"GCC suffers from layering problems and leaky abstractions: the back end walks front-end ASTs to generate debug info, the front ends generate back-end data structures, and the entire compiler depends on global data structures set up by the command line interface."

Chris Lattner, "The Design of LLVM" http://www.drdobbs.com/architecture-and-design/the-design-of...

(Seems possibly relevant to the boundary issue.)