riscv: print .2byte or .4byte before an unknown instruction encoding

When the RISC-V disassembler encounters an unknown instruction, it
currently just prints the value of the bytes, like this:

  Dump of assembler code for function custom_insn:
     0x00010132 <+0>:	addi	sp,sp,-16
     0x00010134 <+2>:	sw	s0,12(sp)
     0x00010136 <+4>:	addi	s0,sp,16
     0x00010138 <+6>:	0x52018b
     0x0001013c <+10>:	0x9c45

My proposal, in this patch, is to change the behaviour to this:

  Dump of assembler code for function custom_insn:
     0x00010132 <+0>:	addi	sp,sp,-16
     0x00010134 <+2>:	sw	s0,12(sp)
     0x00010136 <+4>:	addi	s0,sp,16
     0x00010138 <+6>:	.4byte	0x52018b
     0x0001013c <+10>:	.2byte	0x9c45

Adding the .4byte and .2byte opcodes.  The benefit that I see here is
that in the patched version of the tools, the disassembler output can
be fed back into the assembler and it should assemble to the same
binary format.  Before the patch, the disassembler output is invalid
assembly.

I've started a RISC-V specific test file under binutils so that I can
add a test for this change.

binutils/ChangeLog:

	* testsuite/binutils-all/riscv/riscv.exp: New file.
	* testsuite/binutils-all/riscv/unknown.d: New file.
	* testsuite/binutils-all/riscv/unknown.s: New file.

opcodes/ChangeLog:

	* riscv-dis.c (riscv_disassemble_insn): Print a .%dbyte opcode
	before an unknown instruction, '%d' is replaced with the
	instruction length.

3 files changed, 67 insertions, 0 deletions

diff --git a/binutils/testsuite/binutils-all/riscv/riscv.exp b/binutils/testsuite/binutils-all/riscv/riscv.exp
new file mode 100644
index 0000000..8f0b7dc
--- /dev/null
+++ b/binutils/testsuite/binutils-all/riscv/riscv.exp
@@ -0,0 +1,29 @@
+# Copyright (C) 2021 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston, MA 02110-1301, USA.
+
+if ![istarget "riscv*-*-*"] then {
+    return
+}
+
+set tempfile tmpdir/riscvtemp.o
+set copyfile tmpdir/riscvcopy
+
+set test_list [lsort [glob -nocomplain $srcdir/$subdir/*.d]]
+foreach t $test_list {
+    # We need to strip the ".d", but can leave the dirname.
+    verbose [file rootname $t]
+    run_dump_test [file rootname $t]
+}
diff --git a/binutils/testsuite/binutils-all/riscv/unknown.d b/binutils/testsuite/binutils-all/riscv/unknown.d
new file mode 100644
index 0000000..6479116
--- /dev/null
+++ b/binutils/testsuite/binutils-all/riscv/unknown.d
@@ -0,0 +1,11 @@
+#as: -march=rv32ic
+#objdump: -d
+# Test the disassembly of unknown instruction encodings, specifically,
+# ensure that we generate a .?byte opcode.
+
+#...
+Disassembly of section \.text:
+
+[0-9a-f]+ <\.text>:
+   [0-9a-f]+:	0052018b          	\.4byte	0x52018b
+   [0-9a-f]+:	9c45                	\.2byte	0x9c45
diff --git a/binutils/testsuite/binutils-all/riscv/unknown.s b/binutils/testsuite/binutils-all/riscv/unknown.s
new file mode 100644
index 0000000..df92904
--- /dev/null
+++ b/binutils/testsuite/binutils-all/riscv/unknown.s
@@ -0,0 +1,27 @@
+/* Copyright (C) 2021 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+        .text
+	/* The following instruction is in the area set aside for
+	   custom instruction extensions.  As such it is unlikely that
+           an upstream extension should ever clash with this.  */
+	.insn r 0x0b, 0x0, 0x0, x3, x4, x5
+        /* Unlike the above, the following is just a reserved
+	   instruction encoding.  This means that in the future an
+	   extension to the compressed instruction set might use this
+	   encoding.  If/when that happens we'll need to find a
+	   different unused encoding within the compressed instruction
+	   space.  */
+	.insn ca 0x1, 0x27, 0x2, x8, x9