GNU Compiler Collection:

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.^[2] Front ends were later developed for Fortran, Pascal, Objective C, Java, and Ada, among others.^[3]

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,^[4] and released in 1987^[5] 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.^[6]

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.^[7]^[8]

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.^[9]

Languages

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^[11] for Unified Parallel C

Architectures

Additional processors have been supported by GCC versions maintained separately from the FSF version:

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.

Nearly all of GCC is written in C with the exception of the Ada frontend; much of the Ada frontend is written in Ada.

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

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^[13], 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

GNU Compiler Collection

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