GNU Compiler Collection (GCC) Internals: Target Fragment

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### 20.1 Target Makefile Fragments

Target makefile fragments can set these Makefile variables.

`LIBGCC2_CFLAGS`

  Compiler flags to use when compiling `libgcc2.c`.

`LIB2FUNCS_EXTRA`

  A list of source file names to be compiled or assembled and inserted into `libgcc.a`.

`CRTSTUFF_T_CFLAGS`

  Special flags used when compiling `crtstuff.c`. See Initialization.

`CRTSTUFF_T_CFLAGS_S`

  Special flags used when compiling `crtstuff.c` for shared linking. Used if you use `crtbeginS.o` and `crtendS.o` in `EXTRA-PARTS`. See Initialization.

`MULTILIB_OPTIONS`

  For some targets, invoking GCC in different ways produces objects that cannot be linked together. For example, for some targets GCC produces both big and little endian code. For these targets, you must arrange for multiple versions of `libgcc.a` to be compiled, one for each set of incompatible options. When GCC invokes the linker, it arranges to link in the right version of `libgcc.a`, based on the command line options used.

  The `MULTILIB_OPTIONS` macro lists the set of options for which special versions of `libgcc.a` must be built. Write options that are mutually incompatible side by side, separated by a slash. Write options that may be used together separated by a space. The build procedure will build all combinations of compatible options.

  For example, if you set `MULTILIB_OPTIONS` to ‘`m68000/m68020 msoft-float`’, `Makefile` will build special versions of `libgcc.a` using the following sets of options: `-m68000`, `-m68020`, `-msoft-float`, ‘`-m68000 -msoft-float`’, and ‘`-m68020 -msoft-float`’.

`MULTILIB_DIRNAMES`

  If `MULTILIB_OPTIONS` is used, this variable specifies the directory names that should be used to hold the various libraries. Write one element in `MULTILIB_DIRNAMES` for each element in `MULTILIB_OPTIONS`. If `MULTILIB_DIRNAMES` is not used, the default value will be `MULTILIB_OPTIONS`, with all slashes treated as spaces.

  `MULTILIB_DIRNAMES` describes the multilib directories using GCC conventions and is applied to directories that are part of the GCC installation. When multilib-enabled, the compiler will add a subdirectory of the form prefix/multilib before each directory in the search path for libraries and crt files.

  For example, if `MULTILIB_OPTIONS` is set to ‘`m68000/m68020 msoft-float`’, then the default value of `MULTILIB_DIRNAMES` is ‘`m68000 m68020 msoft-float`’. You may specify a different value if you desire a different set of directory names.

`MULTILIB_MATCHES`

  Sometimes the same option may be written in two different ways. If an option is listed in `MULTILIB_OPTIONS`, GCC needs to know about any synonyms. In that case, set `MULTILIB_MATCHES` to a list of items of the form ‘`option=option`’ to describe all relevant synonyms. For example, ‘`m68000=mc68000 m68020=mc68020`’.

`MULTILIB_EXCEPTIONS`

  Sometimes when there are multiple sets of `MULTILIB_OPTIONS` being specified, there are combinations that should not be built. In that case, set `MULTILIB_EXCEPTIONS` to be all of the switch exceptions in shell case syntax that should not be built.

  For example the ARM processor cannot execute both hardware floating point instructions and the reduced size THUMB instructions at the same time, so there is no need to build libraries with both of these options enabled. Therefore `MULTILIB_EXCEPTIONS` is set to:

      *mthumb/*mhard-float*

`MULTILIB_REQUIRED`

  Sometimes when there are only a few combinations are required, it would be a big effort to come up with a `MULTILIB_EXCEPTIONS` list to cover all undesired ones. In such a case, just listing all the required combinations in `MULTILIB_REQUIRED` would be more straightforward.

  The way to specify the entries in `MULTILIB_REQUIRED` is same with the way used for `MULTILIB_EXCEPTIONS`, only this time what are required will be specified. Suppose there are multiple sets of `MULTILIB_OPTIONS` and only two combinations are required, one for ARMv7-M and one for ARMv7-R with hard floating-point ABI and FPU, the `MULTILIB_REQUIRED` can be set to:

      MULTILIB_REQUIRED =  mthumb/march=armv7-m
      MULTILIB_REQUIRED += march=armv7-r/mfloat-abi=hard/mfpu=vfpv3-d16

  The `MULTILIB_REQUIRED` can be used together with `MULTILIB_EXCEPTIONS`. The option combinations generated from `MULTILIB_OPTIONS` will be filtered by `MULTILIB_EXCEPTIONS` and then by `MULTILIB_REQUIRED`.

`MULTILIB_REUSE`

  Sometimes it is desirable to reuse one existing multilib for different sets of options. Such kind of reuse can minimize the number of multilib variants. And for some targets it is better to reuse an existing multilib than to fall back to default multilib when there is no corresponding multilib. This can be done by adding reuse rules to `MULTILIB_REUSE`.

