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

[Jan Beulich <jbeulich@suse.com>]

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?

Jan