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GNU Compiler Collection
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The GNU Compiler Collection (usually shortened to GCC) is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain. As well as being the official compiler of the GNU system, GCC has been adopted as the standard compiler by most other modern Unix-like computer operating systems, including GNU/Linux, the BSD family and Mac OS X. GCC has been ported to a wide variety of processor architectures, and is widely deployed as a tool in commercial, proprietary and closed source software development environments.
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Encyclopedia
The GNU Compiler Collection (usually shortened to GCC) is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain. As well as being the official compiler of the GNU system, GCC has been adopted as the standard compiler by most other modern Unix-like computer operating systems, including GNU/Linux, the BSD family and Mac OS X. GCC has been ported to a wide variety of processor architectures, and is widely deployed as a tool in commercial, proprietary and closed source software development environments. GCC is also available for most embedded platforms, for example Symbian, AMCC and Freescale Power Architecture-based chips. The compiler can target a wide variety of platforms, including videogame consoles such as the Playstation 2 and Sega Dreamcast. Several companies make a business out of supplying and supporting gcc ports to various platforms, and chip manufacturers today consider a gcc port almost essential to the success of an architecture.
Originally named the GNU C Compiler, because it only handled the C programming language, GCC 1.0 was released in 1987, and the compiler was extended to compile C++ in December of that year. Front ends were later developed for Fortran, Pascal, Objective C, Java, and Ada, among others.
The Free Software Foundation (FSF) distributes GCC under the GNU General Public License (GNU GPL) and the GNU Lesser General Public License (GNU LGPL). GCC is free software.
History
Richard Stallman started GCC in 1985. He extended an existing compiler to compile C. The compiler originally compiled Pastel, an extended, nonportable dialect of Pascal, and was written in Pastel. It was rewritten in C by Len Tower and Stallman, and released in 1987 as the compiler for the GNU Project, in order to have a compiler available that was free software. Its development was supervised by the Free Software Foundation.
EGCS
By 1991, GCC 1.x had reached a point of stability, but architectural limitations prevented many desired improvements, so the Free Software Foundation (FSF) started work on GCC 2.x. But during the mid-1990s, the FSF kept such close control on what was added to the official version of GCC 2.x that GCC was used as one example of the "cathedral" development model in Eric S. Raymond's essay The Cathedral and the Bazaar.
As GCC was free software, programmers wanting to work in other directions—particularly those writing interfaces for languages other than C—were free to develop their own fork of the compiler. Multiple forks proved inefficient and unwieldy, however, and the difficulty in getting work accepted by the official GCC project was greatly frustrating for many.
In 1997, a group of developers formed EGCS (Experimental/Enhanced GNU Compiler System), to merge several experimental forks into a single project. The basis of the merger was a gcc development snapshot taken between the 2.7 and 2.81 releases. Projects merged included g77 (Fortran), PGCC (Pentium-optimized GCC), many C++ improvements, and many new architectures and operating system variants.
EGCS development proved considerably more vigorous than GCC development, so much so that the FSF officially halted development on their GCC 2.x compiler, "blessed" EGCS as the official version of GCC and appointed the EGCS project as the GCC maintainers in April 1999. Furthermore, the project explicitly adopted the "bazaar" model over the "cathedral" model. With the release of GCC 2.95 in July 1999, the two projects were once again united.
Uses
GCC is often the compiler of choice for developing software that is required to execute on a wide variety of hardware. Differences in native compilers lead to difficulties in developing code that will compile correctly on all the compilers and build scripts that will run for all the platforms. By using GCC, the same parser is used for all platforms, so if the code compiles on one, chances are high that it compiles on all.
GCC is now maintained by a varied group of programmers from around the world. It has been ported to more kinds of processors and operating systems than any other compiler.
Languages
The standard compiler release 4.3 includes front ends for C, C++ (G++), Java (GCJ), Ada (GNAT), Objective-C, Objective-C++, and Fortran (GFortran). Also available, but not in standard are Modula-2, Modula-3, Pascal, PL/I, D (gdc), Mercury, VHDL (GHDL). A popular parallel language extension, OpenMP, is also supported.
The Fortran front end was g77 before version 4.0, which only supports Fortran 77. In newer versions, g77 is dropped in favor of the new GFortran front end that supports Fortran 95. A front end for CHILL was previously included, but has been dropped owing to a lack of maintenance.
A few experimental branches exist to support additional languages, such as the GCC UPC compiler for Unified Parallel C.
Architectures
GCC target processor families as of version 4.3 include:
Lesser-known target processors supported in the standard release have included:
Additional processors have been supported by GCC versions maintained separately from the FSF version:
When retargeting GCC to a new platform, bootstrapping is often used.
Structure
GCC's external interface is generally standard for a UNIX compiler. Users invoke a driver program named gcc, which interprets command arguments, decides which language compilers to use for each input file, runs the assembler on their output, and then possibly runs the linker to produce a complete executable binary.
