[PATCH] x86: Avoid abort on invalid broadcast

[H.J. Lu]

On Thu, Aug 19, 2021 at 9:03 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 19.08.2021 17:35, H.J. Lu wrote:
> > On Thu, Aug 19, 2021 at 8:27 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 19.08.2021 16:45, H.J. Lu wrote:
> >>> On Thu, Aug 19, 2021 at 7:18 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>> On 19.08.2021 16:02, H.J. Lu via Binutils wrote:
> >>>>> --- /dev/null
> >>>>> +++ b/gas/testsuite/gas/i386/bad-bcast.s
> >>>>> @@ -0,0 +1,2 @@
> >>>>> +     .text
> >>>>> +     .byte 0x62, 0xc3, 0x8c, 0x1d, 0x66, 0x90, 0x66, 0x90, 0x66, 0x90
> >>>>
> >>>> Would you mind adding a comment indicating what this resembles?
> >>>
> >>> I will add some comments.
> >>
> >> Hmm, I guess I wasn't explicit enough: While the comment you add
> >> now states the EVEX.W aspect, what I was after is to save readers
> >> from having to look up what opcode this is and hence why broadcast
> >> is invalid here with EVEX.W in the first place. Due to the rubbish
> >> the disassembler produces its output isn't any help here either.
> >> So what I was looking to have is a commented instruction with
> >> operands (which the assembler would reject), plus expressing of
> >> the EVEX.W aspect in whatever way is suitable. And I actually
> >> wonder whether the insn you've chosen isn't invalid even without
> >> broadcast, but with EVEX.W set. In which case I wouldn't be
> >> convinced of the usefulness of the test - it would become invalid
> >> once EVEX.W gains a meaning for the major opcode in question,
> >> would be removed at that point, and the logic in question wouldn't
> >> be tested anymore at all.
> >
> > The input bytes aren't valid x86 opcodes.   But abort isn't nice.
> > If people run into the this, ".byte" should give a clue for invalid
> > opcodes.
>
> So a comment along the lines of
>
> # "vfpclasspsx $0x90, (%eax){1to8}, %k0" but with EVEX.W set
>
> is not helpful, you would think?
>

No.   We need a different byte sequence for vfpclasspsx since the
first 2 bytes, 0x62, 0xc3, are invalid.

-- 
H.J.