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

[Jan Beulich <jbeulich at suse dot com>]

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.

Jan