fragments.texi

理解和实践操作系统的一本好书

@c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
@c 1999, 2000, 2001, 2003, 2004, 2005 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node Fragments
@chapter Makefile Fragments
@cindex makefile fragment

When you configure GCC using the @file{configure} script, it will
construct the file @file{Makefile} from the template file
@file{Makefile.in}.  When it does this, it can incorporate makefile
fragments from the @file{config} directory.  These are used to set
Makefile parameters that are not amenable to being calculated by
autoconf.  The list of fragments to incorporate is set by
@file{config.gcc} (and occasionally @file{config.build}
and @file{config.host}); @xref{System Config}.

Fragments are named either @file{t-@var{target}} or @file{x-@var{host}},
depending on whether they are relevant to configuring GCC to produce
code for a particular target, or to configuring GCC to run on a
particular host.  Here @var{target} and @var{host} are mnemonics
which usually have some relationship to the canonical system name, but
no formal connection.

If these files do not exist, it means nothing needs to be added for a
given target or host.  Most targets need a few @file{t-@var{target}}
fragments, but needing @file{x-@var{host}} fragments is rare.

@menu
* Target Fragment:: Writing @file{t-@var{target}} files.
* Host Fragment::   Writing @file{x-@var{host}} files.
@end menu

@node Target Fragment
@section Target Makefile Fragments
@cindex target makefile fragment
@cindex @file{t-@var{target}}

Target makefile fragments can set these Makefile variables.

@table @code
@findex LIBGCC2_CFLAGS
@item LIBGCC2_CFLAGS
Compiler flags to use when compiling @file{libgcc2.c}.

@findex LIB2FUNCS_EXTRA
@item LIB2FUNCS_EXTRA
A list of source file names to be compiled or assembled and inserted
into @file{libgcc.a}.

@findex Floating Point Emulation
@item Floating Point Emulation
To have GCC include software floating point libraries in @file{libgcc.a}
define @code{FPBIT} and @code{DPBIT} along with a few rules as follows:
@smallexample
# 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
@end smallexample

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


@findex CRTSTUFF_T_CFLAGS
@item CRTSTUFF_T_CFLAGS
Special flags used when compiling @file{crtstuff.c}.
@xref{Initialization}.

@findex CRTSTUFF_T_CFLAGS_S
@item CRTSTUFF_T_CFLAGS_S
Special flags used when compiling @file{crtstuff.c} for shared
linking.  Used if you use @file{crtbeginS.o} and @file{crtendS.o}
in @code{EXTRA-PARTS}.
@xref{Initialization}.

@findex MULTILIB_OPTIONS
@item MULTILIB_OPTIONS
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 @file{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 @file{libgcc.a}, based on
the command line options used.

The @code{MULTILIB_OPTIONS} macro lists the set of options for which
special versions of @file{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 @code{MULTILIB_OPTIONS} to @samp{m68000/m68020
msoft-float}, @file{Makefile} will build special versions of
@file{libgcc.a} using the following sets of options:  @option{-m68000},
@option{-m68020}, @option{-msoft-float}, @samp{-m68000 -msoft-float}, and
@samp{-m68020 -msoft-float}.

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

For example, if @code{MULTILIB_OPTIONS} is set to @samp{m68000/m68020
msoft-float}, then the default value of @code{MULTILIB_DIRNAMES} is
@samp{m68000 m68020 msoft-float}.  You may specify a different value if
you desire a different set of directory names.

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

@findex MULTILIB_EXCEPTIONS
@item MULTILIB_EXCEPTIONS
Sometimes when there are multiple sets of @code{MULTILIB_OPTIONS} being
specified, there are combinations that should not be built.  In that
case, set @code{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 @code{MULTILIB_EXCEPTIONS} is set to:
@smallexample
*mthumb/*mhard-float*
@end smallexample

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

@findex NATIVE_SYSTEM_HEADER_DIR
@item NATIVE_SYSTEM_HEADER_DIR
If the default location for system headers is not @file{/usr/include},
you must set this to the directory containing the headers.  This value
should match the value of the @code{SYSTEM_INCLUDE_DIR} macro.

@findex SPECS
@item SPECS
Unfortunately, setting @code{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
@code{DRIVER_SELF_SPECS} to bring options from the @file{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 @file{specs} file that will
be used during the toolchain build, while you still install the
original, built-in @file{specs}.  The trick is to set @code{SPECS} to
some other filename (say @file{specs.install}), that will then be
created out of the built-in specs, and introduce a @file{Makefile}
rule to generate the @file{specs} file that's going to be used at
build time out of your @file{specs.install}.
@end table

@node Host Fragment
@section Host Makefile Fragments
@cindex host makefile fragment
@cindex @file{x-@var{host}}

The use of @file{x-@var{host}} fragments is discouraged.  You should do
so only if there is no other mechanism to get the behavior desired.
Host fragments should never forcibly override variables set by the
configure script, as they may have been adjusted by the user.

Variables provided for host fragments to set include:

@table @code

@item X_CFLAGS
@itemx X_CPPFLAGS
These are extra flags to pass to the C compiler and preprocessor,
respectively.  They are used both when building GCC, and when compiling
things with the just-built GCC@.

@item XCFLAGS
These are extra flags to use when building the compiler.  They are not
used when compiling @file{libgcc.a}.  However, they @emph{are} used when
recompiling the compiler with itself in later stages of a bootstrap.

@item BOOT_LDFLAGS
Flags to be passed to the linker when recompiling the compiler with
itself in later stages of a bootstrap.  You might need to use this if,
for instance, one of the front ends needs more text space than the
linker provides by default.

@item EXTRA_PROGRAMS
A list of additional programs required to use the compiler on this host,
which should be compiled with GCC and installed alongside the front
ends.  If you set this variable, you must also provide rules to build
the extra programs.

@end table