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19.1 Target Makefile Fragments

Target makefile fragments can set these Makefile variables.

Compiler flags to use when compiling libgcc2.c.

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

Floating Point Emulation
To have GCC include software floating point libraries in libgcc.a define FPBIT and DPBIT along with a few rules as follows:
          # We want fine grained libraries, so use the new code
          # to build the floating point emulation libraries.
          FPBIT = fp-bit.c
          DPBIT = dp-bit.c
          fp-bit.c: $(srcdir)/config/fp-bit.c
                  echo '#define FLOAT' > fp-bit.c
                  cat $(srcdir)/config/fp-bit.c >> fp-bit.c
          dp-bit.c: $(srcdir)/config/fp-bit.c
                  cat $(srcdir)/config/fp-bit.c > dp-bit.c

You may need to provide additional #defines at the beginning of fp-bit.c and dp-bit.c to control target endianness and other options.

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

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

For some targets, invoking GCC in different ways produces objects that can not 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'.

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.

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.

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'.

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:


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.

If the default location for system headers is not /usr/include, you must set this to the directory containing the headers. This value should match the value of the SYSTEM_INCLUDE_DIR macro.

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.
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.