Re: [PATCH] x86: fix "REP RET" with -madd-bnd-prefix

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Wed, May 30, 2018 at 7:23 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>> On 30.05.18 at 14:35, <hjl.tools@gmail.com> wrote:
>> On Wed, May 30, 2018 at 12:40 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>> --- a/gas/testsuite/gas/i386/x86-64-mpx-add-bnd-prefix.s
>>> +++ b/gas/testsuite/gas/i386/x86-64-mpx-add-bnd-prefix.s
>>> @@ -8,7 +8,9 @@
>>>         jmp     *(%rbx)
>>>         ret
>>>  foo:
>>> -       # Use of REPNE prefix - we shouldn't get any error
>>> +       # Use of any REP prefix - we shouldn't get any error
>>> +       rep ret
>>> +       repe ret
>>>         repne ret
>>>         # BND prefix already exists - we shouldn't get any error here
>>>         bnd ret
>>>
>>
>> Assembly code asks for F3 prefix and F2 prefix is generated.
>> It does't look right to me.
>
> Not emitting a BND prefix despite -madd-bnd-prefix looks even worse
> to me: Which REP prefix is used doesn't matter without MPX, but it
> very much does matter with MPX.

Assembler should give a warning in this case.

-- 
H.J.