[MonoAOT] [x64] Return non-void in Function of void return type

[jkurdek]

Found while fixing #103628

Found return instr that returns non-void in Function of void return type!
  ret void undef, !dbg !23
 void

Mono IR:

*** Unoptimized LLVM IR for HelloWorld.Program:GetS1 () ***
define hidden monocc void @HelloWorld_Program_GetS1() #2 !dbg !4 {
BB0:
  %vreg_loc_19 = alloca %S1, align 1, !dbg !8
  %vreg_loc_16 = alloca %S1, align 1, !dbg !8
  %ret_buf = alloca ptr, align 8, !dbg !8
  br label %INIT_BB1, !dbg !8

INIT_BB1:                                         ; preds = %BB0
  br label %INITED_BB2, !dbg !8

INITED_BB2:                                       ; preds = %INIT_BB1
  br label %BB2, !dbg !8

BB2:                                              ; preds = %INITED_BB2
  call void @llvm.memset.p0.i32(ptr %vreg_loc_19, i8 0, i32 0, i1 false), !dbg !9
  br label %BB1, !dbg !9

BB1:                                              ; preds = %BB2
  call void @llvm.memset.p0.i32(ptr %ret_buf, i8 0, i32 ptrtoint (ptr getelementptr (void, ptr null, i32 1) to i32), i1 false), !dbg !8
  call void @llvm.memcpy.p0.p0.i32(ptr %ret_buf, ptr %vreg_loc_19, i32 0, i1 false), !dbg !8
  ret void undef, !dbg !8
}

***

Steps to reproduce:

Build the following program with mono aot:

using System;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential)]
public struct S1
{
}

namespace HelloWorld
{
    internal class Program
    {
        public static S1 GetS1() => new S1();
        private static void Main(string[] args)
        {
            S1 s1 = GetS1();
        }
    }
}

Error is most likely caused by wrong logic here

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

Line 1596 in 15e96fa

ret_type = LLVMVoidType ();

[steveisok]

Would LLVMStructType(NULL, 0, FALSE) fix it?

[jkurdek]

This look very hopeful. Thanks Steve! Added it to: #106013