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Subject: Re: Should std::atomic<T*> have operator->?
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2019-10-21 12:33:02


On 2019-10-21 18:36, Thiago Macieira via Std-Discussion wrote:
> On Monday, 21 October 2019 00:44:24 PDT Jeremy Ong via Std-Discussion wrote:
>> You have now done an atomic load 3 times and it is possible that stores or
>> loads to foo have occurred in between. The pattern for atomic usage is to
>> typically load once, perform a change, and possibly compare/swap in a loop
>> in a manner that prevents ABA bugs.
>
> If the compiler can inline those three function calls, it may also coalesce
> the three loads into one. AFAIK, no compiler currently does it, but it could.
> If it can't inline, then the regular rules of visibility apply and if the
> atomic pointer's own address is not accessible by other threads and the called
> functions, coalescing can happen again.
>
> But if it can change, then the code is actually optimal, since you do have tot
> reload it every time. Except that you have to write:
>
> foo.load()->do_one_thing();
> foo.load()->do_another_thing();
> foo.load()->do_a_final_thing();

I don't think the compiler can ever prove that the pointer is not
accessed by any other threads or signal handlers. Even in case of LTO,
the compiler can't be sure external threads spawned by OS or shared
libraries or signal handlers don't modify the atomic pointer.


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