A back end for a target architecture in GCC has the following parts:
A directory machine under gcc/config, containing a
machine description machine.md file (see Machine Descriptions), header files machine.h and
machine-protos.h and a source file machine.c
(see Target Description Macros and Functions),
possibly a target Makefile fragment t-machine
(see The Target Makefile Fragment), and maybe
some other files. The names of these files may be changed from the
defaults given by explicit specifications in config.gcc.
If necessary, a file machine-modes.def in the
machine directory, containing additional machine modes to
represent condition codes. See Condition Code, for further details.
Entries in config.gcc (see The config.gcc File) for the systems with this target
architecture.
Documentation in gcc/doc/invoke.texi for any command-line
options supported by this target (see Run-time Target Specification). This means both entries in the summary table
of options and details of the individual options.
Documentation in gcc/doc/extend.texi for any target-specific
attributes supported (see Defining target-specific uses of __attribute__), including where the
same attribute is already supported on some targets, which are
enumerated in the manual.
Documentation in gcc/doc/extend.texi for any target-specific
pragmas supported.
Documentation in gcc/doc/extend.texi of any target-specific
built-in functions supported.
Documentation in gcc/doc/extend.texi of any target-specific
format checking styles supported.
A note in gcc/doc/contrib.texi under the person or people who
contributed the target support.
Entries in gcc/doc/install.texi for all target triplets
supported with this target architecture, giving details of any special
notes about installation for this target, or saying that there are no
special notes if there are none.
Possibly other support outside the gcc directory for runtime
libraries. FIXME: reference docs for this. The libstdc++ porting
manual needs to be installed as info for this to work, or to be a
chapter of this manual.
If the back end is added to the official GCC CVS repository, the
following are also necessary:
An entry for the target architecture in readings.php on the
GCC web site, with any relevant links.
Details of the properties of the back end and target architecture in
backends.php on the GCC web site.
A news item about the contribution of support for that target
architecture, in index.php on the GCC web site.
Normally, one or more maintainers of that target listed in
MAINTAINERS. Some existing architectures may be unmaintained,
but it would be unusual to add support for a target that does not have
a maintainer when support is added.