Open Dylan has 3 compiler back-ends currently:
The HARP back-end generates native code for x86 processors and is used on Windows, FreeBSD and Linux. While for many years, this was our best and most mature compiler back-end, it is falling by the wayside and will be replaced by the LLVM back-end.
The issues with the HARP back-end are that it is 32-bit only and is more difficult to add support for things like new instructions, better scheduling, atomic operations and SSE support.
The C back-end generates C which is then compiled by either clang or gcc. We use the C back-end on Mac OS X, as well as Linux and FreeBSD on x86_64 processors.
The C back-end is relatively easy to work with and extend and is the easiest option for bringing up Open Dylan on a new platform. We are currently doing this with Linux on the ARM processor.
The C back-end used to generate code that was slower than HARP although in recent months, the C back-end has been optimized and many parts of it produce faster results than HARP.
The C back-end allows debugging the generated C (rather than at the Dylan source level).
The LLVM back-end is still a work in progress. This will replace the use of the HARP and C back-ends on Mac OS X, Linux, FreeBSD and Windows on both x86 and x86_64 processors.
The LLVM compiler back-end is much easier to maintain and generates significantly better code than either of the other back-ends. It is also much easier than HARP to extend to support new processors and features, like atomic and SSE operations.
The LLVM back-end will enable debugging at the Dylan source level as it emits DWARF debug data and will be functional with LLDB.
We hope that the LLVM back-end will be ready for use this calendar year although some challenges remain on Windows and with debugger support.