gdb/riscv: Don't error when decoding a 6 or 8 byte instruction

If the RISC-V prologue scanner finds a 6 or 8 byte instruction we
currently throw an internal error, which is not great for the user.

A mechanism already exists in the prologue scanner to leave
instructions marked as unknown so that we can stop the prologue scan
without raising an error, this is used for all 2 and 4 byte
instructions that are not part of the small set the prologue scanner
actually understands.

This commit changes GDB so that all 6 and 8 byte instructions are
marked as unknown, rather than causing an error.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_insn::decode): Gracefully ignore
	instructions of lengths 6 or 8 bytes.

gdb/testsuite/ChangeLog:

	* gdb.arch/riscv-unwind-long-insn-6.s: New file.
	* gdb.arch/riscv-unwind-long-insn-8.s: New file.
	* gdb.arch/riscv-unwind-long-insn.c: New file.
	* gdb.arch/riscv-unwind-long-insn.exp: New file.

1 files changed, 25 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.arch/riscv-unwind-long-insn.c b/gdb/testsuite/gdb.arch/riscv-unwind-long-insn.c
new file mode 100644
index 0000000..d601e2d
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/riscv-unwind-long-insn.c
@@ -0,0 +1,25 @@
+/* Copyright 2019 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/>.  */
+
+extern void func (void);
+extern void bar (void);
+
+int
+main ()
+{
+  bar ();
+  func ();
+  return 0;
+}