LLVM IR is a Complete Code RepresentationIn particular, LLVM IR is both well specified and the only interface to the optimizer. This property means that all you need to know to write a front end for LLVM is what LLVM IR is, how it works, and the invariants it expects. Since LLVM IR has a first-class textual form, it is both possible and reasonable to build a front end that outputs LLVM IR as text, then uses UNIX pipes to send it through the optimizer sequence and code generator of your choice.
It might be surprising, but this is actually a pretty novel property to LLVM and one of the major reasons for its success in a broad range of different applications. Even the widely successful and relatively well-architected GCC compiler does not have this property: its GIMPLE mid-level representation is not a self-contained representation. As a simple example, when the GCC code generator goes to emit DWARF debug information, it reaches back and walks the source level "tree" form. GIMPLE itself uses a "tuple" representation for the operations in the code, but (at least as of GCC 4.5) still represents operands as references back to the source level tree form.
The implications of this are that front-end authors need to know and produce GCC's tree data structures as well as GIMPLE to write a GCC front end. The GCC back end has similar problems, so they also need to know bits and pieces of how the RTL back end works as well. Finally, GCC doesn't have a way to dump out "everything representing my code," or a way to read and write GIMPLE (and the related data structures that form the representation of the code) in text form. The result is that it is relatively hard to experiment with GCC, and therefore it has relatively few front ends.
По-моему, не фатально для GCC.
