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

[Jan Beulich]

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)).

Jan