As of this writing (but perhaps not for very much longer!) the four mainstream compilers on Godbolt Compiler Explorer give four different answers for this simple C++ program:
V. Saraswat, R. Jagadeesan, M. Michael, and C. von Praun. PPoPP '07: Proceedings of the 12th ACM SIGPLAN symposium on Principles and practice of parallel programming, page 161--172. New York, NY, USA, ACM, (2007)