  A reuse rule is comprised of two parts connected by equality sign. The left part is the option set used to build multilib and the right part is the option set that will reuse this multilib. Both parts should only use options specified in `MULTILIB_OPTIONS` and the equality signs found in options name should be replaced with periods. An explicit period in the rule can be escaped by preceding it with a backslash. The order of options in the left part matters and should be same with those specified in `MULTILIB_REQUIRED` or aligned with the order in `MULTILIB_OPTIONS`. There is no such limitation for options in the right part as we don’t build multilib from them.

  `MULTILIB_REUSE` is different from `MULTILIB_MATCHES` in that it sets up relations between two option sets rather than two options. Here is an example to demo how we reuse libraries built in Thumb mode for applications built in ARM mode:

      MULTILIB_REUSE = mthumb/march.armv7-r=marm/march.armv7-r

  Before the advent of `MULTILIB_REUSE`, GCC select multilib by comparing command line options with options used to build multilib. The `MULTILIB_REUSE` is complementary to that way. Only when the original comparison matches nothing it will work to see if it is OK to reuse some existing multilib.

`MULTILIB_EXTRA_OPTS`

  Sometimes it is desirable that when building multiple versions of `libgcc.a` certain options should always be passed on to the compiler. In that case, set `MULTILIB_EXTRA_OPTS` to be the list of options to be used for all builds. If you set this, you should probably set `CRTSTUFF_T_CFLAGS` to a dash followed by it.

`MULTILIB_OSDIRNAMES`

  If `MULTILIB_OPTIONS` is used, this variable specifies a list of subdirectory names, that are used to modify the search path depending on the chosen multilib. Unlike `MULTILIB_DIRNAMES`, `MULTILIB_OSDIRNAMES` describes the multilib directories using operating systems conventions, and is applied to the directories such as `lib` or those in the `LIBRARY_PATH` environment variable. The format is either the same as of `MULTILIB_DIRNAMES`, or a set of mappings. When it is the same as `MULTILIB_DIRNAMES`, it describes the multilib directories using operating system conventions, rather than GCC conventions. When it is a set of mappings of the form gccdir=osdir, the left side gives the GCC convention and the right gives the equivalent OS defined location. If the osdir part begins with a ‘`!`’, GCC will not search in the non-multilib directory and use exclusively the multilib directory. Otherwise, the compiler will examine the search path for libraries and crt files twice; the first time it will add multilib to each directory in the search path, the second it will not.

  For configurations that support both multilib and multiarch, `MULTILIB_OSDIRNAMES` also encodes the multiarch name, thus subsuming `MULTIARCH_DIRNAME`. The multiarch name is appended to each directory name, separated by a colon (e.g. ‘`../lib32:i386-linux-gnu`’).

  Each multiarch subdirectory will be searched before the corresponding OS multilib directory, for example ‘`/lib/i386-linux-gnu`’ before ‘`/lib/../lib32`’. The multiarch name will also be used to modify the system header search path, as explained for `MULTIARCH_DIRNAME`.

`MULTIARCH_DIRNAME`

  This variable specifies the multiarch name for configurations that are multiarch-enabled but not multilibbed configurations.

  The multiarch name is used to augment the search path for libraries, crt files and system header files with additional locations. The compiler will add a multiarch subdirectory of the form prefix/multiarch before each directory in the library and crt search path. It will also add two directories `LOCAL_INCLUDE_DIR`/multiarch and `NATIVE_SYSTEM_HEADER_DIR`/multiarch) to the system header search path, respectively before `LOCAL_INCLUDE_DIR` and `NATIVE_SYSTEM_HEADER_DIR`.

  `MULTIARCH_DIRNAME` is not used for configurations that support both multilib and multiarch. In that case, multiarch names are encoded in `MULTILIB_OSDIRNAMES` instead.

  More documentation about multiarch can be found at https://wiki.debian.org/Multiarch.

`SPECS`

  Unfortunately, setting `MULTILIB_EXTRA_OPTS` is not enough, since it does not affect the build of target libraries, at least not the build of the default multilib. One possible work-around is to use `DRIVER_SELF_SPECS` to bring options from the `specs` file as if they had been passed in the compiler driver command line. However, you don’t want to be adding these options after the toolchain is installed, so you can instead tweak the `specs` file that will be used during the toolchain build, while you still install the original, built-in `specs`. The trick is to set `SPECS` to some other filename (say `specs.install`), that will then be created out of the built-in specs, and introduce a `Makefile` rule to generate the `specs` file that’s going to be used at build time out of your `specs.install`.

`T_CFLAGS`

  These are extra flags to pass to the C compiler. They are used both when building GCC, and when compiling things with the just-built GCC. This variable is deprecated and should not be used.

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