Re: [PATCH] x86: work around clang IAS bug referencing __force_order

[Thomas Gleixner]

On Thu, Aug 13 2020 at 14:09, Arvind Sankar wrote:
> On Thu, Aug 13, 2020 at 10:37:01AM -0700, Paul E. McKenney wrote:
>> > Let me ask (hopefully) useful questions this time:
>> > 
>> >   Is a compiler allowed to reorder two 'asm volatile()'?
>> > 
>> >   Are there compilers (gcc >= 4.9 or other supported ones) which do that?
>> 
>> I would hope that the answer to both of these questions is "no"!
>> 
>> But I freely confess that I have been disappointed before on this sort
>> of thing.  :-/
>> 
>> 							Thanx, Paul
>
> Ok, I found this, so gcc developers consider re-ordering volatile asm
> wrt each other a bug at least.
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602

Yes. It prevents reordering of volatiles, but it does not necessarily
prevent reorder of something like this:

        asm volatile(...);
        foo();
        asm volatile(...);

it might turn that into

        foo();
        asm volatile(...);
        asm volatile(...);

if I understood their discussion correctly. So removing this magic is
not really straight forward.

Thanks,

        tglx