Re: [PATCH v2 5/9] x86: improve handling of insns with ambiguous operand sizes

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

On Mon, Nov 4, 2019 at 2:29 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 31.10.2019 18:26,  H.J. Lu  wrote:
> > On Thu, Oct 31, 2019 at 2:24 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 31.10.2019 00:57, H.J. Lu wrote:
> >>> On Wed, Oct 30, 2019 at 12:59 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>
> >>>> On 29.10.2019 18:55,  H.J. Lu  wrote:
> >>>>> On Mon, Oct 28, 2019 at 1:05 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>>>
> >>>>>> Commit b76bc5d54e ("x86: don't default variable shift count insns to
> >>>>>> 8-bit operand size") pointed out a very bad case, but the underlying
> >>>>>> problem is, as mentioned on various occasions, much larger: Silently
> >>>>>> selecting a (nowhere documented afaict) certain default operand size
> >>>>>> when there's no "sizing" suffix and no suitable register operand(s) is
> >>>>>> simply dangerous (for the programmer to make mistakes).
> >>>>>>
> >>>>>> While in Intel syntax mode such mistakes already lead to an error (which
> >>>>>> is going to remain that way), AT&T syntax mode now gains warnings in
> >>>>>> such cases by default, which can be suppressed or promoted to an error
> >>>>>> if so desired by the programmer. Furthermore at least general purpose
> >>>>>> insns now consistently have a default applied (alongside the warning
> >>>>>> emission), rather than accepting some and refusing others.
> >>>>>>
> >>>>>> No warnings are (as before) to be generated for "DefaultSize" insns as
> >>>>>> well as ones acting on selector and other fixed-width values. The set of
> >>>>>> "DefaultSize" ones gets slightly widened for the purposes here.
> >>>>>
> >>>>> What is the advantage to add DefaultSize vs the alternative?
> >>>>
> >>>> I don't know what alternative you refer to; if you mean some
> >>>> hypothetical one, then the advantage of simply adding
> >>>> DefaultSize as done here is likely that it allows to not add or
> >>>> further complicate logic in tc-i386*.c. Furthermore the ones which
> >>>> get the attribute added should have had it already before, if the
> >>>> comment "default insn size depends on mode" is to be trusted.
> >>>>
> >>>
> >>> DefaultSize is added to some instructions and then they are excluded:
> >>>
> >>> +          /* exclude jmp/ljmp */
> >>> +          && strcmp (i.tm.name, "jmp") && strcmp (i.tm.name, "ljmp")
> >>> +          /* exclude byte-displacement jumps */
> >>> +          && !i.tm.opcode_modifier.jumpbyte
> >>> +          /* exclude lgdt/lidt/sgdt/sidt */
> >>> +          && (i.tm.base_opcode != 0x0f01 || i.tm.extension_opcode > 3)
> >>>            /* exclude fldenv/frstor/fsave/fstenv */
> >>>            && i.tm.opcode_modifier.no_ssuf)
> >>>
> >>> It looks odd.
> >>
> >> But this isn't the only place where defaultsize gets evaluated.
> >> See how lgdt/lidt/sgdt/sidt already have the attribute in the
> >> opcode table, but need exclusion here now too. The alternative
> >> would be two independent attributes - one to be evaluated here,
> >> and the other to be evaluated further down in the function. Yet
> >> again - this dual use has been there before, and just needs
> >> suitable extending of the logic now.
> >>
> >
> > Normally instructions with DefaultSize have i.suffix unset.  Except with
> > .code16gcc, which is used to support 16-bit mode with 32-bit address,
> > i.suffix is set to 'l' for 32-bit address.
>
> I don't follow you here: Since when is there a connection between
> 'l' suffix and addressing mode? All .code16gcc distinguishes from
> plain .code16 is stack pointer width, isn't it? In which case
> using fldenv etc in their 32-bit operand size form looks wrong.

fldenv doesn't use 32-bit operand size.

> Or is this behavior firmly documented? The main gas documentation
> certainly doesn't, afaics.

I don't think code16gcc is well documented.

> > However, iret/fldenv/frstor/fsave/fstenv
> > are exceptions since they need 16-bit variants.  So we need 2 different
> > DefaultSize behaviors for .code16gcc, one uses LONG_MNEM_SUFFIX
> > and the other uses WORD_MNEM_SUFFIX.  We should update
> > DefaultSize to properly encode iret/fldenv/frstor/fsave/fstenv for
> > .code16gcc, instead of checking i.tm.opcode_modifier.no_ssuf.
> > Something like this.
>
> Plausible, but still afaict orthogonal to what I'm doing here, and
> what you look to be unhappy about.
>

DefaultSize only impacts .code16gcc.   Why adding DefaultSize to these
instructions?


-- 
H.J.