  The GCC free compiler is a very large software, compiling source in several
languages for many targets on various systems. It can be extended by plugins,
which may take advantage of its power to provide extra specific functionality
(warnings, optimizations, source refactoring or navigation) by processing
various GCC internal representations (Gimple, Tree, ...). Writing plugins in C
is a complex and time-consuming task, but customizing GCC by using an existing
scripting language inside is impractical. We describe MELT, a specific
Lisp-like DSL which fits well into existing GCC technology and offers
high-level features (functional, object or reflexive programming, pattern
matching). MELT is translated to C fitted for GCC internals and provides
various features to facilitate this. This work shows that even huge, legacy,
software can be a posteriori extended by specifically tailored and translated
high-level DSLs.
