Are the days of Microsoft's proprietary compiler over? Microsoft has announced they've started work on a new .NET compiler using LLVM and targets their CoreCLR — any C# program written for the .NET core class libraries can now run on any OS where CoreCLR and LLVM are supported. Right now the compiler only supports JIT compilation but AOT is being worked on along with other features. The new Microsoft LLVM compiler is called LLILC and is MIT-licensed.