The top level source directory in a GCC distribution contains several files and directories that are shared with other software distributions such as that of GNU Binutils. It also contains several subdirectories that contain parts of GCC and its runtime libraries:
The Boehm conservative garbage collector, optionally used as part of the ObjC runtime library when configured with --enable-objc-gc.
Autoconf macros and Makefile fragments used throughout the tree.
Contributed scripts that may be found useful in conjunction with GCC. One of these, contrib/texi2pod.pl, is used to generate man pages from Texinfo manuals as part of the GCC build process.
The support for fixing system headers to work with GCC. See fixincludes/README for more information. The headers fixed by this mechanism are installed in libsubdir/include-fixed. Along with those headers, README-fixinc is also installed, as libsubdir/include-fixed/README.
The main sources of GCC itself (except for runtime libraries), including optimizers, support for different target architectures, language front ends, and testsuites. See The gcc Subdirectory, for details.
Support tools for GNAT.
Headers for the libiberty library.
GNU libintl, from GNU gettext, for systems which do not
include it in libc.
The Ada runtime library.
The runtime support library for atomic operations (e.g. for __sync
and __atomic).
The C preprocessor library.
The Decimal Float support library.
The libffi library, used as part of the Go runtime library.
The GCC runtime library.
The Fortran runtime library.
The Go runtime library. The bulk of this library is mirrored from the master Go repository.
The GNU Offloading and Multi Processing Runtime Library.
The libiberty library, used for portability and for some
generally useful data structures and algorithms. See Introduction in GNU libiberty, for more information
about this library.
The runtime support library for transactional memory.
The Objective-C and Objective-C++ runtime library.
The runtime support library for quad-precision math operations.
The D standard and runtime library. The bulk of this library is mirrored from the master D repositories.
The Stack protector runtime library.
The C++ runtime library.
Plugin used by the linker if link-time optimizations are enabled.
Scripts used by the gccadmin account on gcc.gnu.org.
The zlib compression library, used for compressing and
uncompressing GCC’s intermediate language in LTO object files.
The build system in the top level directory, including how recursion into subdirectories works and how building runtime libraries for multilibs is handled, is documented in a separate manual, included with GNU Binutils. See GNU configure and build system in The GNU configure and build system, for details.