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, 6 insertions, 4 deletions

diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
index 3fc86ab..bae987c 100644
--- a/gdb/riscv-tdep.c
+++ b/gdb/riscv-tdep.c
@@ -1385,10 +1385,12 @@ riscv_insn::decode (struct gdbarch *gdbarch, CORE_ADDR pc)
 	m_opcode = OTHER;
     }
   else
-    internal_error (__FILE__, __LINE__,
-		    _("unable to decode %d byte instructions in "
-		      "prologue at %s"), m_length,
-		    core_addr_to_string (pc));
+    {
+      /* This must be a 6 or 8 byte instruction, we don't currently decode
+	 any of these, so just ignore it.  */
+      gdb_assert (m_length == 6 || m_length == 8);
+      m_opcode = OTHER;
+    }
 }
 
 /* The prologue scanner.  This is currently only used for skipping the