Re: [PATCH] x86: Update register operand check for AddrPrefixOpReg

[Jan Beulich <jbeulich@suse.com>]

On 02.10.2020 13:33, H.J. Lu wrote:
> On Thu, Oct 1, 2020 at 11:58 PM Jan Beulich <jbeulich@suse.com> wrote:
>> On 01.10.2020 17:08, H.J. Lu wrote:
>>> GNU assembler accommodates other usages.  But it must work with GCC.
>>
>> We agree on this. What you don't want to accept is that gcc also
>> needs to emit correct code for gas to consume.
>>
>>> In this case, it needs to check the register operand for the address size prefix
>>> when the memory operand is (%rip).
>>
>> And we again agree on the checking aspect. What we disagree on is
>> what to do if the check finds a discrepancy. Without your recent
>> change, the checking you talk about did already happen. And the
>> assembler correctly diagnosed it. The assembler may not _ever_
>> guess upon what the programmer meant, when finding an ambiguity
>> (exceptions to this rule may be warranted for backwards
>> compatibility in very rare cases, but this clearly doesn't apply
>> here). We've had this discussion before in different contexts,
>> and I have to admit I'm struggling with how you drive things in
>> this regard. If there was a way for gcc to indicate "I've done my
>> best, but please fix it up for me", I could accept there being
>> such a (non-default) mode. I wouldn't like it very much, but I
>> could live with it. But for hand-written code (be it entire .s
>> files or inline assembly) inconsistencies and ambiguities need to
>> be diagnosed by default.
>>
>> Note in particular that your change isn't even limited to x32
>> mode, where one _might_ view the adjustment as benign.
> 
> The address size prefix isn't limited to x32.   For RIP-relative
> addressing, GCC always uses (%rip).

But the issue at hand is limited to x32, and the conversion you're
doing is also only half-way safe for x32.

>  The assembler will never
> guess correctly what the assembly code is trying to do in all
> cases.

Hence it shouldn't guess. It should tell the programmer to change
the code so guessing isn't needed.

>  In this particular case, the assembler needs to look
> at the register operand to extract the address size prefix.

And how does it know _that's_ the part which is correct (as per
the programmer's intentions), and not the other operand?

Jan