[llvm-dev] O2 Aggressive Optimization by GCC

[Umesh Kalappa via llvm-dev]

Hi Tim ,

Thank you for the suggestions,we need the side effects(assignment and
increment) of the test() take place and observable for  other threads
,without using the memory model construct .

Otherway,how we can say optimization not to consider the UB and
optimization way the statements,any clang options for the same ?

Thank you
~Umesh

On Fri, Jul 20, 2018 at 6:18 PM, Tim Northover <t.p.northover at gmail.com> wrote:
> On Fri, 20 Jul 2018 at 13:20, Umesh Kalappa via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> Anyone from the community ,please share their insights why above
>> transformation is right ?
>
> In addition to the UB you mentioned yourself, there's no way any other
> thread could detect that j has been modified without introducing a
> race (because this thread doesn't do any synchronization and j is
> non-atomic).
>
> I sort of doubt that the optimizer is making use of that fact because
> it's very hard in general to prove that no synchronization occurs
> elsewhere. But it should probably give you pause over just what you're
> trying to do.
>
>> and without using volatile or memory barrier ,how we can stop the
>> above transformation.
>
> You could use a (possibly memory_order_relaxed) atomic operation to
> store to j. That would then be something the compiler has to ensure
> reaches other threads.
>
> Cheers.
>
> Tim.