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>

2 files changed, 21 insertions, 2 deletions

diff --git a/bfd/elfxx-riscv.c b/bfd/elfxx-riscv.c
index 7eda177..cb3a980 100644
--- a/bfd/elfxx-riscv.c
+++ b/bfd/elfxx-riscv.c
@@ -1222,12 +1222,18 @@ static struct riscv_supported_ext riscv_supported_std_zxm_ext[] =
   {NULL, 0, 0, 0, 0}
 };
 
+static struct riscv_supported_ext riscv_supported_vendor_x_ext[] =
+{
+  {NULL, 0, 0, 0, 0}
+};
+
 const struct riscv_supported_ext *riscv_all_supported_ext[] =
 {
   riscv_supported_std_ext,
   riscv_supported_std_z_ext,
   riscv_supported_std_s_ext,
   riscv_supported_std_zxm_ext,
+  riscv_supported_vendor_x_ext,
   NULL
 };
 
@@ -1483,8 +1489,7 @@ riscv_get_default_ext_version (enum riscv_spec_class *default_isa_spec,
     case RV_ISA_CLASS_ZXM: table = riscv_supported_std_zxm_ext; break;
     case RV_ISA_CLASS_Z: table = riscv_supported_std_z_ext; break;
     case RV_ISA_CLASS_S: table = riscv_supported_std_s_ext; break;
-    case RV_ISA_CLASS_X:
-      break;
+    case RV_ISA_CLASS_X: table = riscv_supported_vendor_x_ext; break;
     default:
       table = riscv_supported_std_ext;
     }
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