What tools and libraries are available for implementing compilers targeting the CLR?

What compiler tools and supporting libraries are available that would help in implementing a compiler targeting .NET and the CLR? Preferably open source. I am especially interested in frameworks that implement a high-level IR and support for common high-level and middl开发者_Python百科e-level optimizations.

There's the Phoenix framework, which is now a bit dated (and not strictly open-source). A more modern but still very incomplete system is the common compiler infrastructure. Sadly, no support for optimizations yet, but the "compiler as a service" system is expected in .NET 5.