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#### 6.3.9 Anatomy of a Target Back End

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.
- An optional `machine.opt` file in the `machine` directory, containing a list of target-specific options. You can also add other option files using the `extra_options` variable in `config.gcc`. See Options.
- 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.
- Documentation in `gcc/doc/md.texi` of any target-specific constraint letters (see Constraints for Particular Machines).
- 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.

The `machine.h` header is included very early in GCC’s standard sequence of header files, while `machine-protos.h` is included late in the sequence. Thus `machine-protos.h` can include declarations referencing types that are not defined when `machine.h` is included, specifically including those from `rtl.h` and `tree.h`. Since both RTL and tree types may not be available in every context where `machine-protos.h` is included, in this file you should guard declarations using these types inside appropriate `#ifdef RTX_CODE` or `#ifdef TREE_CODE` conditional code segments.

If the backend uses shared data structures that require `GTY` markers for garbage collection (see Type Information), you must declare those in `machine.h` rather than `machine-protos.h`. Any definitions required for building libgcc must also go in `machine.h`.

GCC uses the macro `IN_TARGET_CODE` to distinguish between machine-specific `.c` and `.cc` files and machine-independent `.c` and `.cc` files. Machine-specific files should use the directive:

    #define IN_TARGET_CODE 1

before including `config.h`.

If the back end is added to the official GCC source repository, the following are also necessary:

- An entry for the target architecture in `readings.html` on the GCC web site, with any relevant links.
- Details of the properties of the back end and target architecture in `backends.html` on the GCC web site.
- A news item about the contribution of support for that target architecture, in `index.html` 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.
- Target triplets covering all `config.gcc` stanzas for the target, in the list in `contrib/config-list.mk`.

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