[PATCH v2 9/9] x86: correct VFPCLASSP{S,D} operand size handling

[H.J. Lu]

On Thu, Oct 31, 2019 at 2:39 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 30.10.2019 23:40,  H.J. Lu  wrote:
> > On Wed, Oct 30, 2019 at 1:03 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 29.10.2019 20:16, H.J. Lu wrote:
> >>> On Mon, Oct 28, 2019 at 1:10 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>
> >>>> With AVX512VL disabled (e.g. when writing code for the Knights family
> >>>> of processors) these insns aren't ambiguous when used with a memory
> >>>> source, and hence should be accepted without suffix or operand size
> >>>> specifier. When AVX512VL is enabled, to be consistent with this as
> >>>> well as other ambiguous operand size handling it seems better to just
> >>>> wanrn about the ambiguity in AT&T mode, and still default to 512-bit
> >>>> operands (on the assumption that the code may have been written without
> >>>> AVX512VL in mind yet).
> >>>
> >>> There is no need for this.   Memory size or suffix is always needed for them.
> >>
> >> It is not - what you say (again) is your private opinion, not (afaict)
> >> something based on some objective criteria. If I'm missing something
> >> here, please clarify, but recall that we've been discussing this before.
> >>
> >
> > As I remembered, I didn't believe they should be treated differently,
> > depending on AVX512VL.  One should be able to tell what the operand
> > size is without checking if AVX512VL was disabled or not when the assembly
> > file was assembled.
>
> Well - I understand this position of yours, but I can only repeat: To
> me this is your personal preference, not an objective criteria. Let's
> take PUSH/POP for comparison: Original 16-bit code may have been
> written without the i386 in mind. Why would you enforce on people to
> append W suffixes just because the assembler becomes i386 (and hence
> 32-bit operand) aware? An indeed - gas doesn't require suffixes here,
> silently defaulting to the default size implied by the mode specified.
> It's even more relaxed there - the defaults get used irrespective of
> any .arch settings.
>
> IOW I continue to think that it should be the programmer's choice
> whether to imply CPU capability restrictions for their own code. It's
> not like I'm forbidding the use of the suffix / operand size specifier
> when none is actually needed.
>

Assembler accepts all instructions by default.   Interpret an instruction
based on ISA doesn't buy much and may lead confusions.

-- 
H.J.