Each of the language compilers is a separate program that inputs source code and outputs assembly code. All have a common internal structure. A per-language front end parses the source code in that language and produces an abstract syntax tree ("tree" for short), and a back end converts the trees to GCC's Register Transfer Language (RTL). Compiler optimizations and static code analysis techniques (such as FORTIFY_SOURCE, a compiler directive which attempts to discover some buffer overflows) are applied to the code. Finally, assembly language is produced using architecture-specific pattern matching originally based on an algorithm of Jack Davidson and Chris Fraser.
GCC is written primarily in C, although the C++, Ada and Java frontends contain much code written in those languages.
Front-ends
Frontends vary internally, having to produce trees that can be handled by the backend. The parsers are hand-coded recursive descent parsers.
Until recently, the tree representation of the program was not fully independent of the processor being targeted. Confusingly, the meaning of a tree was somewhat different for different language front-ends, and front-ends could provide their own tree codes.
In 2005, two new forms of language-independent trees were introduced. These new tree formats are called GENERIC and GIMPLE. Parsing now creates temporary language-dependent trees, which are converting to GENERIC. The so-called "gimplifier" then lowers this more complex form into the simpler SSA-based GIMPLE form which is the common language for a large number of new powerful language- and architecture-independent global (function scope) optimizations.
The C and C++ front ends currently (ver. 4.3.3) convert directly from front end
trees to GIMPLE, and hand that off to the back end rather than first
converting to GENERIC.
Optimization
Optimization on trees does not generally fit into what most compiler developers would consider a front end task, as it is not language dependent and does not involve parsing. GCC developers have given this part of the compiler the somewhat contradictory name the "middle end." Some of these optimizations performed at this level include dead code elimination, partial redundancy elimination, global value numbering, sparse conditional constant propagation, and scalar replacement of aggregates. Array dependence based optimizations such as automatic vectorization and automatic parallelization are also performed. Profile-guided optimization is also possible as demonstrated here: http://gcc.gnu.org/install/build.html#TOC4
Back-end
The behavior of GCC's back end is partly specified by preprocessor macros and functions specific to a target architecture, for instance to define the endianness, word size, and calling conventions. The front part of the back end uses these to help decide RTL generation, so although GCC's RTL is nominally processor-independent, the initial sequence of abstract instructions is already adapted to the target.
The exact set of GCC optimizations varies from release to release as it develops, but includes the standard algorithms, such as loop optimization, jump threading, common subexpression elimination, instruction scheduling, and so forth. The RTL optimizations are of less importance with the addition of global SSA-based optimizations on GIMPLE trees, as RTL optimizations have a much more limited scope, and have less high-level information.
A "reloading" phase changes abstract (pseudo-) registers into real machine registers, using data collected from the patterns describing the target's instruction set. This is a somewhat complicated phase, because it must account for the vagaries of all of GCC's targets.
The final phase is somewhat anticlimactic, because the patterns to match were generally chosen during reloading, and so the assembly code is simply built by running substitutions of registers and addresses into the strings specifying the instructions.
Debugging GCC programs
The primary tool used to debug GCC code is the GNU Debugger (gdb). Among more specialized tools are Valgrind for finding memory errors and leaks. The GNU Profiler (gprof) can determine how much time is spent in which routines, and how often they are called; this requires programs to be compiled with profiling options.
License
"GCC 4.2.1 was the last release of GCC covered by version 2 of the GNU General Public License. All subsequent releases are released under GPL version 3."
Criticism
GCC has received criticism from OpenBSD developers such as Theo de Raadt and Otto Moerbeek for being large, buggy, and slow, and for generating poor code. Due to this criticism, and the relatively restrictive GPL that GCC is licensed under (as compared to the BSD license preferred by the various BSD projects), there was an attempt to replace GCC with compilers such as PCC in NetBSD and OpenBSD or LLVM in FreeBSD.
See also
Open Source compilers:
Formerly proprietary compilers:
Diverse:
Further reading
- Richard M. Stallman: , Free Software Foundation, ISBN 0-595-10035-X
- Richard M. Stallman: , Free Software Foundation, ISBN 1-882114-39-6
- Brian J. Gough: , Network Theory Ltd., ISBN 0-9541617-9-3
- Arthur Griffith, GCC: The Complete Reference. McGrawHill/Osborne. ISBN 0-07-222405-3.
- The Jem Report: More on OpenBSD's New Compiler:
External links
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- , an essay covering GCC development for the 1990s, with 30 monthly reports for in the "Inside Cygnus Engineering" section near the end.
- , by Brian Gough
- , by the GCC developers
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- , an essay by Rick Moen recording seven well-known forks, including the GCC/EGCS one
- [irc://irc.gnu.org/gcc #gcc] IRC channel
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