[v3] libstdc++/24469

[Paolo Carlini]

Richard Guenther wrote:

>>I have also to study again this long standing issue vs the brancd new
>>performance improvements contributed by Boreham and Bridges, but in
>>fact, in the ""old"" code the issue was rather simple: _M_used, and only _M_used, could be changed simultaneously by different threads. Thus, a possible fix could involve using __exchange_and_add/__atomic_add on _M_used.
>>    
>>
>I do not see this concurrent updating of _M_used.
>
Did you study the PR? Sorry, Richard, but this "assault" of yours in the 
occasion of the patch, with no previous discussions and participation to 
the development of the code, in some areas definitely rather subtle, 
seems to me rather weird and naive.

Paolo.