Re: processor errata vs encodings produced by gas

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

On Wed, Dec 19, 2018 at 11:40 PM Jan Beulich <JBeulich@suse.com> wrote:
>
> >>> On 19.12.18 at 18:27, <hjl.tools@gmail.com> wrote:
> > On Wed, Dec 19, 2018 at 8:59 AM Jan Beulich <JBeulich@suse.com> wrote:
> >>
> >> Hi H.J.,
> >>
> >> having had to deal with such SandyBridge errata (BT36, BT41, and
> >> BT230) in a different context, it occurred to me that it's not ideal
> >> for gas to (allow to) produce such encodings in the first place. The
> >> problem has been there virtually forever for VCVT{,T}S{S,D}2SI
> >
> > True.
> >
> >> and the use of -mavxscalar=256, but our combined recent effort
> >> to improve VEX.W handling has extended it to VZERO{ALL,UPPER}
> >> and (in the 32-bit case) VPEXTRD when of using -mvexwig=1.
> >>
> >> These are errata, so I'm not suggesting to revert what has been
> >> done. Instead I'm trying to think of ways to point out the issue
> >> to the programmer and/or default to safe encodings but still allow
> >> a means to override them. One question of course is what the
> >> target audience for options like -mavxscalar= was meant to be,
> >
> > They are used to generate different encodings to test disassembler.
>
> So flagging them (in documentation) as "don't use for generating
> production code" would be enough - are you fine with just doing
> that?

If we fix i386-opc.tbl so that -mavxscalar= works everywhere,
do we still need it?

> >> when these options got introduced. Hence a variant of the first
> >> approach could be to simply flag use of these options as
> >> "dangerous" (and hence a suggestion to not use them for building
> >> production code).
> >>
> >> As to -mavxscalar=256 in particular, the SDM now recommends
> >> to set VEX.L to zero for all scalar insns.
> >
> > We need to properly mark all instructions, including errata, with WIG.
>
> I'm afraid this is ambiguous to me: "Properly" might mean "according
> to the SDM" or "according to what actually works everywhere".
> Would you mind clarifying?

According to what actually works everywhere with comments in
i386-opc.tbl explaining why we differs from SDM.

-- 
H.J.