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`libgcc.a' and Cross-Compilers
------------------------------

   Code compiled by GNU CC uses certain runtime support functions
implicitly.  Some of these functions can be compiled successfully with
GNU CC itself, but a few cannot be.  These problem functions are in the
source file `libgcc1.c'; the library made from them is called
`libgcc1.a'.

   When you build a native compiler, these functions are compiled with
some other compiler-the one that you use for bootstrapping GNU CC.
Presumably it knows how to open code these operations, or else knows how
to call the run-time emulation facilities that the machine comes with.
But this approach doesn't work for building a cross-compiler.  The
compiler that you use for building knows about the host system, not the
target system.

   So, when you build a cross-compiler you have to supply a suitable
library `libgcc1.a' that does the job it is expected to do.

   To compile `libgcc1.c' with the cross-compiler itself does not work.
The functions in this file are supposed to implement arithmetic
operations that GNU CC does not know how to open code for your target
machine.  If these functions are compiled with GNU CC itself, they will
compile into infinite recursion.

   On any given target, most of these functions are not needed.  If GNU
CC can open code an arithmetic operation, it will not call these
functions to perform the operation.  It is possible that on your target
machine, none of these functions is needed.  If so, you can supply an
empty library as `libgcc1.a'.

   Many targets need library support only for multiplication and
division.  If you are linking with a library that contains functions for
multiplication and division, you can tell GNU CC to call them directly
by defining the macros `MULSI3_LIBCALL', and the like.  These macros
need to be defined in the target description macro file.  For some
targets, they are defined already.  This may be sufficient to avoid the
need for libgcc1.a; if so, you can supply an empty library.

   Some targets do not have floating point instructions; they need other
functions in `libgcc1.a', which do floating arithmetic.  Recent
versions of GNU CC have a file which emulates floating point.  With a
certain amount of work, you should be able to construct a floating
point emulator that can be used as `libgcc1.a'.  Perhaps future
versions will contain code to do this automatically and conveniently.
That depends on whether someone wants to implement it.

   Some embedded targets come with all the necessary `libgcc1.a'
routines written in C or assembler.  These targets build `libgcc1.a'
automatically and you do not need to do anything special for them.
Other embedded targets do not need any `libgcc1.a' routines since all
the necessary operations are supported by the hardware.

   If your target system has another C compiler, you can configure GNU
CC as a native compiler on that machine, build just `libgcc1.a' with
`make libgcc1.a' on that machine, and use the resulting file with the
cross-compiler.  To do this, execute the following on the target
machine:

     cd TARGET-BUILD-DIR
     ./configure --host=sparc --target=sun3
     make libgcc1.a

And then this on the host machine:

     ftp TARGET-MACHINE
     binary
     cd TARGET-BUILD-DIR
     get libgcc1.a
     quit

   Another way to provide the functions you need in `libgcc1.a' is to
define the appropriate `perform_...' macros for those functions.  If
these definitions do not use the C arithmetic operators that they are
meant to implement, you should be able to compile them with the
cross-compiler you are building.  (If these definitions already exist
for your target file, then you are all set.)

   To build `libgcc1.a' using the perform macros, use
`LIBGCC1=libgcc1.a OLDCC=./xgcc' when building the compiler.
Otherwise, you should place your replacement library under the name
`libgcc1.a' in the directory in which you will build the
cross-compiler, before you run `make'.