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GNU Compiler Collection (GCC) Internals

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12.1 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 section 10.20.5 How Initialization Functions Are Handled.

Special flags used when compiling `crtstuff.c' for shared linking. Used if you use `crtbeginS.o' and `crtendS.o' in EXTRA-PARTS. See section 10.20.5 How Initialization Functions Are Handled.

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, in the PowerPC embedded ABI support, it is not desirable to build libraries compiled with the `-mcall-aix' option and either of the `-fleading-underscore' or `-mlittle' options at the same time. Therefore MULTILIB_EXCEPTIONS is set to
*mcall-aix/*fleading-underscore* *mlittle/*mcall-aix*

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.

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