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 Fri, Nov 8, 2019 at 8:55 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 08.11.2019 16:54,  H.J. Lu  wrote:
> > On Fri, Nov 8, 2019 at 12:09 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 07.11.2019 18:47,  H.J. Lu  wrote:
> >>> 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.
> >>
> >> Then _again_ - what is your alternative suggestion?
> >
> > Don't add DefaultSize to more instructions.
>
> H.J., excuse me. This doesn't help the situation. I'm not adding these
> attributes just for the fun of it. If I'm not to add them, I need some
> other solution to whatever breaks without them (I'm sorry, this is 3
> or 4 year old code, so I don't recall exactly what doesn't work anymore
> without adding any one of them). What I'm really not looking forward to
> doing are further opcode-based special cases elsewhere in the code.
>

Please open a bug for the problem you are trying to solve.


-- 
H.J.