aarch64 nop instruction

Andrew Pinski pinskia@gmail.com
Wed Nov 20 22:09:00 GMT 2019

On Wed, Nov 20, 2019 at 1:31 PM Eliot Miranda <eliot.miranda@gmail.com> wrote:
> 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

Resent out with the attached picture so it would goto the gdb list.

Wait I think you are reading the number wrong and the ARM ARM incorrectly.
I see
(gdb) p/t 0xd503201f
$2 = 11010101000000110010000000011111

This is exactly what is written in the ARM ARM as far as I can see.
See attached picture.
It ends in 5 1's which is 0x1f (you missed the one 1 too).  starts
with 1101 which is 0xd.  I don't see where you get 6a from either.
Are you thinking the value in memory; so you are reading it as a
little-endian value or something different?

Andrew Pinski

> 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 (), {},
> with the 0ther 0x6a opcodes, e.g.
>   CORE_INSN ("bics", 0x6a200000, 0x7f200000, log_shift, 0, OP3 (Rd, Rn,
> 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

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