README.C4X: New file with information about the c4x ports.

        * README.C4X: New file with information about the c4x ports.
        * ginclude/va-c4x.h: New file for c4x varargs support.
        * config/c4x: New directory with c4x port files.

From-SVN: r22475

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+This file describes the implementation notes of the GNU C Compiler for
+the Texas Instruments Floating Point Digital Signal Processor
+families, TMS320C3x and TMS320C4x (including the C30, C31, C32, C40,
+and C44 chips).
+
+
+Currently, only two code variants are generated---those for the C3x
+and C4x architectures.  Note that the new operand combinations for
+parallel instructions, included in newer silicon revisions, are not
+yet supported.  These should be trivial to add for someone with the
+newer chips and the inclination.
+
+
+While the generated assembly code is fairly similar to that recognised
+by the TI assembler, there are a few differences (currently the machine
+option -mti, designed to enfore compatibility, is not fully
+implemented).  The major difference is the use of the ^ operator to
+load the 16 MSBs of an address or constant for the C4x.
+
+
+The generated assembly code requires the GNU assembler (GAS).  This is
+not currently included as part of the binutils package, due to the
+many hacks required to be compatible with TI's kludged COFF
+implementation, and the binutils not being designed for 32-bit bytes.
+Patches against binutils-2.7.2 can be obtained from
+ftp://ongaonga.chch.cri.nz/pub/c4x.  This site also has patches for
+the GNU debugger (GDB), incoporating a cycle accurate simulator that
+can display profiles and histories of code execution, detailing
+pipeline conflicts etc.
+
+
+GCC can be configured as a cross compiler for both the C3x and C4x
+architectures on the same system.  Use `configure --target=c4x' to
+configure GCC for both the C3x and C4x.  Then use the -m30 option to
+generate code for the C30 or -m40 (the default) for the C40.
+
+
+Further installation notes and other optimization patches for GCC can
+also be obtained from ftp://ongaonga.chch.cri.nz/pub/c4x.
+
+
+A Majordomo mailing list, gcc_c40@atlantek.com.au, exists to discuss
+related issues and suggestions for further optimizations.  To
+subscribe send a message with `subscribe gcc_c40' in the body to
+majordomo@atlantek.com.au.
+
+
+Michael Hayes,  16 Sep 98