[PATCH] Set -fprofile-update=atomic when -pthread is present

[Richard Biener]

On August 18, 2016 5:54:49 PM GMT+02:00, Jakub Jelinek <jakub@redhat.com> wrote:
>On Thu, Aug 18, 2016 at 08:51:31AM -0700, Andi Kleen wrote:
>> > I'd prefer to make updates atomic in multi-threaded applications.
>> > The best proxy we have for that is -pthread.
>> > 
>> > Is it slower, most definitely, but odds are we're giving folks
>> > garbage data otherwise, which in many ways is even worse.
>> 
>> It will likely be catastrophically slower in some cases. 
>> 
>> Catastrophically as in too slow to be usable.
>> 
>> An atomic instruction is a lot more expensive than a single
>increment. Also
>> they sometimes are really slow depending on the state of the machine.
>
>Can't we just have thread-local copies of all the counters (perhaps
>using
>__thread pointer as base) and just atomically merge at thread
>termination?

I suggested that as well but of course it'll have its own class of issues (short lived threads, so we need to somehow re-use counters from terminated threads, large number of threads and thus using too much memory for the counters)

Richard.

>	Jakub