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 Thu, Nov 7, 2019 at 2:27 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 06.11.2019 23:54,  H.J. Lu  wrote:
> > On Mon, Nov 4, 2019 at 11:45 PM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 04.11.2019 18:12, H.J. Lu wrote:
> >>> 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.
> >>
> >> Then what is it that DefaultSize is needed for on its template?
> >>
> >>>> Or is this behavior firmly documented? The main gas documentation
> >>>> certainly doesn't, afaics.
> >>>
> >>> I don't think code16gcc is well documented.
> >>
> >> Which is not very helpful.
> >>
> >>>>> 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?
> >>
> >> There are two uses in process_suffix(), and only one of them is
> >> .coge16gcc related afaict. The other also affects 64-bit mode,
> >> or else I don't understand why various Cpu64 templates also have
> >> the attribute.
> >
> > DefaultSize makes no difference on Cpu64 push/pop in AT&T syntax.
> > It is only used by Intel syntax.  In AT&T syntax, only i.tm.opcode_modifier.w
> > instructions need suffix.   I don't think we should add DefaultSize to more
> > instructions.
>
> What I continue to miss is what you suggest as an alternative. Did
> you mean to commit that other change, widening the attribute to 2
> bits, and you'd then expect me to re-base over it? Aiui this would
> improve the situation, but not necessarily avoid adding exceptions
> (I'd have to check if it actually does). I'd also like to note that
> your mention of only push/pop for 64-bit looks incomplete to me -
> as said, call, ret, enter etc also have that attribute.

I don't think DefaultSize matters for them either in AT&T syntax.
and I don't think we should add DefaultSize to more instructions.

-- 
H.J.