Re: [PATCH] x86/Intel: SHLD/SHRD have dual meaning

[Jan Beulich <jbeulich@suse.com>]

On 22.04.2024 09:47, Hongtao Liu wrote:
> On Mon, Apr 22, 2024 at 3:44 PM Cui, Lili <lili.cui@intel.com> wrote:
>>> On 22.04.2024 06:09, Hongtao Liu wrote:
>>>> On Fri, Apr 19, 2024 at 5:29 PM Jan Beulich <jbeulich@suse.com> wrote:
>>>>>
>>>>> Since we uniformly permit D suffixes in Intel mode whenever in AT&T
>>>>> mode an L suffix may be used, we need to be consistent with this.
>>>> I think we need to forbid the D suffix for APX NDD SHL/SHR under Intel
>>>> mode to avoid ambiguity.
>>>
>>> Hmm. Special casing just two insns is out of question imo (in fact that's what
>>> is - unintentionally - partly happening prior to the change here).
>>>
> I know  the suffix L is needed in AT&T mode to specify the operand
> size to avoid ambiguity.
> But why do we need the suffix D for Intel mode?

We don't need it. But that's not just here; the vast majority of insns should
never have permitted suffixes in Intel syntax. Yet here we are. And the
question is what to do about it. What I've been considering in the meantime
is to disallow suffixes where not needed by default, but have a command line
option to restore original behavior (perhaps right away with a deprecation
warning), just to allow people to get their (bogus) code to build again (or,
if need be, to work around issues we might introduce, i.e. mistakenly
refusing use of a suffix where one might in fact be needed). Thoughts?

Jan