [0] == LLVMArgNone && cinfo->ret.pair_storage [1] == LLVMArgNone) {
 			/* Empty struct */
-			ret_type = LLVMVoidType ();
+			ret_type = LLVMStructType (NULL, 0, FALSE);
 		} else if (cinfo->ret.pair_storage [0] == LLVMArgInIReg && cinfo->ret.pair_storage [1] == LLVMArgInIReg) {
 			LLVMTypeRef members [2];
 
@@ -1610,7 +1610,11 @@ sig_to_llvm_sig_full (EmitContext *ctx, MonoMethodSignature *sig, LLVMCallInfo *
 	case LLVMArgVtypeAsScalar: {
 		int size = mono_class_value_size (mono_class_from_mono_type_internal (rtype), NULL);
 		/* LLVM models this by returning an int */
-		if (size < TARGET_SIZEOF_VOID_P) {
+		if (size == 0) {
+			/* Empty struct with LayoutKind attribute and without specified size */
+			g_assert(cinfo->ret.nslots == 0);
+			ret_type = LLVMIntType (8);
+		} else if (size < TARGET_SIZEOF_VOID_P) {
 			g_assert (cinfo->ret.nslots == 1);
 			ret_type = LLVMIntType (size * 8);
 		} else {
@@ -4946,6 +4950,9 @@ process_call (EmitContext *ctx, MonoBasicBlock *bb, LLVMBuilderRef *builder_ref,
 			/* Empty struct */
 			break;
 
+		if (LLVMTypeOf (lcall) == LLVMStructType (NULL, 0, FALSE))
+			break;
+
 		if (!addresses [ins->dreg])
 			addresses [ins->dreg] = build_alloca_address (ctx, sig->ret);
 

src/tests/JIT/Regression/JitBlue/Runtime_103628/Runtime_103628.cs

@@ -0,0 +1,29 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.InteropServices;
+using Xunit;
+
+[StructLayout(LayoutKind.Sequential)]
+public struct S1
+{
+}
+
+[StructLayout(LayoutKind.Auto)]
+public struct S2
+{
+}
+
+public class Runtime_103628
+{
+    public static S1 Get_S1() => new S1();
+
+    public static S2 Get_S2() => new S2();
+
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        S1 s1 = Get_S1();
+        S2 s2 = Get_S2();
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_103628/Runtime_103628.csproj

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