[ios-8.0/9.0] Cannot compile project with AOT enabled (Assertion at mini-llvm.c, condition `cinfo->ret.nslots == 1' not met)

[vyacheslav-volkov]

Description

I'm developing a large project and at the very beginning the code compiled without problems, but after a while I checked again and got this error, I don’t have time to look for which specific type breaks the assembly, is it possible to display a more specific error message and how can I debug it?

I tried the latest net9.0-preview 5 but the result is the same.

Reproduction Steps

I can't provide the source code as this is a commercial product.

Expected behavior

No compilation errors

Actual behavior

Mono Ahead of Time compiler - compiling assembly ..../obj/Release/net9.0-ios/ios-arm64/linked/MugenMvvm.dll
AOTID 0D0A964D-FAF6-A1E4-FADC-1F08E0767345

• Assertion at /Users/runner/work/1/s/src/mono/mono/mini/mini-llvm.c:1605, condition `cinfo->ret.nslots == 1' not met

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

[ivanpovazan]

Hi @vyacheslav-volkov, you mentioned

at the very beginning the code compiled without problems

Are you referring to .NET8 release or something else?
I am trying to understand if the problem is a regression introduced in .NET8 or .NET9 previews?

---

Since you cannot provide a smaller repro, you could try increasing the verbosity of the AOT compiler when it compiles the problematic assembly (in your case: MugenMvvm.dll).

To do so:

1. Build your app with:

dotnet build -c Release -r ios-arm64 -tl:false -v:diag > build.log

2. Open build.log and search for mono-aot-cross
3. You should find messages like:

Tool /usr/local/share/dotnet/packs/Microsoft.NETCore.App.Runtime.AOT.osx-arm64.Cross.ios-arm64/9.0.0-preview.6.24327.7/Sdk/../tools/mono-aot-cross 
execution started with arguments: ... 

• These are the invocations of the AOT compiler with command line arguments for different assemblies

4. Find the one invocation which compiles MugenMvvm.dll
5. Get the AOT command

• Pay attention to the message format and remove command-irrelevant parts:

Tool <path_to_aot_compier> execution started with arguments: <args> `obj/Release/net9.0-ios/ios-arm64/linked/MugenMvvm.dll`

• Your command should be just:

<path_to_aot_compier> <args> `obj/Release/net9.0-ios/ios-arm64/linked/MugenMvvm.dll` 

• Include the -v arguments to increase AOT compiler verbosity:

<path_to_aot_compier> <args> -v `obj/Release/net9.0-ios/ios-arm64/linked/MugenMvvm.dll` 

6. Open the terminal
7. Run the command
   • This should run the AOT compiler and only compile the problematic assembly with increased verbosity and your output should include which LLVM methods got compiled

   converting llvm method void <Module>:.cctor ()
   void <Module>:.cctor () emitted as Sample__Module__cctor
   converting llvm method void Program:<Main>$ (string[])
   ...
   

   The idea is that you would see the one just before the compiler asserts so hopefully you could backtrack the problem and provide a smaller repro.

Hope this helps and please let me know if you have troubles with the listed steps to increase verbosity.

[vyacheslav-volkov]

Hi @ivanpovazan, I think I've found the issue, I had ref type with no fields and marked with [StructLayout(LayoutKind.Auto)] once I removed the attribute the error went away.

    [StructLayout(LayoutKind.Auto)]
    public ref struct Builder<T>
    {
    }

I am trying to understand if the problem is a regression introduced in .NET8 or .NET9 previews?

both versions are affected by the issue

[ivanpovazan]

Is this then still blocking you?

In any case we would appreciate if you could provide a smaller repro, so we could look into generating better error messages.

[vyacheslav-volkov]

@ivanpovazan thanks for the help, no blockers atm, I just removed [StructLayout(LayoutKind.Auto)] as it is redundant for this struct, I'll try to create a small repo to reproduce the issue.

[vyacheslav-volkov]

@ivanpovazan I've reproduced this with a simple example:

using System.Runtime.InteropServices;

namespace iOSApp1;

public static class Builder
{
    public static Builder<T> Get<T>() => new();
}

[StructLayout(LayoutKind.Auto)]
public ref struct Builder<T>
{
    public object Build() => "";
}

[Register("AppDelegate")]
public class AppDelegate : UIApplicationDelegate
{
    public override UIWindow? Window { get; set; }

    public override bool FinishedLaunching(UIApplication application, NSDictionary launchOptions)
    {
        Builder.Get<string>().Build();
        return true;
    }
}

[vitek-karas]

@jkurdek could you please try to repo and look what's going on?

[jkurdek]

Minimal repro:

[StructLayout(LayoutKind.Auto)] // Also occurs with [StructLayout(LayoutKind.Sequential)] 
public struct Builder
{
    public string Build() => "Hello";
}

namespace HelloWorld
{
    internal class Program
    {
        public static Builder Get() => new Builder();
        private static void Main(string[] args)
        {

        }
    }
}

---

Initial investigation:

The issue seems to be caused by placing StructLayoutAttribute on a struct with no fields. The size of such struct is then computed to be 0. Then during handling of Get() method , 0 registers are allocated for the return value. Which then leads to failing assertion.

Will follow up with more detailed analyis