Re: [PATCH 3/6] x86: harmonize disp with imm handling

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

On Fri, Jun 18, 2021 at 7:52 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 18.06.2021 16:12, H.J. Lu wrote:
> > On Fri, Jun 18, 2021 at 2:03 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 17.06.2021 18:12, H.J. Lu wrote:
> >>> On Thu, Jun 17, 2021 at 9:05 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>
> >>>> On 17.06.2021 18:00, H.J. Lu wrote:
> >>>>> On Thu, Jun 17, 2021 at 7:57 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>>>
> >>>>>> On 17.06.2021 16:46, H.J. Lu wrote:
> >>>>>>> On Mon, Jun 14, 2021 at 3:25 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>>>>> --- /dev/null
> >>>>>>>> +++ b/gas/testsuite/gas/i386/disp-imm-32.s
> >>>>>>>> @@ -0,0 +1,17 @@
> >>>>>>>> +       .text
> >>>>>>>> +disp_imm:
> >>>>>>>> +       mov     -0xffffffff(%eax), %eax
> >>>>>>>
> >>>>>>> I don't think we should treat  -0xffffffff(%eax) as 1(%eax).
> >>>>>>> We allow addresses to wraparound. I don't see a need for
> >>>>>>> displacements to wraparound.
> >>>>>>
> >>>>>> This then is entirely unexpected to the programmer. In fact the
> >>>>>> same (abstracted away behind some defines or equates) constant
> >>>>>> could be used for both purposes (and should be usable both ways,
> >>>>>> imo).
> >>>>>
> >>>>> Since hardware wraparound on DISP + BASE + INDEX * SCALE, not
> >>>>> on DISP, it is wrong to change DISP + BASE + INDEX * SCALE to
> >>>>> wraparound (DISP) + BASE + INDEX * SCALE.
> >>>>
> >>>> But this is true regardless of how small (or big) the displacement.
> >>>> Without knowing the register values, you can't know at what
> >>>> displacement values wraparound occurs. Also, unless I'm mistaken,
> >>>> wrapround(a + b) == wrapround(wrapround(a) + wrapround(b)).
> >>>
> >>> Hardware does wraparound (DISP + BASE + INDEX * SCALE).
> >>> Assembler and linker should only wraparound on the final address.
> >>
> >> I'm afraid this last sentence makes no sense to me: The assembler
> >> (or linker) can't know the final address. Instead, both immediates
> >> and displacements should allow for anything the programmer might
> >> sensibly use. If 0xffffffff as a displacement is fine (meaning
> >> -1 really), -0xffffffff (meaning 1) ought to be, too. Or else
> >> where do you draw the boundary of which displacements are
> >> "legitimate" and which are not?
> >
> > If the program wants 1, use 1.
>
> You realize that for the purpose of the test case the -0xffffffff is
> an over-simplification of what might appear in "real" code, don't
> you? It also feels as if you didn't really read my earlier remark:

I don't think treating -0xffffffff as 1 here helps anyone.

> "In fact the same (abstracted away behind some defines or equates)
>  constant could be used for both purposes (and should be usable
>  both ways, imo)."
>
> If what you say were true for disp, why would it not also apply to
> imm? In the end, an imm loaded into a register may very well be
> used to access memory, i.e. possibly fulfills the purpose of a disp.
> We shouldn't force people to write quirky code just because gas
> accepts certain things for imm which it doesn't accept for disp.
>

Loading imm into register is similar to wraparound in C.  DISP
is different.

-- 
H.J.