Representation of 0-tuples

[andyjansson]

Tuples can be represented with DebugStructType, which works fine with one exception: 0-tuples. When attempting to create an empty struct, I'm greeted with the argument exception 'Type must be sized to get target size information'. This leaves me with the question of how I would best go about representing this structure. Any advice?

[smaillet]

Can you provide more specific examples of your code? Empty structs are a valid concept in many languages and should be possible in LLVM (and the DWARF inspired debug info it uses).

LLVM is unfortunately not particularly good about documenting it's parameter requirements and doesn't generate errors on invalid ones - it just crashes in release builds or asserts and aborts the process in a debug build. So a lot of the work in the .NET bindings is in doing parameter checks and creating meaningful exceptions and exception messages. It's possible this is a case of overly aggressive checks in the .NET projection.

[andyjansson]

The issue is demonstrated by the following snippet:

new DebugStructType(
  module,
  "tuple",
  module.DICompileUnit,
  "tuple",
  null,
  0,
  DebugInfoFlags.None,
  Enumerable.Empty<DebugMemberInfo>()
);

Here's the relevant exception and stack trace:

Exception has occurred: CLR/System.ArgumentException
Exception thrown: 'System.ArgumentException' in Ubiquity.NET.Llvm.dll: 'Type must be sized to get target size information'
   at Ubiquity.NET.Llvm.DataLayout.VerifySized(ITypeRef type, String name)
   at Ubiquity.NET.Llvm.DataLayout.BitSizeOf(ITypeRef typeRef)
   at Ubiquity.NET.Llvm.DebugInfo.DebugStructType..ctor(BitcodeModule module, String nativeName, DIScope scope, String name, DIFile file, UInt32 line, DebugInfoFlags debugFlags, IEnumerable`1 debugElements, DIType derivedFrom, Boolean packed, Nullable`1 bitSize, UInt32 bitAlignment)

[smaillet]

OK, that looks like a bug, Context.CreateStructType(string,bool,params ITypeRef[]) skips setting the body, when the element count is 0. This is unnecessary as the underlying LLVM APIs will accept the 0 length and mark the type as sized (with a size of 0).

Possible workaround: (I haven't tested it as I need to get to my day job [Been looking at this while waiting on a machine OS update]) But you should be able to create a native struct type, set the body to 0, then use that to create the DebugStruct type (e.g. attach the debug info from the existing native IR type that is a sized empty struct)

var lirType = module.Context.CreateStructType("tuple");
irType.SetBody(false);
var debugType = new DebugStructType(
  llvmType: irType,
 module: module,
 scope: null,
 name:"tuple",
 file: null,
 line: 0,
debugFlags: default
derivedFrom: null
elements: Enumerable.Empty<DIType>());

NOTE: There are two overloads of the DebugStructType constructor that take an IStructType as the first parameter. The one with fewer parameters creates a replaceable composite type, which you probably don't want. (Which is why the example above uses the named parameters to be clearer on which one you need)

[andyjansson]

That worked a treat, thank you very much.