Re: [PATCH v7] x86: replace adhoc (partly wrong) ambiguous operand checking for MOVSX/MOVZX

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

On Thu, Feb 13, 2020 at 8:26 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 13.02.2020 17:18, H.J. Lu wrote:
> > On Thu, Feb 13, 2020 at 7:02 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 13.02.2020 15:34, H.J. Lu wrote:
> >>> On Thu, Feb 13, 2020 at 1:21 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>> @@ -6551,6 +6558,32 @@ process_suffix (void)
> >>>>         }
> >>>>      }
> >>>>
> >>>> +  if ((i.tm.base_opcode | 8) == 0xfbe
> >>>> +      || (i.tm.base_opcode == 0x63 && i.tm.cpu_flags.bitfield.cpu64))
> >>>> +    {
> >>>> +      /* In Intel syntax, movsx/movzx must have a "suffix" (checked above).
> >>>> +        In AT&T syntax, if there is no suffix (warned about above), the default
> >>>> +        will be byte extension.  */
> >>>
> >>> Please drop the warning for AT&T syntax and document it instead.
> >>
> >> But it is wrong to make a choice silently. When there are multiple
> >> options, we ought to inform the user, i.e. it needs to be at least
> >
> > I have seen
> >
> > movzx 7(%eax), %ecx
> >
> > from 2010.   We need to live with this oddity of movsx/movzx.
>
> Right, hence not an error, but just a warning. Not giving a warning

If a project checks assembler warnings, it will fail to build with
the new assembler.   We don't want it.

> here, just to repeat myself, would be against the idea of the
> overall set of changes. We stopped silently assuming what the
> programmer has meant for all cases except ones where it's pretty
> clear what is wanted by default (push, pop, call, jmp, etc). There
> is no meaningful difference between, say, mov, add, or shl (which
> now get warned about) and movsx/movzx. The sensible exception
> (which is already being made, as the test cases confirm) are
>
>         movsx   (%eax), %ax
>         movzx   (%eax), %ax
>
> where the 16- or 32-bit source operand size case isn't considered
> useful, and hence the defaulting to "byte" is silent.
>
> Jan



-- 
H.J.