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The '''GNU Compiler Collection''' (usually shortened to '''GCC''') is a set of [[compiler]]s produced for various [[programming language]]s by the [[GNU Project]]. 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 system]]s, including [[Linux]], the [[Berkeley Software Distribution|BSD]] family and [[Mac OS X]]. GCC has been [[porting|ported]] to a wide variety of [[computer architecture]]s, and is widely deployed as a tool in commercial, [[proprietary software|proprietary]] and [[closed source software]] development environments. GCC is also available for most embedded platforms, for example [[Symbian]]<ref>{{cite web|url=http://www.inf.u-szeged.hu/symbian-gcc/|title=Symbian GCC Improvement Project|accessdate=2007-11-08}}</ref>, [[AMCC]] and [[Freescale]] [[Power Architecture]]-based chips<ref>{{cite web|url=http://www.freescale.com/webapp/sps/site/overview.jsp?code=CW_BSP&fsrch=1|title=Linux Board Support Packages|accessdate=2008-08-07}}</ref>, [[Playstation]] and [[Sega Dreamcast]].{{Fact|date=February 2007}}. Companies such as [[CodeSourcery]] 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.
The '''GNU Compiler Collection''' (usually shortened to '''GCC''') is a [[compiler]] system supporting various [[programming language]]s by the [[GNU Project]]. 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 system]]s, including [[Linux]], the [[Berkeley Software Distribution|BSD]] family and [[Mac OS X]]. GCC has been [[porting|ported]] to a wide variety of [[computer architecture]]s, and is widely deployed as a tool in commercial, [[proprietary software|proprietary]] and [[closed source software]] development environments. GCC is also available for most embedded platforms, for example [[Symbian]]<ref>{{cite web|url=http://www.inf.u-szeged.hu/symbian-gcc/|title=Symbian GCC Improvement Project|accessdate=2007-11-08}}</ref>, [[AMCC]] and [[Freescale]] [[Power Architecture]]-based chips<ref>{{cite web|url=http://www.freescale.com/webapp/sps/site/overview.jsp?code=CW_BSP&fsrch=1|title=Linux Board Support Packages|accessdate=2008-08-07}}</ref>, [[Playstation]] and [[Sega Dreamcast]].{{Fact|date=February 2007}}. Companies such as [[CodeSourcery]] 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)|C programming language]], GCC 1.0 was released in 1987, and the compiler was extended to compile [[C++]] in December of that year.<ref>{{ cite web | url = http://www.gnu.org/software/gcc/releases.html | title = GCC Releases | accessdate = 2006-12-27 | publisher = GNU Project }}</ref> [[Front end]]s were later developed for [[Fortran]], [[Pascal (programming language)|Pascal]], [[Objective C]], [[Java (programming language)|Java]], and [[Ada (programming language)|Ada]], among others.<ref>{{cite web|url=http://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/G_002b_002b-and-GCC.html#G_002b_002b-and-GCC|title=Programming Languages Supported by GCC|accessdate=2006-12-27|publisher=GNU Project}}</ref>
Originally named the '''GNU C Compiler''', because it only handled the [[C (programming language)|C programming language]], GCC 1.0 was released in 1987, and the compiler was extended to compile [[C++]] in December of that year.<ref>{{ cite web | url = http://www.gnu.org/software/gcc/releases.html | title = GCC Releases | accessdate = 2006-12-27 | publisher = GNU Project }}</ref> [[Front end]]s were later developed for [[Fortran]], [[Pascal (programming language)|Pascal]], [[Objective C]], [[Java (programming language)|Java]], and [[Ada (programming language)|Ada]], among others.<ref>{{cite web|url=http://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/G_002b_002b-and-GCC.html#G_002b_002b-and-GCC|title=Programming Languages Supported by GCC|accessdate=2006-12-27|publisher=GNU Project}}</ref>

Revision as of 23:26, 9 November 2008

GNU Compiler Collection
Developer(s)GNU Project
Stable release
4.3.2 / August 27, 2008 (2008-08-27)
Repository
Operating systemCross-platform
PlatformGNU
TypeCompiler
LicenseGNU General Public License, GNU Lesser General Public License
Websitehttp://gcc.gnu.org/

The GNU Compiler Collection (usually shortened to GCC) is a compiler system supporting various programming languages by the GNU Project. 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 Linux, the BSD family and Mac OS X. GCC has been ported to a wide variety of computer 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[1], AMCC and Freescale Power Architecture-based chips[2], Playstation and Sega Dreamcast.[citation needed]. Companies such as CodeSourcery 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.[3] Front ends were later developed for Fortran, Pascal, Objective C, Java, and Ada, among others.[4]

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,[5] and released in 1987[6] 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.[7] A history of the release schedule is available at: http://gcc.gnu.org/releases.html

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, 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.[8][9]

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.[10]

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).[11] 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[12] for Unified Parallel C.

Architectures

GCC target processors 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[13], 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.[14]

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.

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." These optimizations 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 are currently being developed.

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 recent addition of global SSA-based optimizations on GIMPLE trees[15], 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 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 find out 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." [16]

Criticism

GCC has received criticism from OpenBSD developers such as Theo de Raadt and Otto Moerbeek for being large, buggy, slow, and generating poor code.[17] Due to this criticism, plus the relatively restrictive GPL that GCC is licensed under (as compared to the BSD license preferred by the various BSD projects), there has been an attempt to replace GCC with compilers such as PCC in NetBSD and OpenBSD.[18]

See also

References

  1. ^ "Symbian GCC Improvement Project". Retrieved 2007-11-08.
  2. ^ "Linux Board Support Packages". Retrieved 2008-08-07.
  3. ^ "GCC Releases". GNU Project. Retrieved 2006-12-27.
  4. ^ "Programming Languages Supported by GCC". GNU Project. Retrieved 2006-12-27.
  5. ^ Stallman, Richard M. (February 1986). "GNU Status". GNU's Bulletin. 1 (1). Free Software Foundation. Retrieved 2006-09-26.
  6. ^ Tower, Leonard (1987) "GNU C compiler beta test release," comp.lang.misc USENET newsgroup; see also http://gcc.gnu.org/releases.html#timeline
  7. ^ Stallman, Richard M. (2001) "Contributors to GCC," in Using and Porting the GNU Compiler Collection (GCC) for gcc version 2.95 (Cambridge, Mass.: Free Software Foundation)
  8. ^ "A Brief History of GCC".
  9. ^ "The Short History of GCC development".
  10. ^ Linux Information Project (LINFO) accessed 2007-03-20
  11. ^ GCC Front Ends, GCC.org, Retrieved May 11, 2008.
  12. ^ GCC UPC (GCC Unified Parallel C) | intrepid.com
  13. ^ fedoraproject.org: Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE)
  14. ^ "languages used to make GCC".
  15. ^ Tree SSA: Main Page
  16. ^ "Tools: GCC 4.2.1 Released". {{cite web}}: Text "KernelTrap" ignored (help) 080208 kerneltrap.org
  17. ^ http://www.thejemreport.com/content/view/369/
  18. ^ http://it.toolbox.com/blogs/bsd-guru/towards-a-bsdlicensed-compiler-19289

Further reading

  • The Jem Report: More on OpenBSD's New Compiler: [1]
Wikibooks has a book on the topic of: GNU C Compiler Internals
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