aarch64 nop instruction

[Eliot Miranda <eliot.miranda@gmail.com>]

Hi All,

    I use gdb for processor simulation in a Smalltalk JIT.  I am using gdb
8.3.1 to derive an aarch64 (ARMV8) simulator.  I have noticed that 8.3.1
does not disassemble Arm's "approved" not instruction.

The approved nop is 2r1101010100000011001000000001111, 0x6a81900f, see
C6.2.202 on p 252 of
https://developer.arm.com/docs/ddi0487/ea/arm-architecture-reference-manual-armv8-for-armv8-a-architecture-profile.
When I use the 8.3.1 derived system to disassemble that nop I get e.g.
00000000: .inst 0x6a81900f ; undefined


The entry in opcodes/aarch64-tbl.h for the nop that does disassemble is

CORE_INSN ("nop", 0xd503201f, 0xffffffff, ic_system, 0, OP0 (), {}, F_ALIAS)

I've tried adding
CORE_INSN ("nop", 0x6a81900f, 0xffffffff, ic_system, 0, OP0 (), {}, F_ALIAS)
which leaves 0xd503201f unaffected but still yields "undefined" for the
:"approved" nop.

I've tried adding
  CORE_INSN ("nop", 0x6a81900f, 0xffffffff, log_shift, 0, OP0 (), {},
F_HAS_ALIAS),
with the 0ther 0x6a opcodes, e.g.
  CORE_INSN ("bics", 0x6a200000, 0x7f200000, log_shift, 0, OP3 (Rd, Rn,
Rm_SFT), QL_I3SAMER, F_SF),
and this breaks the other nop, 0xd503201f, and both forms print as
undefined.

While I think that not disassembling the official nop is a bug I'm really
more interested in finding out what the minimal change to the table is to
get the "approved" nop to disassemble.  Can anyone help?
_,,,^..^,,,_
best, Eliot