[llvm-dev] O2 Aggressive Optimization by GCC

[Matthias Braun via llvm-dev]

You need to use volatile variables if you need assignments to be visible side effects.

- Matthias

> On Jul 22, 2018, at 8:17 AM, Umesh Kalappa via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 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.
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