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

["Cui, Lili" <lili.cui@intel.com>]

> 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).
> 
> > Neither SHL (always SAL), nor SHR with a D suffix (Intel mode) is
> > generated by GCC.
> 
> While this may be deemed helpful, I actually view it as a mistake, even if only
> from a cosmetic perspective: SHL is the main insn; SAL is merely an alias
> (questionably using extension opcode 4 rather than 6 in gas).
> 
> Still extending what you suggest - limiting the restriction to APX - may be an
> option, albeit ...
> 
> >> ---
> >> The alternative, more intrusive and more risky (in terms of perceived
> >> or even real regressions) route would be to mark the few insns which
> >> permit suffixes even in Intel syntax, and reject suffix uses when
> >> that indicator isn't set.
> 
> ... as implied here I'd still consider this inconsistent.
> 
> Furthermore the point in time when suffixes are processed off of the
> incoming mnemonic is too early to know whether an insn is having an APX
> representation. Hence doing said extension of what you suggest would likely
> end up quite hacky.
> 

If the user messes them up, we can't figure out the original intention because the input is exactly the same, maybe we can find a suitable place to inform the user that shl + apx ndd does not support suffix?

Lili.