RISC-V: Add generic support for vendor extensions

This patch introduces changes that allow the integration of vendor ISA
extensions:
* Define a list of vendor extensions (riscv_supported_vendor_x_ext)
  where vendor extensions can be added
* Introduce a section with a table in the documentation where vendor
  extensions can be added

To add a vendor extension that consists of instructions only,
the following things need to be done:
* Add the extension to the riscv_supported_vendor_x_ext list
* Add lookup entry in riscv_multi_subset_supports
* Documenting the extension in c-riscv.texti
* Add test cases for all instructions
* Add MATCH*/MASK* constants and DECLARE_INSN() for all instructions
* Add new instruction class to enum riscv_insn_class
* Define the instructions in riscv_opcodes
* Additional changes if necessary (depending on the instructions)

Co-developed-by: Lifang Xia <lifang_xia@linux.alibaba.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>

1 files changed, 14 insertions, 0 deletions

diff --git a/gas/doc/c-riscv.texi b/gas/doc/c-riscv.texi
index 21d867e..30afd20 100644
--- a/gas/doc/c-riscv.texi
+++ b/gas/doc/c-riscv.texi
@@ -20,6 +20,7 @@
 * RISC-V-Modifiers::      RISC-V Assembler Modifiers
 * RISC-V-Formats::        RISC-V Instruction Formats
 * RISC-V-ATTRIBUTE::      RISC-V Object Attribute
+* RISC-V-CustomExts::     RISC-V Custom (Vendor-Defined) Extensions
 @end menu
 
 @node RISC-V-Options
@@ -692,3 +693,16 @@ the privileged specification.  It will report errors if object files of
 different privileged specification versions are merged.
 
 @end table
+
+@node RISC-V-CustomExts
+@section RISC-V Custom (Vendor-Defined) Extensions
+@cindex custom (vendor-defined) extensions, RISC-V
+@cindex RISC-V custom (vendor-defined) extensions
+
+The following table lists the custom (vendor-defined) RISC-V
+extensions supported and provides the location of their
+publicly-released documentation:
+
+@table @r
+
+@end table