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

[H.J. Lu]

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.

-- 
H.J.