GCC MELT
gcc-melt.org

Overview

GCC MELT is a high-level domain specific language for extending or customizing the Gnu Compiler Collection (i.e. gcc, g++, gfortran, gnat, gccgo, gobjc, ...). It is implemented as a GCC plugin (for GNU/Linux systems, with GCC 4.6, 4.7 or later) in a free software (under GPLv3+ license and FSF copyright).
GCC MELT targets advanced GCC users developing, compiling, improving, evaluating, maintaining, or designing important software (coded in C, C++, Fortran, Ada, Go, Objective C or any language supported by a recent straight- or cross- GCC compiler). For such power users, spending a few days or weeks to customize or extend their compiler for their specific needs is worthwhile, and GCC MELT should lower the required effort, because coding in MELT a GCC extension is much easier than manually developping a GCC plugin in C or C++ (these are efficient languages, but not well fit to easily work on compiler's internal representations).

GCC MELT uses existing GCC plugin hooks to work on GCC internal middle-end representations, notably GCC Generic Trees and GCC Gimples, and the control flow graph. These powerful internal representations are common to all source languages accepted by GCC, so your extension coded in MELT will be able to handle any source code compiled by GCC. In addition, the (crude) GCC MELT probe provides a graphical interface to interactively query some internal GCC representations.

Probably, GCC MELT won't be very useful to casual GCC users working on tiny programming projects, unless they simply use some existing GCC extensions already developed with GCC MELT.

Why customize or extend GCC?

The GCC compiler can be customized or extended (with plugins or MELT extensions) for:

• specific warnings, e.g. for typing your variadic functions (like json_pack in Jansson library, or g_object_set in GTK toolkit), or checking some properties at every call of some given function;
• specific coding rules validation (e.g. requiring that pthread_mutex_lock and pthread_mutex_unlock are balanced in the same function, or that every fopen gets its resulted tested);
• metrics on the GCC internal representations;
• help on code refactoring or source code navigation;
• adding new (application- specific) builtins or attributes to the compiler;
• (application- or corporation-) specific optimizations (e.g. fprintf(stdout,format,...) → printf(format,...), etc.);
• etc....

From the GCC user's point of view, a customization of GCC is easier to use than external tools. He/She needs only to add extra compilation flags (.e.g. CFLAGS += -fplugin=melt -fplugin-arg-melt-extra=extension -fplugin-arg-melt-mode=mode in a Makefile), but don't need to change workflow habits.

How MELT helps to extend GCC?

MELT is a domain-specific language specifically designed to ease the extension (or customization) of GCC because of the following features:

• strong connection with existing GCC internals using available plugin machinery: new GCC passes, attributes, builtins, ... can be implemented in MELT
• simple, orthogonal, Lisp-like syntax
• efficient garbage collection (compatible with GCC implementation), using state of the art generational copying GC techniques.
• dynamically typed first-class values (including lists, tuples, closures, objects in a rich single-inheritance meta-class based system with dynamic message dispatching, homogenous hash-tables, boxed integers and boxed GCC data -Gimples, Trees, ..., etc)
• handle natively any arbitrary GCC data stuff (e.g. tree-s, gimple-s, edge-s, loop-s in a statically typed manner
• anonymous functions (and named ones) and a rich object system enabling functional, object, reflective programming style
• very powerful pattern-matching, notably on GCC internal stuff like tree-s and gimple-s etc.
• dynamic run-time evaluation and read-eval-print-loop
• dynamically translated to C or C++ code (compiled and dynamically loaded at runtime thru dlopen)
• ability to mix your small chunks of C code with your main code in MELT
• bootstrapped implementation: The MELT → C translator is implemented in MELT

The above powerful features of MELT lower the cost of customizing GCC, hence enabling you to leverage on GCC powerful abilities with GCC MELT. However, even using MELT you need to understand some of GCC internals.

The emitted C or C++ code is carefully generated to fit into what your GCC plugin interface expects (GCC switched from C to C++ in its 4.7 version, and GCC MELT is careful to generate a code acceptable by your version of GCC).

Looking into GCC internal representations with the GCC MELT probe

the MELT 0.9.7-post-snap-r192893 probe
on its gcc/melt-runtime.c

(click on the picture to get the full view)

A practical way to understand more on main GCC internal representations is to have a relevant (small) source code (e.g. in C, C++, Fortran, or Ada, etc), then compile it with the -fdump-tree-all.

For instance (assuming you have some example.cc in some playground directory) compile with g++ -fdump-tree-all -O -c example.cc. This produce hundreds of dump files, such as perhaps example.cc.073t.phiopt1 which might contain (like dump files suffixed .phiopt*) a dump of Gimple/SSA (Static Single Assignment) statements, that you can browse using an editor or pager (e.g. less example.cc.073t.phiopt1 or gedit example.cc.073t.phiopt1).

However, GCC MELT also provides you with a graphical probe: you could simply run gcc -fplugin=melt -fplugin-arg-melt-mode=probe -O -c example.cc and see a graphical window, with the ability to interactively look at GCC internal representations tied to a particular source code location. The probe is a small (GtkMM or PyQt) GUI application communicating, thru asynchronous textual pipes, with a MELT enhanced GCC compiler.

The GCC MELT graphical probe may interest any people curious of GCC internal representations.

GCC MELT is downloadable here (under GPLv3 license, for GNU/Linux systems with a recent GCC), where you can get installation instructions, and also examples of code written in MELT.

Please download and install GCC MELT before reading other material.

GCC MELT has some documentation here , where you can get tutorial, and also reference about MELT, with publications and slides of talks given on it.

GCC MELT is an improvement to GCC itself, so follows the same contribution policies as the GCC compiler. In particular, it demands the same legal prerequisites (which take a lot of time). We cannot accept even small patches without your contribution belonging legally to the FSF (as GCC MELT does).

Thanks to (in alphabetical order) Romain Geissler, Emmanuel Haucourt, Alexandre Lissy, Jérémie Salvucci, Pierre Vittet for their contributions to GCC MELT.

My (=Basile Starynkevitch) work at CEA, LIST on GCC MELT has been partially funded by DGCIS French public agency thru various projects.

Participating to projects

Please contact me by email at Basile dot Starynkevitch at cea dot fr for industrial contracts (thru CEA, LIST) or collaborative research projects related to GCC MELT

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