On 08/18/2016 05:53 PM, Jeff Law wrote:
> On 08/18/2016 09:51 AM, 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.
> And for those cases there's a way to override.
> 
> The default should be set for correctness.
> 
> jeff

I would to somehow resolve the discussion related to default value selection.
Is the prevailing consensus that we should set -fprofile-update=atomic when
-pthread is set? If so, I'll prepare a patch. I tend to do it this way.

Moreover, I also have a patch that provides a warning, which can be also useful
even though we would change the default behavior:

$ ./xgcc -B. /tmp/a.c -fprofile-update=single -pthread -fprofile-generate
xgcc: warning: -profile-update=atomic should be used to generate a valid profile for a multithreaded application

Ideas?
Martin